When possible, avoid taking wal file read-lock 0 in sqlite3_snapshot_get().

FossilOrigin-Name: 34b6ac3d76dbc6819778ec2a0f81cbcdcc0cd1a6303381d97f1c479e4ecdd132

2 files changed, 20 insertions, 2 deletions

diff --git a/src/main.c b/src/main.c
index 6ab09c556..fceb7957b 100644
--- a/src/main.c
+++ b/src/main.c
@@ -4963,7 +4963,11 @@ int sqlite3_snapshot_get(
     if( iDb==0 || iDb>1 ){
       Btree *pBt = db->aDb[iDb].pBt;
       if( SQLITE_TXN_WRITE!=sqlite3BtreeTxnState(pBt) ){
+        Pager *pPager = sqlite3BtreePager(pBt);
+        i64 dummy = 0;
+        sqlite3PagerSnapshotOpen(pPager, (sqlite3_snapshot*)&dummy);
         rc = sqlite3BtreeBeginTrans(pBt, 0, 0);
+        sqlite3PagerSnapshotOpen(pPager, 0);
         if( rc==SQLITE_OK ){
           rc = sqlite3PagerSnapshotGet(sqlite3BtreePager(pBt), ppSnapshot);
         }
diff --git a/src/wal.c b/src/wal.c
index 89106038b..f4aa663fd 100644
--- a/src/wal.c
+++ b/src/wal.c
@@ -541,6 +541,7 @@ struct Wal {
 #endif
 #ifdef SQLITE_ENABLE_SNAPSHOT
   WalIndexHdr *pSnapshot;    /* Start transaction here if not NULL */
+  int bGetSnapshot;          /* Transaction opened for sqlite3_get_snapshot() */
 #endif
 #ifdef SQLITE_ENABLE_SETLK_TIMEOUT
   sqlite3 *db;
@@ -3097,7 +3098,7 @@ static int walTryBeginRead(Wal *pWal, int *pChanged, int useWal, int *pCnt){
   SEH_INJECT_FAULT;
   if( !useWal && AtomicLoad(&pInfo->nBackfill)==pWal->hdr.mxFrame
 #ifdef SQLITE_ENABLE_SNAPSHOT
-   && (pWal->pSnapshot==0 || pWal->hdr.mxFrame==0)
+   && ((pWal->bGetSnapshot==0 && pWal->pSnapshot==0) || pWal->hdr.mxFrame==0)
 #endif
   ){
     /* The WAL has been completely backfilled (or it is empty).
@@ -4497,7 +4498,20 @@ void sqlite3WalSnapshotOpen(
   Wal *pWal,
   sqlite3_snapshot *pSnapshot
 ){
-  pWal->pSnapshot = (WalIndexHdr*)pSnapshot;
+  if( pSnapshot && ((WalIndexHdr*)pSnapshot)->iVersion==0 ){
+    /* iVersion==0 means that this is a call to sqlite3_snapshot_get().  In
+    ** this case set the bGetSnapshot flag so that if the call to
+    ** sqlite3_snapshot_get() is about to read transaction on this wal 
+    ** file, it does not take read-lock 0 if the wal file has been completely
+    ** checkpointed. Taking read-lock 0 would work, but then it would be
+    ** possible for a subsequent writer to destroy the snapshot even while 
+    ** this connection is holding its read-transaction open. This is contrary
+    ** to user expectations, so we avoid it by not taking read-lock 0. */
+    pWal->bGetSnapshot = 1;
+  }else{
+    pWal->pSnapshot = (WalIndexHdr*)pSnapshot;
+    pWal->bGetSnapshot = 0;
+  }
 }
 
 /*