When searching the wal file for a frame, do not search that part that was already checkpointed when the snapshot being read was at the head of the wal file.

FossilOrigin-Name: 90760e72327eb0593cbfa6d7058b554198cd8044

src/wal.c

@@ -428,6 +428,7 @@ struct Wal {
   u8 syncHeader;             /* Fsync the WAL header if true */
   u8 padToSectorBoundary;    /* Pad transactions out to the next sector */
   WalIndexHdr hdr;           /* Wal-index header for current transaction */
+  u32 minFrame;              /* Ignore wal frames before this one */
   const char *zWalName;      /* Name of WAL file */
   u32 nCkpt;                 /* Checkpoint sequence counter in the wal-header */
 #ifdef SQLITE_DEBUG
@@ -2296,12 +2297,27 @@ static int walTryBeginRead(Wal *pWal, int *pChanged, int useWal, int cnt){
     ** pWal->hdr.mxFrame risks reading a corrupted snapshot. So, retry
     ** instead.
     **
-    ** This does not guarantee that the copy of the wal-index header is up to
-    ** date before proceeding. That would not be possible without somehow
-    ** blocking writers. It only guarantees that a dangerous checkpoint or 
-    ** log-wrap (either of which would require an exclusive lock on
-    ** WAL_READ_LOCK(mxI)) has not occurred since the snapshot was valid.
+    ** Before checking that the live wal-index header has not changed
+    ** since it was read, set Wal.minFrame to the first frame in the wal
+    ** file that has not yet been checkpointed. This client will not need
+    ** to read any frames earlier than minFrame from the wal file - they
+    ** can be safely read directly from the database file.
+    **
+    ** Because a ShmBarrier() call is made between taking the copy of 
+    ** nBackfill and checking that the wal-header in shared-memory still
+    ** matches the one cached in pWal->hdr, it is guaranteed that the 
+    ** checkpointer that set nBackfill was not working with a wal-index
+    ** header newer than that cached in pWal->hdr. If it were, that could
+    ** cause a problem. The checkpointer could omit to checkpoint
+    ** a version of page X that lies before pWal->minFrame (call that version
+    ** A) on the basis that there is a newer version (version B) of the same
+    ** page later in the wal file. But if version B happens to like past
+    ** frame pWal->hdr.mxFrame - then the client would incorrectly assume
+    ** that it can read version A from the database file. However, since
+    ** we can guarantee that the checkpointer that set nBackfill could not
+    ** see any pages past pWal->hdr.mxFrame, this problem does not come up.
     */
+    pWal->minFrame = pInfo->nBackfill+1;
     walShmBarrier(pWal);
     if( pInfo->aReadMark[mxI]!=mxReadMark
      || memcmp((void *)walIndexHdr(pWal), &pWal->hdr, sizeof(WalIndexHdr))
@@ -2372,7 +2388,6 @@ int sqlite3WalFindFrame(
   u32 iRead = 0;                  /* If !=0, WAL frame to return data from */
   u32 iLast = pWal->hdr.mxFrame;  /* Last page in WAL for this reader */
   int iHash;                      /* Used to loop through N hash tables */
-  u32 iFirst;
   int iMinHash;
 
   /* This routine is only be called from within a read transaction. */
@@ -2384,10 +2399,7 @@ int sqlite3WalFindFrame(
   ** then the WAL is ignored by the reader so return early, as if the 
   ** WAL were empty.
   */
-  if( iLast==0 
-   || pWal->readLock==0 
-   || iLast==(iFirst = walCkptInfo(pWal)->nBackfill)
-  ){
+  if( iLast==0 || pWal->readLock==0 ){
     *piRead = 0;
     return SQLITE_OK;
   }
@@ -2417,7 +2429,7 @@ int sqlite3WalFindFrame(
   **     This condition filters out entries that were added to the hash
   **     table after the current read-transaction had started.
   */
-  iMinHash = walFramePage(iFirst);
+  iMinHash = walFramePage(pWal->minFrame);
   for(iHash=walFramePage(iLast); iHash>=iMinHash && iRead==0; iHash--){
     volatile ht_slot *aHash;      /* Pointer to hash table */
     volatile u32 *aPgno;          /* Pointer to array of page numbers */
@@ -2433,7 +2445,7 @@ int sqlite3WalFindFrame(
     nCollide = HASHTABLE_NSLOT;
     for(iKey=walHash(pgno); aHash[iKey]; iKey=walNextHash(iKey)){
       u32 iFrame = aHash[iKey] + iZero;
-      if( iFrame<=iLast && iFrame>iFirst && aPgno[aHash[iKey]]==pgno ){
+      if( iFrame<=iLast && iFrame>=pWal->minFrame && aPgno[aHash[iKey]]==pgno ){
         assert( iFrame>iRead || CORRUPT_DB );
         iRead = iFrame;
       }