Add the opfs-sahpool sqlite3_vfs implementation to JS, offering an alternative to the other OPFS VFS (with tradeoffs).

FossilOrigin-Name: d2e602cda44bf35e76167143262b4f91826d25780d0e095e680a31d5dedb2018

7 files changed, 1270 insertions, 56 deletions

diff --git a/ext/wasm/api/README.md b/ext/wasm/api/README.md
index 6618666ae..eb0f073cf 100644
--- a/ext/wasm/api/README.md
+++ b/ext/wasm/api/README.md
@@ -83,15 +83,18 @@ browser client:
   helpers for use by downstream code which creates `sqlite3_vfs`
   and `sqlite3_module` implementations.
 - **`sqlite3-vfs-opfs.c-pp.js`**\  
-  is an sqlite3 VFS implementation which supports Google Chrome's
-  Origin-Private FileSystem (OPFS) as a storage layer to provide
-  persistent storage for database files in a browser. It requires...
+  is an sqlite3 VFS implementation which supports the Origin-Private
+  FileSystem (OPFS) as a storage layer to provide persistent storage
+  for database files in a browser. It requires...
     - **`sqlite3-opfs-async-proxy.js`**\  
       is the asynchronous backend part of the OPFS proxy. It speaks
       directly to the (async) OPFS API and channels those results back
       to its synchronous counterpart. This file, because it must be
       started in its own Worker, is not part of the amalgamation.
-- **`api/sqlite3-api-cleanup.js`**\  
+- **`sqlite3-vfs-opfs-sahpool.c-pp.js`**\  
+  is another sqlite3 VFS supporting the OPFS, but uses a completely
+  different approach that the above-listed one.
+- **`sqlite3-api-cleanup.js`**\  
   The previous files do not immediately extend the library. Instead
   they add callback functions to be called during its
   bootstrapping. Some also temporarily create global objects in order
@@ -108,13 +111,15 @@ browser client:
 with `c-pp`](#c-pp), noting that such preprocessing may be applied
 after all of the relevant files are concatenated. That extension is
 used primarily to keep the code maintainers cognisant of the fact that
-those files contain constructs which will not run as-is in JavaScript.
+those files contain constructs which may not run as-is in any given
+JavaScript environment.
 
 The build process glues those files together, resulting in
-`sqlite3-api.js`, which is everything except for the `post-js-*.js`
-files, and `sqlite3.js`, which is the Emscripten-generated amalgamated
-output and includes the `post-js-*.js` parts, as well as the
-Emscripten-provided module loading pieces.
+`sqlite3-api.js`, which is everything except for the
+`pre/post-js-*.js` files, and `sqlite3.js`, which is the
+Emscripten-generated amalgamated output and includes the
+`pre/post-js-*.js` parts, as well as the Emscripten-provided module
+loading pieces.
 
 The non-JS outlier file is `sqlite3-wasm.c`: it is a proxy for
 `sqlite3.c` which `#include`'s that file and adds a couple more
@@ -152,8 +157,8 @@ Preprocessing of Source Files
 ------------------------------------------------------------------------
 
 Certain files in the build require preprocessing to filter in/out
-parts which differ between vanilla JS builds and ES6 Module
-(a.k.a. esm) builds. The preprocessor application itself is in
+parts which differ between vanilla JS, ES6 Modules, and node.js
+builds. The preprocessor application itself is in
 [`c-pp.c`](/file/ext/wasm/c-pp.c) and the complete technical details
 of such preprocessing are maintained in
 [`GNUMakefile`](/file/ext/wasm/GNUmakefile).
diff --git a/ext/wasm/api/sqlite3-api-prologue.js b/ext/wasm/api/sqlite3-api-prologue.js
index f3d0a7c40..0179091c0 100644
--- a/ext/wasm/api/sqlite3-api-prologue.js
+++ b/ext/wasm/api/sqlite3-api-prologue.js
@@ -88,9 +88,9 @@
      can be replaced with (e.g.) empty functions to squelch all such
      output.
 
-   - `wasmfsOpfsDir`[^1]: As of 2022-12-17, this feature does not
-     currently work due to incompatible Emscripten-side changes made
-     in the WASMFS+OPFS combination. This option is currently ignored.
+   - `wasmfsOpfsDir`[^1]: Specifies the "mount point" of the OPFS-backed
+     filesystem in WASMFS-capable builds.
+
 
    [^1] = This property may optionally be a function, in which case
           this function calls that function to fetch the value,
@@ -125,11 +125,11 @@ globalThis.sqlite3ApiBootstrap = function sqlite3ApiBootstrap(
     log: console.log.bind(console),
     wasmfsOpfsDir: '/opfs',
     /**
-       useStdAlloc is just for testing an allocator discrepancy. The
+       useStdAlloc is just for testing allocator discrepancies. The
        docs guarantee that this is false in the canonical builds. For
        99% of purposes it doesn't matter which allocators we use, but
-       it becomes significant with, e.g., sqlite3_deserialize()
-       and certain wasm.xWrap.resultAdapter()s.
+       it becomes significant with, e.g., sqlite3_deserialize() and
+       certain wasm.xWrap.resultAdapter()s.
     */
     useStdAlloc: false
   }, apiConfig || {});
@@ -1861,6 +1861,9 @@ globalThis.sqlite3ApiBootstrap = function sqlite3ApiBootstrap(
     client: undefined,
 
     /**
+       This function is not part of the public interface, but a
+       piece of internal bootstrapping infrastructure.
+
        Performs any optional asynchronous library-level initialization
        which might be required. This function returns a Promise which
        resolves to the sqlite3 namespace object. Any error in the
@@ -1876,27 +1879,19 @@ globalThis.sqlite3ApiBootstrap = function sqlite3ApiBootstrap(
        then it must be called by client-level code, which must not use
        the library until the returned promise resolves.
 
-       Bug: if called while a prior call is still resolving, the 2nd
-       call will resolve prematurely, before the 1st call has finished
-       resolving. The current build setup precludes that possibility,
-       so it's only a hypothetical problem if/when this function
-       ever needs to be invoked by clients.
+       If called multiple times it will return the same promise on
+       subsequent calls. The current build setup precludes that
+       possibility, so it's only a hypothetical problem if/when this
+       function ever needs to be invoked by clients.
 
        In Emscripten-based builds, this function is called
        automatically and deleted from this object.
     */
-    asyncPostInit: async function(){
-      let lip = sqlite3ApiBootstrap.initializersAsync;
+    asyncPostInit: async function ff(){
+      if(ff.isReady instanceof Promise) return ff.isReady;
+      let lia = sqlite3ApiBootstrap.initializersAsync;
       delete sqlite3ApiBootstrap.initializersAsync;
-      if(!lip || !lip.length) return Promise.resolve(sqlite3);
-      lip = lip.map((f)=>{
-        const p = (f instanceof Promise) ? f : f(sqlite3);
-        return p.catch((e)=>{
-          console.error("an async sqlite3 initializer failed:",e);
-          throw e;
-        });
-      });
-      const postInit = ()=>{
+      const postInit = async ()=>{
         if(!sqlite3.__isUnderTest){
           /* Delete references to internal-only APIs which are used by
              some initializers. Retain them when running in test mode
@@ -1905,23 +1900,25 @@ globalThis.sqlite3ApiBootstrap = function sqlite3ApiBootstrap(
           /* It's conceivable that we might want to expose
              StructBinder to client-side code, but it's only useful if
              clients build their own sqlite3.wasm which contains their
-             one C struct types. */
+             own C struct types. */
           delete sqlite3.StructBinder;
         }
         return sqlite3;
       };
-      if(1){
-        /* Run all initializers in sequence. The advantage is that it
-           allows us to have post-init cleanup defined outside of this
-           routine at the end of the list and have it run at a
-           well-defined time. */
-        let p = lip.shift();
-        while(lip.length) p = p.then(lip.shift());
-        return p.then(postInit);
-      }else{
-        /* Run them in an arbitrary order. */
-        return Promise.all(lip).then(postInit);
+      const catcher = (e)=>{
+        config.error("an async sqlite3 initializer failed:",e);
+        throw e;
+      };
+      if(!lia || !lia.length){
+        return ff.isReady = postInit().catch(catcher);
       }
+      lia = lia.map((f)=>{
+        return (f instanceof Function) ? async x=>f(sqlite3) : f;
+      });
+      lia.push(postInit);
+      let p = Promise.resolve(sqlite3);
+      while(lia.length) p = p.then(lia.shift());
+      return ff.isReady = p.catch(catcher);
     },
     /**
        scriptInfo ideally gets injected into this object by the
@@ -1981,7 +1978,7 @@ globalThis.sqlite3ApiBootstrap.initializers = [];
   specifically for initializers which are asynchronous. All entries in
   this list must be either async functions, non-async functions which
   return a Promise, or a Promise. Each function in the list is called
-  with the sqlite3 ojbect as its only argument.
+  with the sqlite3 object as its only argument.
 
   The resolved value of any Promise is ignored and rejection will kill
   the asyncPostInit() process (at an indeterminate point because all
diff --git a/ext/wasm/api/sqlite3-opfs-async-proxy.js b/ext/wasm/api/sqlite3-opfs-async-proxy.js
index 179a81635..cafd296c6 100644
--- a/ext/wasm/api/sqlite3-opfs-async-proxy.js
+++ b/ext/wasm/api/sqlite3-opfs-async-proxy.js
@@ -35,6 +35,9 @@
 
   https://developer.chrome.com/blog/sync-methods-for-accesshandles/
 
+  Firefox v111 and Safari 16.4, both released in March 2023, also
+  include this.
+
   We cannot change to the sync forms at this point without breaking
   clients who use Chrome v104-ish or higher. truncate(), getSize(),
   flush(), and close() are now (as of v108) synchronous. Calling them
diff --git a/ext/wasm/api/sqlite3-v-helper.js b/ext/wasm/api/sqlite3-v-helper.js
index 80ab7c5b0..e63da8afc 100644
--- a/ext/wasm/api/sqlite3-v-helper.js
+++ b/ext/wasm/api/sqlite3-v-helper.js
@@ -100,7 +100,7 @@ globalThis.sqlite3ApiBootstrap.initializers.push(function(sqlite3){
 
      ACHTUNG: because we cannot generically know how to transform JS
      exceptions into result codes, the installed functions do no
-     automatic catching of exceptions. It is critical, to avoid 
+     automatic catching of exceptions. It is critical, to avoid
      undefined behavior in the C layer, that methods mapped via
      this function do not throw. The exception, as it were, to that
      rule is...
@@ -295,7 +295,8 @@ globalThis.sqlite3ApiBootstrap.initializers.push(function(sqlite3){
 
      - If `struct.$zName` is falsy and the entry has a string-type
        `name` property, `struct.$zName` is set to the C-string form of
-       that `name` value before registerVfs() is called.
+       that `name` value before registerVfs() is called. That string
+       gets added to the on-dispose state of the struct.
 
      On success returns this object. Throws on error.
   */
@@ -608,7 +609,7 @@ globalThis.sqlite3ApiBootstrap.initializers.push(function(sqlite3){
      This is to facilitate creation of those methods inline in the
      passed-in object without requiring the client to explicitly get a
      reference to one of them in order to assign it to the other
-     one. 
+     one.
 
      The `catchExceptions`-installed handlers will account for
      identical references to the above functions and will install the
diff --git a/ext/wasm/api/sqlite3-vfs-opfs-sahpool.c-pp.js b/ext/wasm/api/sqlite3-vfs-opfs-sahpool.c-pp.js
new file mode 100644
index 000000000..28524b613
--- /dev/null
+++ b/ext/wasm/api/sqlite3-vfs-opfs-sahpool.c-pp.js
@@ -0,0 +1,1199 @@
+//#ifnot target=node
+/*
+  2023-07-14
+
+  The author disclaims copyright to this source code.  In place of a
+  legal notice, here is a blessing:
+
+  *   May you do good and not evil.
+  *   May you find forgiveness for yourself and forgive others.
+  *   May you share freely, never taking more than you give.
+
+  ***********************************************************************
+
+  This file holds a sqlite3_vfs backed by OPFS storage which uses a
+  different implementation strategy than the "opfs" VFS. This one is a
+  port of Roy Hashimoto's OPFS SyncAccessHandle pool:
+
+  https://github.com/rhashimoto/wa-sqlite/blob/master/src/examples/AccessHandlePoolVFS.js
+
+  As described at:
+
+  https://github.com/rhashimoto/wa-sqlite/discussions/67
+
+  with Roy's explicit permission to permit us to port his to our
+  infrastructure rather than having to clean-room reverse-engineer it:
+
+  https://sqlite.org/forum/forumpost/e140d84e71
+
+  Primary differences from the "opfs" VFS include:
+
+  - This one avoids the need for a sub-worker to synchronize
+  communication between the synchronous C API and the
+  only-partly-synchronous OPFS API.
+
+  - It does so by opening a fixed number of OPFS files at
+  library-level initialization time, obtaining SyncAccessHandles to
+  each, and manipulating those handles via the synchronous sqlite3_vfs
+  interface. If it cannot open them (e.g. they are already opened by
+  another tab) then the VFS will not be installed.
+
+  - Because of that, this one lacks all library-level concurrency
+  support.
+
+  - Also because of that, it does not require the SharedArrayBuffer,
+  so can function without the COOP/COEP HTTP response headers.
+
+  - It can hypothetically support Safari 16.4+, whereas the "opfs" VFS
+  requires v17 due to a subworker/storage bug in 16.x which makes it
+  incompatible with that VFS.
+
+  - This VFS requires the "semi-fully-sync" FileSystemSyncAccessHandle
+  (hereafter "SAH") APIs released with Chrome v108 (and all other
+  major browsers released since March 2023). If that API is not
+  detected, the VFS is not registered.
+*/
+globalThis.sqlite3ApiBootstrap.initializers.push(function(sqlite3){
+  'use strict';
+  const toss = sqlite3.util.toss;
+  const toss3 = sqlite3.util.toss3;
+  const initPromises = Object.create(null);
+  const capi = sqlite3.capi;
+  const wasm = sqlite3.wasm;
+  // Config opts for the VFS...
+  const SECTOR_SIZE = 4096;
+  const HEADER_MAX_PATH_SIZE = 512;
+  const HEADER_FLAGS_SIZE = 4;
+  const HEADER_DIGEST_SIZE = 8;
+  const HEADER_CORPUS_SIZE = HEADER_MAX_PATH_SIZE + HEADER_FLAGS_SIZE;
+  const HEADER_OFFSET_FLAGS = HEADER_MAX_PATH_SIZE;
+  const HEADER_OFFSET_DIGEST = HEADER_CORPUS_SIZE;
+  const HEADER_OFFSET_DATA = SECTOR_SIZE;
+  /* Bitmask of file types which may persist across sessions.
+     SQLITE_OPEN_xyz types not listed here may be inadvertently
+     left in OPFS but are treated as transient by this VFS and
+     they will be cleaned up during VFS init. */
+  const PERSISTENT_FILE_TYPES =
+        capi.SQLITE_OPEN_MAIN_DB |
+        capi.SQLITE_OPEN_MAIN_JOURNAL |
+        capi.SQLITE_OPEN_SUPER_JOURNAL |
+        capi.SQLITE_OPEN_WAL /* noting that WAL support is
+                                unavailable in the WASM build.*/;
+
+  /** Subdirectory of the VFS's space where "opaque" (randomly-named)
+      files are stored. Changing this effectively invalidates the data
+      stored under older names (orphaning it), so don't do that. */
+  const OPAQUE_DIR_NAME = ".opaque";
+
+  /**
+     Returns short a string of random alphanumeric characters
+     suitable for use as a random filename.
+  */
+  const getRandomName = ()=>Math.random().toString(36).slice(2);
+
+  const textDecoder = new TextDecoder();
+  const textEncoder = new TextEncoder();
+
+  const optionDefaults = Object.assign(Object.create(null),{
+    name: 'opfs-sahpool',
+    directory: undefined /* derived from .name */,
+    initialCapacity: 6,
+    clearOnInit: false,
+    /* Logging verbosity 3+ == everything, 2 == warnings+errors, 1 ==
+       errors only. */
+    verbosity: 2
+  });
+
+  /** Logging routines, from most to least serious. */
+  const loggers = [
+    sqlite3.config.error,
+    sqlite3.config.warn,
+    sqlite3.config.log
+  ];
+  const log = sqlite3.config.log;
+  const warn = sqlite3.config.warn;
+  const error = sqlite3.config.error;
+
+  /* Maps (sqlite3_vfs*) to OpfsSAHPool instances */
+  const __mapVfsToPool = new Map();
+  const getPoolForVfs = (pVfs)=>__mapVfsToPool.get(pVfs);
+  const setPoolForVfs = (pVfs,pool)=>{
+    if(pool) __mapVfsToPool.set(pVfs, pool);
+    else __mapVfsToPool.delete(pVfs);
+  };
+  /* Maps (sqlite3_file*) to OpfsSAHPool instances */
+  const __mapSqlite3File = new Map();
+  const getPoolForPFile = (pFile)=>__mapSqlite3File.get(pFile);
+  const setPoolForPFile = (pFile,pool)=>{
+    if(pool) __mapSqlite3File.set(pFile, pool);
+    else __mapSqlite3File.delete(pFile);
+  };
+
+  /**
+     Impls for the sqlite3_io_methods methods. Maintenance reminder:
+     members are in alphabetical order to simplify finding them.
+  */
+  const ioMethods = {
+    xCheckReservedLock: function(pFile,pOut){
+      const pool = getPoolForPFile(pFile);
+      pool.log('xCheckReservedLock');
+      pool.storeErr();
+      wasm.poke32(pOut, 1);
+      return 0;
+    },
+    xClose: function(pFile){
+      const pool = getPoolForPFile(pFile);
+      pool.storeErr();
+      const file = pool.getOFileForS3File(pFile);
+      if(file) {
+        try{
+          pool.log(`xClose ${file.path}`);
+          pool.mapS3FileToOFile(pFile, false);
+          file.sah.flush();
+          if(file.flags & capi.SQLITE_OPEN_DELETEONCLOSE){
+            pool.deletePath(file.path);
+          }
+        }catch(e){
+          pool.storeErr(e);
+          return capi.SQLITE_IOERR;
+        }
+      }
+      return 0;
+    },
+    xDeviceCharacteristics: function(pFile){
+      return capi.SQLITE_IOCAP_UNDELETABLE_WHEN_OPEN;
+    },
+    xFileControl: function(pFile, opId, pArg){
+      return capi.SQLITE_NOTFOUND;
+    },
+    xFileSize: function(pFile,pSz64){
+      const pool = getPoolForPFile(pFile);
+      pool.log(`xFileSize`);
+      const file = pool.getOFileForS3File(pFile);
+      const size = file.sah.getSize() - HEADER_OFFSET_DATA;
+      //log(`xFileSize ${file.path} ${size}`);
+      wasm.poke64(pSz64, BigInt(size));
+      return 0;
+    },
+    xLock: function(pFile,lockType){
+      const pool = getPoolForPFile(pFile);
+      pool.log(`xLock ${lockType}`);
+      pool.storeErr();
+      const file = pool.getOFileForS3File(pFile);
+      file.lockType = lockType;
+      return 0;
+    },
+    xRead: function(pFile,pDest,n,offset64){
+      const pool = getPoolForPFile(pFile);
+      pool.storeErr();
+      const file = pool.getOFileForS3File(pFile);
+      pool.log(`xRead ${file.path} ${n} @ ${offset64}`);
+      try {
+        const nRead = file.sah.read(
+          wasm.heap8u().subarray(pDest, pDest+n),
+          {at: HEADER_OFFSET_DATA + Number(offset64)}
+        );
+        if(nRead < n){
+          wasm.heap8u().fill(0, pDest + nRead, pDest + n);
+          return capi.SQLITE_IOERR_SHORT_READ;
+        }
+        return 0;
+      }catch(e){
+        pool.storeErr(e);
+        return capi.SQLITE_IOERR;
+      }
+    },
+    xSectorSize: function(pFile){
+      return SECTOR_SIZE;
+    },
+    xSync: function(pFile,flags){
+      const pool = getPoolForPFile(pFile);
+      pool.log(`xSync ${flags}`);
+      pool.storeErr();
+      const file = pool.getOFileForS3File(pFile);
+      //log(`xSync ${file.path} ${flags}`);
+      try{
+        file.sah.flush();
+        return 0;
+      }catch(e){
+        pool.storeErr(e);
+        return capi.SQLITE_IOERR;
+      }
+    },
+    xTruncate: function(pFile,sz64){
+      const pool = getPoolForPFile(pFile);
+      pool.log(`xTruncate ${sz64}`);
+      pool.storeErr();
+      const file = pool.getOFileForS3File(pFile);
+      //log(`xTruncate ${file.path} ${iSize}`);
+      try{
+        file.sah.truncate(HEADER_OFFSET_DATA + Number(sz64));
+        return 0;
+      }catch(e){
+        pool.storeErr(e);
+        return capi.SQLITE_IOERR;
+      }
+    },
+    xUnlock: function(pFile,lockType){
+      const pool = getPoolForPFile(pFile);
+      pool.log('xUnlock');
+      const file = pool.getOFileForS3File(pFile);
+      file.lockType = lockType;
+      return 0;
+    },
+    xWrite: function(pFile,pSrc,n,offset64){
+      const pool = getPoolForPFile(pFile);
+      pool.storeErr();
+      const file = pool.getOFileForS3File(pFile);
+      pool.log(`xWrite ${file.path} ${n} ${offset64}`);
+      try{
+        const nBytes = file.sah.write(
+          wasm.heap8u().subarray(pSrc, pSrc+n),
+          { at: HEADER_OFFSET_DATA + Number(offset64) }
+        );
+        return nBytes === n ? 0 : capi.SQLITE_IOERR;
+      }catch(e){
+        pool.storeErr(e);
+        return capi.SQLITE_IOERR;
+      }
+    }
+  }/*ioMethods*/;
+
+  const opfsIoMethods = new capi.sqlite3_io_methods();
+  opfsIoMethods.$iVersion = 1;
+  sqlite3.vfs.installVfs({
+    io: {struct: opfsIoMethods, methods: ioMethods}
+  });
+
+  /**
+     Impls for the sqlite3_vfs methods. Maintenance reminder: members
+     are in alphabetical order to simplify finding them.
+  */
+  const vfsMethods = {
+    xAccess: function(pVfs,zName,flags,pOut){
+      //log(`xAccess ${wasm.cstrToJs(zName)}`);
+      const pool = getPoolForVfs(pVfs);
+      pool.storeErr();
+      try{
+        const name = pool.getPath(zName);
+        wasm.poke32(pOut, pool.hasFilename(name) ? 1 : 0);
+      }catch(e){
+        /*ignored*/
+        wasm.poke32(pOut, 0);
+      }
+      return 0;
+    },
+    xCurrentTime: function(pVfs,pOut){
+      wasm.poke(pOut, 2440587.5 + (new Date().getTime()/86400000),
+                'double');
+      return 0;
+    },
+    xCurrentTimeInt64: function(pVfs,pOut){
+      wasm.poke(pOut, (2440587.5 * 86400000) + new Date().getTime(),
+                'i64');
+      return 0;
+    },
+    xDelete: function(pVfs, zName, doSyncDir){
+      const pool = getPoolForVfs(pVfs);
+      pool.log(`xDelete ${wasm.cstrToJs(zName)}`);
+      pool.storeErr();
+      try{
+        pool.deletePath(pool.getPath(zName));
+        return 0;
+      }catch(e){
+        pool.storeErr(e);
+        return capi.SQLITE_IOERR_DELETE;
+      }
+    },
+    xFullPathname: function(pVfs,zName,nOut,pOut){
+      //const pool = getPoolForVfs(pVfs);
+      //pool.log(`xFullPathname ${wasm.cstrToJs(zName)}`);
+      const i = wasm.cstrncpy(pOut, zName, nOut);
+      return i<nOut ? 0 : capi.SQLITE_CANTOPEN;
+    },
+    xGetLastError: function(pVfs,nOut,pOut){
+      const pool = getPoolForVfs(pVfs);
+      pool.log(`xGetLastError ${nOut}`);
+      const e = pool.popErr();
+      if(e){
+        const scope = wasm.scopedAllocPush();
+        try{
+          const [cMsg, n] = wasm.scopedAllocCString(e.message, true);
+          wasm.cstrncpy(pOut, cMsg, nOut);
+          if(n > nOut) wasm.poke8(pOut + nOut - 1, 0);
+        }catch(e){
+          return capi.SQLITE_NOMEM;
+        }finally{
+          wasm.scopedAllocPop(scope);
+        }
+      }
+      return 0;
+    },
+    //xSleep is optionally defined below
+    xOpen: function f(pVfs, zName, pFile, flags, pOutFlags){
+      const pool = getPoolForVfs(pVfs);
+      try{
+        pool.log(`xOpen ${wasm.cstrToJs(zName)} ${flags}`);
+        // First try to open a path that already exists in the file system.
+        const path = (zName && wasm.peek8(zName))
+              ? pool.getPath(zName)
+              : getRandomName();
+        let sah = pool.getSAHForPath(path);
+        if(!sah && (flags & capi.SQLITE_OPEN_CREATE)) {
+          // File not found so try to create it.
+          if(pool.getFileCount() < pool.getCapacity()) {
+            // Choose an unassociated OPFS file from the pool.
+            sah = pool.nextAvailableSAH();
+            pool.setAssociatedPath(sah, path, flags);
+          }else{
+            // File pool is full.
+            toss('SAH pool is full. Cannot create file',path);
+          }
+        }
+        if(!sah){
+          toss('file not found:',path);
+        }
+        // Subsequent I/O methods are only passed the sqlite3_file
+        // pointer, so map the relevant info we need to that pointer.
+        const file = {path, flags, sah};
+        pool.mapS3FileToOFile(pFile, file);
+        file.lockType = capi.SQLITE_LOCK_NONE;
+        const sq3File = new capi.sqlite3_file(pFile);
+        sq3File.$pMethods = opfsIoMethods.pointer;
+        sq3File.dispose();
+        wasm.poke32(pOutFlags, flags);
+        return 0;
+      }catch(e){
+        pool.storeErr(e);
+        return capi.SQLITE_CANTOPEN;
+      }
+    }/*xOpen()*/
+  }/*vfsMethods*/;
+
+  /**
+     Creates and initializes an sqlite3_vfs instance for an
+     OpfsSAHPool. The argument is the VFS's name (JS string).
+
+     Throws if the VFS name is already registered or if something
+     goes terribly wrong via sqlite3.vfs.installVfs().
+
+     Maintenance reminder: the only detail about the returned object
+     which is specific to any given OpfsSAHPool instance is the $zName
+     member. All other state is identical.
+  */
+  const createOpfsVfs = function(vfsName){
+    if( sqlite3.capi.sqlite3_vfs_find(vfsName)){
+      toss3("VFS name is already registered:", vfsName);
+    }
+    const opfsVfs = new capi.sqlite3_vfs();
+    /* We fetch the default VFS so that we can inherit some
+       methods from it. */
+    const pDVfs = capi.sqlite3_vfs_find(null);
+    const dVfs = pDVfs
+          ? new capi.sqlite3_vfs(pDVfs)
+          : null /* dVfs will be null when sqlite3 is built with
+                    SQLITE_OS_OTHER. */;
+    opfsVfs.$iVersion = 2/*yes, two*/;
+    opfsVfs.$szOsFile = capi.sqlite3_file.structInfo.sizeof;
+    opfsVfs.$mxPathname = HEADER_MAX_PATH_SIZE;
+    opfsVfs.addOnDispose(
+      opfsVfs.$zName = wasm.allocCString(vfsName),
+      ()=>setPoolForVfs(opfsVfs.pointer, 0)
+    );
+
+    if(dVfs){
+      /* Inherit certain VFS members from the default VFS,
+         if available. */
+      opfsVfs.$xRandomness = dVfs.$xRandomness;
+      opfsVfs.$xSleep = dVfs.$xSleep;
+      dVfs.dispose();
+    }
+    if(!opfsVfs.$xRandomness && !vfsMethods.xRandomness){
+      /* If the default VFS has no xRandomness(), add a basic JS impl... */
+      vfsMethods.xRandomness = function(pVfs, nOut, pOut){
+        const heap = wasm.heap8u();
+        let i = 0;
+        for(; i < nOut; ++i) heap[pOut + i] = (Math.random()*255000) & 0xFF;
+        return i;
+      };
+    }
+    if(!opfsVfs.$xSleep && !vfsMethods.xSleep){
+      vfsMethods.xSleep = (pVfs,ms)=>0;
+    }
+    sqlite3.vfs.installVfs({
+      vfs: {struct: opfsVfs, methods: vfsMethods}
+    });
+    return opfsVfs;
+  };
+
+  /**
+     Class for managing OPFS-related state for the
+     OPFS SharedAccessHandle Pool sqlite3_vfs.
+  */
+  class OpfsSAHPool {
+    /* OPFS dir in which VFS metadata is stored. */
+    vfsDir;
+    /* Directory handle to this.vfsDir. */
+    #dhVfsRoot;
+    /* Directory handle to the subdir of this.#dhVfsRoot which holds
+       the randomly-named "opaque" files. This subdir exists in the
+       hope that we can eventually support client-created files in
+       this.#dhVfsRoot. */
+    #dhOpaque;
+    /* Directory handle to this.dhVfsRoot's parent dir. Needed
+       for a VFS-wipe op. */
+    #dhVfsParent;
+    /* Maps SAHs to their opaque file names. */
+    #mapSAHToName = new Map();
+    /* Maps client-side file names to SAHs. */
+    #mapFilenameToSAH = new Map();
+    /* Set of currently-unused SAHs. */
+    #availableSAH = new Set();
+    /* Maps (sqlite3_file*) to xOpen's file objects. */
+    #mapS3FileToOFile_ = new Map();
+
+    /* Maps SAH to an abstract File Object which contains
+       various metadata about that handle. */
+    //#mapSAHToMeta = new Map();
+
+    /** Buffer used by [sg]etAssociatedPath(). */
+    #apBody = new Uint8Array(HEADER_CORPUS_SIZE);
+    // DataView for this.#apBody
+    #dvBody;
+
+    // associated sqlite3_vfs instance
+    #cVfs;
+
+    // Logging verbosity. See optionDefaults.verbosity.
+    #verbosity;
+
+    constructor(options = Object.create(null)){
+      this.#verbosity = options.verbosity ?? optionDefaults.verbosity;
+      this.vfsName = options.name || optionDefaults.name;
+      this.#cVfs = createOpfsVfs(this.vfsName);
+      setPoolForVfs(this.#cVfs.pointer, this);
+      this.vfsDir = options.directory || ("."+this.vfsName);
+      this.#dvBody =
+        new DataView(this.#apBody.buffer, this.#apBody.byteOffset);
+      this.isReady = this
+        .reset(!!(options.clearOnInit ?? optionDefaults.clearOnInit))
+        .then(()=>{
+          if(this.$error) throw this.$error;
+          return this.getCapacity()
+            ? Promise.resolve(undefined)
+            : this.addCapacity(options.initialCapacity
+                               || optionDefaults.initialCapacity);
+        });
+    }
+
+    #logImpl(level,...args){
+      if(this.#verbosity>level) loggers[level](this.vfsName+":",...args);
+    };
+    log(...args){this.#logImpl(2, ...args)};
+    warn(...args){this.#logImpl(1, ...args)};
+    error(...args){this.#logImpl(0, ...args)};
+
+    getVfs(){return this.#cVfs}
+
+    /* Current pool capacity. */
+    getCapacity(){return this.#mapSAHToName.size}
+
+    /* Current number of in-use files from pool. */
+    getFileCount(){return this.#mapFilenameToSAH.size}
+
+//    #createFileObject(sah,clientName,opaqueName){
+//      const f = Object.assign(Object.create(null),{
+//        clientName, opaqueName
+//      });
+//      this.#mapSAHToMeta.set(sah, f);
+//      return f;
+//    }
+//    #unmapFileObject(sah){
+//      this.#mapSAHToMeta.delete(sah);
+//    }
+
+    /**
+       Adds n files to the pool's capacity. This change is
+       persistent across settings. Returns a Promise which resolves
+       to the new capacity.
+    */
+    async addCapacity(n){
+      for(let i = 0; i < n; ++i){
+        const name = getRandomName();
+        const h = await this.#dhOpaque.getFileHandle(name, {create:true});
+        const ah = await h.createSyncAccessHandle();
+        this.#mapSAHToName.set(ah,name);
+        this.setAssociatedPath(ah, '', 0);
+        //this.#createFileObject(ah,undefined,name);
+      }
+      return this.getCapacity();
+    }
+
+    /**
+       Reduce capacity by n, but can only reduce up to the limit
+       of currently-available SAHs. Returns a Promise which resolves
+       to the number of slots really removed.
+    */
+    async reduceCapacity(n){
+      let nRm = 0;
+      for(const ah of Array.from(this.#availableSAH)){
+        if(nRm === n || this.getFileCount() === this.getCapacity()){
+          break;
+        }
+        const name = this.#mapSAHToName.get(ah);
+        //this.#unmapFileObject(ah);
+        ah.close();
+        await this.#dhOpaque.removeEntry(name);
+        this.#mapSAHToName.delete(ah);
+        this.#availableSAH.delete(ah);
+        ++nRm;
+      }
+      return nRm;
+    }
+
+    /**
+       Releases all currently-opened SAHs. The only legal
+       operation after this is acquireAccessHandles().
+    */
+    releaseAccessHandles(){
+      for(const ah of this.#mapSAHToName.keys()) ah.close();
+      this.#mapSAHToName.clear();
+      this.#mapFilenameToSAH.clear();
+      this.#availableSAH.clear();
+    }
+
+    /**
+       Opens all files under this.vfsDir/this.#dhOpaque and acquires
+       a SAH for each. returns a Promise which resolves to no value
+       but completes once all SAHs are acquired. If acquiring an SAH
+       throws, SAHPool.$error will contain the corresponding
+       exception.
+
+       If clearFiles is true, the client-stored state of each file is
+       cleared when its handle is acquired, including its name, flags,
+       and any data stored after the metadata block.
+    */
+    async acquireAccessHandles(clearFiles){
+      const files = [];
+      for await (const [name,h] of this.#dhOpaque){
+        if('file'===h.kind){
+          files.push([name,h]);
+        }
+      }
+      return Promise.all(files.map(async([name,h])=>{
+        try{
+          const ah = await h.createSyncAccessHandle()
+          this.#mapSAHToName.set(ah, name);
+          if(clearFiles){
+            ah.truncate(HEADER_OFFSET_DATA);
+            this.setAssociatedPath(ah, '', 0);
+          }else{
+            const path = this.getAssociatedPath(ah);
+            if(path){
+              this.#mapFilenameToSAH.set(path, ah);
+            }else{
+              this.#availableSAH.add(ah);
+            }
+          }
+        }catch(e){
+          this.storeErr(e);
+          this.releaseAccessHandles();
+          throw e;
+        }
+      }));
+    }
+
+    /**
+       Given an SAH, returns the client-specified name of
+       that file by extracting it from the SAH's header.
+
+       On error, it disassociates SAH from the pool and
+       returns an empty string.
+    */
+    getAssociatedPath(sah){
+      sah.read(this.#apBody, {at: 0});
+      // Delete any unexpected files left over by previous
+      // untimely errors...
+      const flags = this.#dvBody.getUint32(HEADER_OFFSET_FLAGS);
+      if(this.#apBody[0] &&
+         ((flags & capi.SQLITE_OPEN_DELETEONCLOSE) ||
+          (flags & PERSISTENT_FILE_TYPES)===0)){
+        warn(`Removing file with unexpected flags ${flags.toString(16)}`,
+             this.#apBody);
+        this.setAssociatedPath(sah, '', 0);
+        return '';
+      }
+
+      const fileDigest = new Uint32Array(HEADER_DIGEST_SIZE / 4);
+      sah.read(fileDigest, {at: HEADER_OFFSET_DIGEST});
+      const compDigest = this.computeDigest(this.#apBody);
+      if(fileDigest.every((v,i) => v===compDigest[i])){
+        // Valid digest
+        const pathBytes = this.#apBody.findIndex((v)=>0===v);
+        if(0===pathBytes){
+          // This file is unassociated, so truncate it to avoid
+          // leaving stale db data laying around.
+          sah.truncate(HEADER_OFFSET_DATA);
+        }
+        return pathBytes
+          ? textDecoder.decode(this.#apBody.subarray(0,pathBytes))
+          : '';
+      }else{
+        // Invalid digest
+        warn('Disassociating file with bad digest.');
+        this.setAssociatedPath(sah, '', 0);
+        return '';
+      }
+    }
+
+    /**
+       Stores the given client-defined path and SQLITE_OPEN_xyz flags
+       into the given SAH. If path is an empty string then the file is
+       disassociated from the pool but its previous name is preserved
+       in the metadata.
+    */
+    setAssociatedPath(sah, path, flags){
+      const enc = textEncoder.encodeInto(path, this.#apBody);
+      if(HEADER_MAX_PATH_SIZE <= enc.written){
+        toss("Path too long:",path);
+      }
+
+      this.#dvBody.setUint32(HEADER_OFFSET_FLAGS, flags);
+
+      const digest = this.computeDigest(this.#apBody);
+      sah.write(this.#apBody, {at: 0});
+      sah.write(digest, {at: HEADER_OFFSET_DIGEST});
+      sah.flush();
+
+      if(path){
+        this.#mapFilenameToSAH.set(path, sah);
+        this.#availableSAH.delete(sah);
+      }else{
+        // This is not a persistent file, so eliminate the contents.
+        sah.truncate(HEADER_OFFSET_DATA);
+        this.#availableSAH.add(sah);
+      }
+    }
+
+    /**
+       Computes a digest for the given byte array and returns it as a
+       two-element Uint32Array. This digest gets stored in the
+       metadata for each file as a validation check. Changing this
+       algorithm invalidates all existing databases for this VFS, so
+       don't do that.
+    */
+    computeDigest(byteArray){
+      let h1 = 0xdeadbeef;
+      let h2 = 0x41c6ce57;
+      for(const v of byteArray){
+        h1 = 31 * h1 + (v * 307);
+        h2 = 31 * h2 + (v * 307);
+      }
+      return new Uint32Array([h1>>>0, h2>>>0]);
+    }
+
+    /**
+       Re-initializes the state of the SAH pool, releasing and
+       re-acquiring all handles.
+
+       See acquireAccessHandles() for the specifics of the clearFiles
+       argument.
+    */
+    async reset(clearFiles){
+      await this.isReady;
+      let h = await navigator.storage.getDirectory();
+      let prev, prevName;
+      for(const d of this.vfsDir.split('/')){
+        if(d){
+          prev = h;
+          h = await h.getDirectoryHandle(d,{create:true});
+        }
+      }
+      this.#dhVfsRoot = h;
+      this.#dhVfsParent = prev;
+      this.#dhOpaque = await this.#dhVfsRoot.getDirectoryHandle(
+        OPAQUE_DIR_NAME,{create:true}
+      );
+      this.releaseAccessHandles();
+      return this.acquireAccessHandles(clearFiles);
+    }
+
+    /**
+       Returns the pathname part of the given argument,
+       which may be any of:
+
+       - a URL object
+       - A JS string representing a file name
+       - Wasm C-string representing a file name
+
+       All "../" parts and duplicate slashes are resolve/removed from
+       the returned result.
+    */
+    getPath(arg) {
+      if(wasm.isPtr(arg)) arg = wasm.cstrToJs(arg);
+      return ((arg instanceof URL)
+              ? arg
+              : new URL(arg, 'file://localhost/')).pathname;
+    }
+
+    /**
+       Removes the association of the given client-specified file
+       name (JS string) from the pool. Returns true if a mapping
+       is found, else false.
+    */
+    deletePath(path) {
+      const sah = this.#mapFilenameToSAH.get(path);
+      if(sah) {
+        // Un-associate the name from the SAH.
+        this.#mapFilenameToSAH.delete(path);
+        this.setAssociatedPath(sah, '', 0);
+      }
+      return !!sah;
+    }
+
+    /**
+       Sets e as this object's current error. Pass a falsy
+       (or no) value to clear it.
+    */
+    storeErr(e){
+      if(e) this.error(e);
+      return this.$error = e;
+    }
+    /**
+       Pops this object's Error object and returns
+       it (a falsy value if no error is set).
+    */
+    popErr(){
+      const rc = this.$error;
+      this.$error = undefined;
+      return rc;
+    }
+
+    /**
+       Returns the next available SAH without removing
+       it from the set.
+    */
+    nextAvailableSAH(){
+      const [rc] = this.#availableSAH.keys();
+      return rc;
+    }
+
+    /**
+       Given an (sqlite3_file*), returns the mapped
+       xOpen file object.
+    */
+    getOFileForS3File(pFile){
+      return this.#mapS3FileToOFile_.get(pFile);
+    }
+    /**
+       Maps or unmaps (if file is falsy) the given (sqlite3_file*)
+       to an xOpen file object and to this pool object.
+    */
+    mapS3FileToOFile(pFile,file){
+      if(file){
+        this.#mapS3FileToOFile_.set(pFile, file);
+        setPoolForPFile(pFile, this);
+      }else{
+        this.#mapS3FileToOFile_.delete(pFile);
+        setPoolForPFile(pFile, false);
+      }
+    }
+
+    /**
+       Returns true if the given client-defined file name is in this
+       object's name-to-SAH map.
+    */
+    hasFilename(name){
+      return this.#mapFilenameToSAH.has(name)
+    }
+
+    /**
+       Returns the SAH associated with the given
+       client-defined file name.
+    */
+    getSAHForPath(path){
+      return this.#mapFilenameToSAH.get(path);
+    }
+
+    /**
+       Removes this object's sqlite3_vfs registration and shuts down
+       this object, releasing all handles, mappings, and whatnot,
+       including deleting its data directory. There is currently no
+       way to "revive" the object and reaquire its resources.
+
+       This function is intended primarily for testing.
+
+       Resolves to true if it did its job, false if the
+       VFS has already been shut down.
+    */
+    async removeVfs(){
+      if(!this.#cVfs.pointer || !this.#dhOpaque) return false;
+      capi.sqlite3_vfs_unregister(this.#cVfs.pointer);
+      this.#cVfs.dispose();
+      try{
+        this.releaseAccessHandles();
+        await this.#dhVfsRoot.removeEntry(OPAQUE_DIR_NAME, {recursive: true});
+        this.#dhOpaque = undefined;
+        await this.#dhVfsParent.removeEntry(
+          this.#dhVfsRoot.name, {recursive: true}
+        );
+        this.#dhVfsRoot = this.#dhVfsParent = undefined;
+      }catch(e){
+        sqlite3.config.error(this.vfsName,"removeVfs() failed:",e);
+        /*otherwise ignored - there is no recovery strategy*/
+      }
+      return true;
+    }
+
+    //! Documented elsewhere in this file.
+    exportFile(name){
+      const sah = this.#mapFilenameToSAH.get(name) || toss("File not found:",name);
+      const n = sah.getSize() - HEADER_OFFSET_DATA;
+      const b = new Uint8Array(n>=0 ? n : 0);
+      if(n>0) sah.read(b, {at: HEADER_OFFSET_DATA});
+      return b;
+    }
+
+    //! Documented elsewhere in this file.
+    importDb(name, bytes){
+      const n = bytes.byteLength;
+      if(n<512 || n%512!=0){
+        toss("Byte array size is invalid for an SQLite db.");
+      }
+      const header = "SQLite format 3";
+      for(let i = 0; i < header.length; ++i){
+        if( header.charCodeAt(i) !== bytes[i] ){
+          toss("Input does not contain an SQLite database header.");
+        }
+      }
+      const sah = this.#mapFilenameToSAH.get(name)
+            || this.nextAvailableSAH()
+            || toss("No available handles to import to.");
+      sah.write(bytes, {at: HEADER_OFFSET_DATA});
+      this.setAssociatedPath(sah, name, capi.SQLITE_OPEN_MAIN_DB);
+    }
+
+  }/*class OpfsSAHPool*/;
+
+
+  /**
+     A OpfsSAHPoolUtil instance is exposed to clients in order to
+     manipulate an OpfsSAHPool object without directly exposing that
+     object and allowing for some semantic changes compared to that
+     class.
+
+     Class docs are in the client-level docs for
+     installOpfsSAHPoolVfs().
+  */
+  class OpfsSAHPoolUtil {
+    /* This object's associated OpfsSAHPool. */
+    #p;
+
+    constructor(sahPool){
+      this.#p = sahPool;
+      this.vfsName = sahPool.vfsName;
+    }
+
+    async addCapacity(n){ return this.#p.addCapacity(n) }
+
+    async reduceCapacity(n){ return this.#p.reduceCapacity(n) }
+
+    getCapacity(){ return this.#p.getCapacity(this.#p) }
+
+    getFileCount(){ return this.#p.getFileCount() }
+
+    async reserveMinimumCapacity(min){
+      const c = this.#p.getCapacity();
+      return (c < min) ? this.#p.addCapacity(min - c) : c;
+    }
+
+    exportFile(name){ return this.#p.exportFile(name) }
+
+    importDb(name, bytes){ return this.#p.importDb(name,bytes) }
+
+    async wipeFiles(){ return this.#p.reset(true) }
+
+    unlink(filename){ return this.#p.deletePath(filename) }
+
+    async removeVfs(){ return this.#p.removeVfs() }
+
+  }/* class OpfsSAHPoolUtil */;
+
+  /**
+     Returns a resolved Promise if the current environment
+     has a "fully-sync" SAH impl, else a rejected Promise.
+  */
+  const apiVersionCheck = async ()=>{
+    const dh = await navigator.storage.getDirectory();
+    const fn = '.opfs-sahpool-sync-check-'+getRandomName();
+    const fh = await dh.getFileHandle(fn, { create: true });
+    const ah = await fh.createSyncAccessHandle();
+    const close = ah.close();
+    await close;
+    await dh.removeEntry(fn);
+    if(close?.then){
+      toss("The local OPFS API is too old for opfs-sahpool:",
+           "it has an async FileSystemSyncAccessHandle.close() method.");
+    }
+    return true;
+  };
+
+  /** Only for testing a rejection case. */
+  let instanceCounter = 0;
+
+  /**
+     installOpfsSAHPoolVfs() asynchronously initializes the OPFS
+     SyncAccessHandle (a.k.a. SAH) Pool VFS. It returns a Promise which
+     either resolves to a utility object described below or rejects with
+     an Error value.
+
+     Initialization of this VFS is not automatic because its
+     registration requires that it lock all resources it
+     will potentially use, even if client code does not want
+     to use them. That, in turn, can lead to locking errors
+     when, for example, one page in a given origin has loaded
+     this VFS but does not use it, then another page in that
+     origin tries to use the VFS. If the VFS were automatically
+     registered, the second page would fail to load the VFS
+     due to OPFS locking errors.
+
+     If this function is called more than once with a given "name"
+     option (see below), it will return the same Promise. Calls for
+     different names will return different Promises which resolve to
+     independent objects and refer to different VFS registrations.
+
+     On success, the resulting Promise resolves to a utility object
+     which can be used to query and manipulate the pool. Its API is
+     described at the end of these docs.
+
+     This function accepts an options object to configure certain
+     parts but it is only acknowledged for the very first call and
+     ignored for all subsequent calls.
+
+     The options, in alphabetical order:
+
+     - `clearOnInit`: (default=false) if truthy, contents and filename
+     mapping are removed from each SAH it is acquired during
+     initalization of the VFS, leaving the VFS's storage in a pristine
+     state. Use this only for databases which need not survive a page
+     reload.
+
+     - `initialCapacity`: (default=6) Specifies the default capacity of
+     the VFS. This should not be set unduly high because the VFS has
+     to open (and keep open) a file for each entry in the pool. This
+     setting only has an effect when the pool is initially empty. It
+     does not have any effect if a pool already exists.
+
+     - `directory`: (default="."+`name`) Specifies the OPFS directory
+     name in which to store metadata for the `"opfs-sahpool"`
+     sqlite3_vfs.  Only one instance of this VFS can be installed per
+     JavaScript engine, and any two engines with the same storage
+     directory name will collide with each other, leading to locking
+     errors and the inability to register the VFS in the second and
+     subsequent engine. Using a different directory name for each
+     application enables different engines in the same HTTP origin to
+     co-exist, but their data are invisible to each other. Changing
+     this name will effectively orphan any databases stored under
+     previous names. The default is unspecified but descriptive.  This
+     option may contain multiple path elements, e.g. "foo/bar/baz",
+     and they are created automatically.  In practice there should be
+     no driving need to change this. ACHTUNG: all files in this
+     directory are assumed to be managed by the VFS. Do not place
+     other files in that directory, as they may be deleted or
+     otherwise modified by the VFS.
+
+     - `name`: (default="opfs-sahpool") sets the name to register this
+     VFS under. Normally this should not be changed, but it is
+     possible to register this VFS under multiple names so long as
+     each has its own separate directory to work from. The storage for
+     each is invisible to all others. The name must be a string
+     compatible with `sqlite3_vfs_register()` and friends and suitable
+     for use in URI-style database file names.
+
+     Achtung: if a custom `name` is provided, a custom `directory`
+     must also be provided if any other instance is registered with
+     the default directory. If no directory is explicitly provided
+     then a directory name is synthesized from the `name` option.
+
+     Peculiarities of this VFS:
+
+     - Paths given to it _must_ be absolute. Relative paths will not
+     be properly recognized. This is arguably a bug but correcting it
+     requires some hoop-jumping in routines which have no business
+     doing tricks.
+
+     - It is possible to install multiple instances under different
+     names, each sandboxed from one another inside their own private
+     directory.  This feature exists primarily as a way for disparate
+     applications within a given HTTP origin to use this VFS without
+     introducing locking issues between them.
+
+
+     The API for the utility object passed on by this function's
+     Promise, in alphabetical order...
+
+     - [async] number addCapacity(n)
+
+     Adds `n` entries to the current pool. This change is persistent
+     across sessions so should not be called automatically at each app
+     startup (but see `reserveMinimumCapacity()`). Its returned Promise
+     resolves to the new capacity.  Because this operation is necessarily
+     asynchronous, the C-level VFS API cannot call this on its own as
+     needed.
+
+     - byteArray exportFile(name)
+
+     Synchronously reads the contents of the given file into a Uint8Array
+     and returns it. This will throw if the given name is not currently
+     in active use or on I/O error. Note that the given name is _not_
+     visible directly in OPFS (or, if it is, it's not from this VFS).
+
+     - number getCapacity()
+
+     Returns the number of files currently contained
+     in the SAH pool. The default capacity is only large enough for one
+     or two databases and their associated temp files.
+
+     - number getFileCount()
+
+     Returns the number of files from the pool currently allocated to
+     slots. This is not the same as the files being "opened".
+
+     - void importDb(name, byteArray)
+
+     Imports the contents of an SQLite database, provided as a byte
+     array, under the given name, overwriting any existing
+     content. Throws if the pool has no available file slots, on I/O
+     error, or if the input does not appear to be a database. In the
+     latter case, only a cursory examination is made.  Note that this
+     routine is _only_ for importing database files, not arbitrary files,
+     the reason being that this VFS will automatically clean up any
+     non-database files so importing them is pointless.
+
+     - [async] number reduceCapacity(n)
+
+     Removes up to `n` entries from the pool, with the caveat that it can
+     only remove currently-unused entries. It returns a Promise which
+     resolves to the number of entries actually removed.
+
+     - [async] boolean removeVfs()
+
+     Unregisters the opfs-sahpool VFS and removes its directory from OPFS
+     (which means that _all client content_ is removed). After calling
+     this, the VFS may no longer be used and there is no way to re-add it
+     aside from reloading the current JavaScript context.
+
+     Results are undefined if a database is currently in use with this
+     VFS.
+
+     The returned Promise resolves to true if it performed the removal
+     and false if the VFS was not installed.
+
+     If the VFS has a multi-level directory, e.g. "/foo/bar/baz", _only_
+     the bottom-most directory is removed because this VFS cannot know for
+     certain whether the higher-level directories contain data which
+     should be removed.
+
+     - [async] number reserveMinimumCapacity(min)
+
+     If the current capacity is less than `min`, the capacity is
+     increased to `min`, else this returns with no side effects. The
+     resulting Promise resolves to the new capacity.
+
+     - boolean unlink(filename)
+
+     If a virtual file exists with the given name, disassociates it from
+     the pool and returns true, else returns false without side
+     effects. Results are undefined if the file is currently in active
+     use.
+
+     - string vfsName
+
+     The SQLite VFS name under which this pool's VFS is registered.
+
+     - [async] void wipeFiles()
+
+     Clears all client-defined state of all SAHs and makes all of them
+     available for re-use by the pool. Results are undefined if any such
+     handles are currently in use, e.g. by an sqlite3 db.
+  */
+  sqlite3.installOpfsSAHPoolVfs = async function(options=Object.create(null)){
+    const vfsName = options.name || optionDefaults.name;
+    if(0 && 2===++instanceCounter){
+      throw new Error("Just testing rejection.");
+    }
+    if(initPromises[vfsName]){
+      //console.warn("Returning same OpfsSAHPool result",options,vfsName,initPromises[vfsName]);
+      return initPromises[vfsName];
+    }
+    if(!globalThis.FileSystemHandle ||
+       !globalThis.FileSystemDirectoryHandle ||
+       !globalThis.FileSystemFileHandle ||
+       !globalThis.FileSystemFileHandle.prototype.createSyncAccessHandle ||
+       !navigator?.storage?.getDirectory){
+      return (initPromises[vfsName] = Promise.reject(new Error("Missing required OPFS APIs.")));
+    }
+
+    /**
+       Maintenance reminder: the order of ASYNC ops in this function
+       is significant. We need to have them all chained at the very
+       end in order to be able to catch a race condition where
+       installOpfsSAHPoolVfs() is called twice in rapid succession,
+       e.g.:
+
+       installOpfsSAHPoolVfs().then(console.warn.bind(console));
+       installOpfsSAHPoolVfs().then(console.warn.bind(console));
+
+       If the timing of the async calls is not "just right" then that
+       second call can end up triggering the init a second time and chaos
+       ensues.
+    */
+    return initPromises[vfsName] = apiVersionCheck().then(async function(){
+      if(options.$testThrowInInit){
+        throw options.$testThrowInInit;
+      }
+      const thePool = new OpfsSAHPool(options);
+      return thePool.isReady.then(async()=>{
+        /** The poolUtil object will be the result of the
+            resolved Promise. */
+        const poolUtil = new OpfsSAHPoolUtil(thePool);
+        if(sqlite3.oo1){
+          const oo1 = sqlite3.oo1;
+          const theVfs = thePool.getVfs();
+          const OpfsSAHPoolDb = function(...args){
+            const opt = oo1.DB.dbCtorHelper.normalizeArgs(...args);
+            opt.vfs = theVfs.$zName;
+            oo1.DB.dbCtorHelper.call(this, opt);
+          };
+          OpfsSAHPoolDb.prototype = Object.create(oo1.DB.prototype);
+          // yes or no? OpfsSAHPoolDb.PoolUtil = poolUtil;
+          poolUtil.OpfsSAHPoolDb = OpfsSAHPoolDb;
+          oo1.DB.dbCtorHelper.setVfsPostOpenSql(
+            theVfs.pointer,
+            function(oo1Db, sqlite3){
+              sqlite3.capi.sqlite3_exec(oo1Db, [
+                /* See notes in sqlite3-vfs-opfs.js */
+                "pragma journal_mode=DELETE;",
+                "pragma cache_size=-16384;"
+              ], 0, 0, 0);
+            }
+          );
+        }/*extend sqlite3.oo1*/
+        thePool.log("VFS initialized.");
+        return poolUtil;
+      }).catch(async (e)=>{
+        await thePool.removeVfs().catch(()=>{});
+        return e;
+      });
+    }).catch((err)=>{
+      //error("rejecting promise:",err);
+      return initPromises[vfsName] = Promise.reject(err);
+    });
+  }/*installOpfsSAHPoolVfs()*/;
+}/*sqlite3ApiBootstrap.initializers*/);
+//#else
+/*
+  The OPFS SAH Pool VFS parts are elided from builds targeting
+  node.js.
+*/
+//#endif target=node
diff --git a/ext/wasm/api/sqlite3-vfs-opfs.c-pp.js b/ext/wasm/api/sqlite3-vfs-opfs.c-pp.js
index 40c6090bf..58c511082 100644
--- a/ext/wasm/api/sqlite3-vfs-opfs.c-pp.js
+++ b/ext/wasm/api/sqlite3-vfs-opfs.c-pp.js
@@ -1,3 +1,4 @@
+//#ifnot target=node
 /*
   2022-09-18
 
@@ -23,7 +24,7 @@ globalThis.sqlite3ApiBootstrap.initializers.push(function(sqlite3){
    installOpfsVfs() returns a Promise which, on success, installs an
    sqlite3_vfs named "opfs", suitable for use with all sqlite3 APIs
    which accept a VFS. It is intended to be called via
-   sqlite3ApiBootstrap.initializersAsync or an equivalent mechanism.
+   sqlite3ApiBootstrap.initializers or an equivalent mechanism.
 
    The installed VFS uses the Origin-Private FileSystem API for
    all file storage. On error it is rejected with an exception
@@ -101,6 +102,10 @@ const installOpfsVfs = function callee(options){
     options = Object.create(null);
   }
   const urlParams = new URL(globalThis.location.href).searchParams;
+  if(urlParams.has('opfs-disable')){
+    //sqlite3.config.warn('Explicitly not installing "opfs" VFS due to opfs-disable flag.');
+    return Promise.resolve(sqlite3);
+  }
   if(undefined===options.verbose){
     options.verbose = urlParams.has('opfs-verbose')
       ? (+urlParams.get('opfs-verbose') || 2) : 1;
@@ -200,9 +205,9 @@ const installOpfsVfs = function callee(options){
       opfsVfs.dispose();
       return promiseReject_(err);
     };
-    const promiseResolve = (value)=>{
+    const promiseResolve = ()=>{
       promiseWasRejected = false;
-      return promiseResolve_(value);
+      return promiseResolve_(sqlite3);
     };
     const W =
 //#if target=es6-bundler-friendly
@@ -236,6 +241,7 @@ const installOpfsVfs = function callee(options){
           ? new sqlite3_vfs(pDVfs)
           : null /* dVfs will be null when sqlite3 is built with
                     SQLITE_OS_OTHER. */;
+    opfsIoMethods.$iVersion = 1;
     opfsVfs.$iVersion = 2/*yes, two*/;
     opfsVfs.$szOsFile = capi.sqlite3_file.structInfo.sizeof;
     opfsVfs.$mxPathname = 1024/*sure, why not?*/;
@@ -1321,10 +1327,10 @@ const installOpfsVfs = function callee(options){
                   sqlite3.opfs = opfsUtil;
                   opfsUtil.rootDirectory = d;
                   log("End of OPFS sqlite3_vfs setup.", opfsVfs);
-                  promiseResolve(sqlite3);
+                  promiseResolve();
                 }).catch(promiseReject);
               }else{
-                promiseResolve(sqlite3);
+                promiseResolve();
               }
             }catch(e){
               error(e);
@@ -1361,7 +1367,10 @@ globalThis.sqlite3ApiBootstrap.initializersAsync.push(async (sqlite3)=>{
     });
   }catch(e){
     sqlite3.config.error("installOpfsVfs() exception:",e);
-    throw e;
+    return Promise.reject(e);
   }
 });
 }/*sqlite3ApiBootstrap.initializers.push()*/);
+//#else
+/* The OPFS VFS parts are elided from builds targeting node.js. */
+//#endif target=node
diff --git a/ext/wasm/api/sqlite3-wasm.c b/ext/wasm/api/sqlite3-wasm.c
index dbe594dc3..94b16d750 100644
--- a/ext/wasm/api/sqlite3-wasm.c
+++ b/ext/wasm/api/sqlite3-wasm.c
@@ -151,7 +151,7 @@
 /**********************************************************************/
 /* SQLITE_T... */
 #ifndef SQLITE_TEMP_STORE
-# define SQLITE_TEMP_STORE 3
+# define SQLITE_TEMP_STORE 2
 #endif
 #ifndef SQLITE_THREADSAFE
 # define SQLITE_THREADSAFE 0