1 files changed, 107 insertions, 29 deletions

diff --git a/src/wal.c b/src/wal.c
index d63c13ffc..fd2eabfd9 100644
--- a/src/wal.c
+++ b/src/wal.c
@@ -2004,6 +2004,19 @@ static int walIteratorInit(Wal *pWal, u32 nBackfill, WalIterator **pp){
 }
 
 #ifdef SQLITE_ENABLE_SETLK_TIMEOUT
+
+
+/*
+** Attempt to enable blocking locks that block for nMs ms. Return 1 if 
+** blocking locks are successfully enabled, or 0 otherwise.
+*/
+static int walEnableBlockingMs(Wal *pWal, int nMs){
+  int rc = sqlite3OsFileControl(
+      pWal->pDbFd, SQLITE_FCNTL_LOCK_TIMEOUT, (void*)&nMs
+  );
+  return (rc==SQLITE_OK);
+}
+
 /*
 ** Attempt to enable blocking locks. Blocking locks are enabled only if (a)
 ** they are supported by the VFS, and (b) the database handle is configured
@@ -2015,11 +2028,7 @@ static int walEnableBlocking(Wal *pWal){
   if( pWal->db ){
     int tmout = pWal->db->busyTimeout;
     if( tmout ){
-      int rc;
-      rc = sqlite3OsFileControl(
-          pWal->pDbFd, SQLITE_FCNTL_LOCK_TIMEOUT, (void*)&tmout
-      );
-      res = (rc==SQLITE_OK);
+      res = walEnableBlockingMs(pWal, tmout);
     }
   }
   return res;
@@ -2068,20 +2077,10 @@ void sqlite3WalDb(Wal *pWal, sqlite3 *db){
   pWal->db = db;
 }
 
-/*
-** Take an exclusive WRITE lock. Blocking if so configured.
-*/
-static int walLockWriter(Wal *pWal){
-  int rc;
-  walEnableBlocking(pWal);
-  rc = walLockExclusive(pWal, WAL_WRITE_LOCK, 1);
-  walDisableBlocking(pWal);
-  return rc;
-}
 #else
 # define walEnableBlocking(x) 0
 # define walDisableBlocking(x)
-# define walLockWriter(pWal) walLockExclusive((pWal), WAL_WRITE_LOCK, 1)
+# define walEnableBlockingMs(pWal, ms) 0
 # define sqlite3WalDb(pWal, db)
 #endif   /* ifdef SQLITE_ENABLE_SETLK_TIMEOUT */
 
@@ -2682,7 +2681,9 @@ static int walIndexReadHdr(Wal *pWal, int *pChanged){
       }
     }else{
       int bWriteLock = pWal->writeLock;
-      if( bWriteLock || SQLITE_OK==(rc = walLockWriter(pWal)) ){
+      if( bWriteLock 
+       || SQLITE_OK==(rc = walLockExclusive(pWal, WAL_WRITE_LOCK, 1)) 
+      ){
         pWal->writeLock = 1;
         if( SQLITE_OK==(rc = walIndexPage(pWal, 0, &page0)) ){
           badHdr = walIndexTryHdr(pWal, pChanged);
@@ -2690,7 +2691,8 @@ static int walIndexReadHdr(Wal *pWal, int *pChanged){
             /* If the wal-index header is still malformed even while holding
             ** a WRITE lock, it can only mean that the header is corrupted and
             ** needs to be reconstructed.  So run recovery to do exactly that.
-            */
+            ** Disable blocking locks first.  */
+            walDisableBlocking(pWal);
             rc = walIndexRecover(pWal);
             *pChanged = 1;
           }
@@ -2901,6 +2903,37 @@ static int walBeginShmUnreliable(Wal *pWal, int *pChanged){
 }
 
 /*
+** The final argument passed to walTryBeginRead() is of type (int*). The
+** caller should invoke walTryBeginRead as follows:
+**
+**   int cnt = 0;
+**   do {
+**     rc = walTryBeginRead(..., &cnt);
+**   }while( rc==WAL_RETRY );
+**
+** The final value of "cnt" is of no use to the caller. It is used by
+** the implementation of walTryBeginRead() as follows:
+**
+**   + Each time walTryBeginRead() is called, it is incremented. Once
+**     it reaches WAL_RETRY_PROTOCOL_LIMIT - indicating that walTryBeginRead()
+**     has many times been invoked and failed with WAL_RETRY - walTryBeginRead()
+**     returns SQLITE_PROTOCOL.
+**
+**   + If SQLITE_ENABLE_SETLK_TIMEOUT is defined and walTryBeginRead() failed
+**     because a blocking lock timed out (SQLITE_BUSY_TIMEOUT from the OS
+**     layer), the WAL_RETRY_BLOCKED_MASK bit is set in "cnt". In this case
+**     the next invocation of walTryBeginRead() may omit an expected call to 
+**     sqlite3OsSleep(). There has already been a delay when the previous call
+**     waited on a lock.
+*/
+#define WAL_RETRY_PROTOCOL_LIMIT 100
+#ifdef SQLITE_ENABLE_SETLK_TIMEOUT
+# define WAL_RETRY_BLOCKED_MASK    0x10000000
+#else
+# define WAL_RETRY_BLOCKED_MASK    0
+#endif
+
+/*
 ** Attempt to start a read transaction.  This might fail due to a race or
 ** other transient condition.  When that happens, it returns WAL_RETRY to
 ** indicate to the caller that it is safe to retry immediately.
@@ -2950,13 +2983,16 @@ static int walBeginShmUnreliable(Wal *pWal, int *pChanged){
 ** so it takes care to hold an exclusive lock on the corresponding
 ** WAL_READ_LOCK() while changing values.
 */
-static int walTryBeginRead(Wal *pWal, int *pChanged, int useWal, int cnt){
+static int walTryBeginRead(Wal *pWal, int *pChanged, int useWal, int *pCnt){
   volatile WalCkptInfo *pInfo;    /* Checkpoint information in wal-index */
   u32 mxReadMark;                 /* Largest aReadMark[] value */
   int mxI;                        /* Index of largest aReadMark[] value */
   int i;                          /* Loop counter */
   int rc = SQLITE_OK;             /* Return code  */
   u32 mxFrame;                    /* Wal frame to lock to */
+#ifdef SQLITE_ENABLE_SETLK_TIMEOUT
+  int nBlockTmout = 0;
+#endif
 
   assert( pWal->readLock<0 );     /* Not currently locked */
 
@@ -2980,14 +3016,34 @@ static int walTryBeginRead(Wal *pWal, int *pChanged, int useWal, int cnt){
   ** so that on the 100th (and last) RETRY we delay for 323 milliseconds.
   ** The total delay time before giving up is less than 10 seconds.
   */
-  if( cnt>5 ){
+  (*pCnt)++;
+  if( *pCnt>5 ){
     int nDelay = 1;                      /* Pause time in microseconds */
-    if( cnt>100 ){
+    int cnt = (*pCnt & ~WAL_RETRY_BLOCKED_MASK);
+    if( cnt>WAL_RETRY_PROTOCOL_LIMIT ){
       VVA_ONLY( pWal->lockError = 1; )
       return SQLITE_PROTOCOL;
     }
-    if( cnt>=10 ) nDelay = (cnt-9)*(cnt-9)*39;
+    if( *pCnt>=10 ) nDelay = (cnt-9)*(cnt-9)*39;
+#ifdef SQLITE_ENABLE_SETLK_TIMEOUT
+    /* In SQLITE_ENABLE_SETLK_TIMEOUT builds, configure the file-descriptor
+    ** to block for locks for approximately nDelay us. This affects three
+    ** locks: (a) the shared lock taken on the DMS slot in os_unix.c (if
+    ** using os_unix.c), (b) the WRITER lock taken in walIndexReadHdr() if the
+    ** first attempted read fails, and (c) the shared lock taken on the 
+    ** read-mark.  
+    **
+    ** If the previous call failed due to an SQLITE_BUSY_TIMEOUT error,
+    ** then sleep for the minimum of 1us. The previous call already provided 
+    ** an extra delay while it was blocking on the lock.
+    */
+    nBlockTmout = (nDelay+998) / 1000;
+    if( !useWal && walEnableBlockingMs(pWal, nBlockTmout) ){
+      if( *pCnt & WAL_RETRY_BLOCKED_MASK ) nDelay = 1;
+    }
+#endif
     sqlite3OsSleep(pWal->pVfs, nDelay);
+    *pCnt &= ~WAL_RETRY_BLOCKED_MASK;
   }
 
   if( !useWal ){
@@ -2995,6 +3051,13 @@ static int walTryBeginRead(Wal *pWal, int *pChanged, int useWal, int cnt){
     if( pWal->bShmUnreliable==0 ){
       rc = walIndexReadHdr(pWal, pChanged);
     }
+#ifdef SQLITE_ENABLE_SETLK_TIMEOUT
+    walDisableBlocking(pWal);
+    if( rc==SQLITE_BUSY_TIMEOUT ){
+      rc = SQLITE_BUSY;
+      *pCnt |= WAL_RETRY_BLOCKED_MASK;
+    }
+#endif
     if( rc==SQLITE_BUSY ){
       /* If there is not a recovery running in another thread or process
       ** then convert BUSY errors to WAL_RETRY.  If recovery is known to
@@ -3109,9 +3172,19 @@ static int walTryBeginRead(Wal *pWal, int *pChanged, int useWal, int cnt){
     return rc==SQLITE_BUSY ? WAL_RETRY : SQLITE_READONLY_CANTINIT;
   }
 
+  (void)walEnableBlockingMs(pWal, nBlockTmout);
   rc = walLockShared(pWal, WAL_READ_LOCK(mxI));
+  walDisableBlocking(pWal);
   if( rc ){
-    return rc==SQLITE_BUSY ? WAL_RETRY : rc;
+#ifdef SQLITE_ENABLE_SETLK_TIMEOUT
+    if( rc==SQLITE_BUSY_TIMEOUT ){
+      *pCnt |= WAL_RETRY_BLOCKED_MASK;
+    }
+#else
+    assert( rc!=SQLITE_BUSY_TIMEOUT );
+#endif
+    assert( (rc&0xFF)!=SQLITE_BUSY||rc==SQLITE_BUSY||rc==SQLITE_BUSY_TIMEOUT );
+    return (rc&0xFF)==SQLITE_BUSY ? WAL_RETRY : rc;
   }
   /* Now that the read-lock has been obtained, check that neither the
   ** value in the aReadMark[] array or the contents of the wal-index
@@ -3299,7 +3372,7 @@ static int walBeginReadTransaction(Wal *pWal, int *pChanged){
 #endif
 
   do{
-    rc = walTryBeginRead(pWal, pChanged, 0, ++cnt);
+    rc = walTryBeginRead(pWal, pChanged, 0, &cnt);
   }while( rc==WAL_RETRY );
   testcase( (rc&0xff)==SQLITE_BUSY );
   testcase( (rc&0xff)==SQLITE_IOERR );
@@ -3480,6 +3553,7 @@ static int walFindFrame(
         iRead = iFrame;
       }
       if( (nCollide--)==0 ){
+        *piRead = 0;
         return SQLITE_CORRUPT_BKPT;
       }
       iKey = walNextHash(iKey);
@@ -3783,7 +3857,7 @@ static int walRestartLog(Wal *pWal){
     cnt = 0;
     do{
       int notUsed;
-      rc = walTryBeginRead(pWal, &notUsed, 1, ++cnt);
+      rc = walTryBeginRead(pWal, &notUsed, 1, &cnt);
     }while( rc==WAL_RETRY );
     assert( (rc&0xff)!=SQLITE_BUSY ); /* BUSY not possible when useWal==1 */
     testcase( (rc&0xff)==SQLITE_IOERR );
@@ -4204,10 +4278,9 @@ int sqlite3WalCheckpoint(
   if( pWal->readOnly ) return SQLITE_READONLY;
   WALTRACE(("WAL%p: checkpoint begins\n", pWal));
 
-  /* Enable blocking locks, if possible. If blocking locks are successfully
-  ** enabled, set xBusy2=0 so that the busy-handler is never invoked. */
+  /* Enable blocking locks, if possible. */
   sqlite3WalDb(pWal, db);
-  (void)walEnableBlocking(pWal);
+  if( xBusy2 ) (void)walEnableBlocking(pWal);
 
   /* IMPLEMENTATION-OF: R-62028-47212 All calls obtain an exclusive
   ** "checkpoint" lock on the database file.
@@ -4248,9 +4321,14 @@ int sqlite3WalCheckpoint(
   /* Read the wal-index header. */
   SEH_TRY {
     if( rc==SQLITE_OK ){
+      /* For a passive checkpoint, do not re-enable blocking locks after
+      ** reading the wal-index header. A passive checkpoint should not block 
+      ** or invoke the busy handler. The only lock such a checkpoint may 
+      ** attempt to obtain is a lock on a read-slot, and it should give up
+      ** immediately and do a partial checkpoint if it cannot obtain it. */
       walDisableBlocking(pWal);
       rc = walIndexReadHdr(pWal, &isChanged);
-      (void)walEnableBlocking(pWal);
+      if( eMode2!=SQLITE_CHECKPOINT_PASSIVE ) (void)walEnableBlocking(pWal);
       if( isChanged && pWal->pDbFd->pMethods->iVersion>=3 ){
         sqlite3OsUnfetch(pWal->pDbFd, 0, 0);
       }