Allow sqlite3_snapshot_open() to be called to change the snapshot aft…

…er a read

transaction is already open on database.

src/main.c

@@ -4209,11 +4209,29 @@ int sqlite3_snapshot_open(
     iDb = sqlite3FindDbName(db, zDb);
     if( iDb==0 || iDb>1 ){
       Btree *pBt = db->aDb[iDb].pBt;
-      if( 0==sqlite3BtreeIsInReadTrans(pBt) ){
-        rc = sqlite3PagerSnapshotOpen(sqlite3BtreePager(pBt), pSnapshot);
+      if( sqlite3BtreeIsInTrans(pBt)==0 ){
+        Pager *pPager = sqlite3BtreePager(pBt);
+        int bUnlock = 0;
+        if( sqlite3BtreeIsInReadTrans(pBt) ){
+          if( db->nVdbeActive==0 ){
+            rc = sqlite3PagerSnapshotCheck(pPager, pSnapshot);
+            if( rc==SQLITE_OK ){
+              bUnlock = 1;
+              rc = sqlite3BtreeCommit(pBt);
+            }
+          }
+        }else{
+          rc = SQLITE_OK;
+        }
+        if( rc==SQLITE_OK ){
+          rc = sqlite3PagerSnapshotOpen(pPager, pSnapshot);
+        }
         if( rc==SQLITE_OK ){
           rc = sqlite3BtreeBeginTrans(pBt, 0, 0);
-          sqlite3PagerSnapshotOpen(sqlite3BtreePager(pBt), 0);
+          sqlite3PagerSnapshotOpen(pPager, 0);
+        }
+        if( bUnlock ){
+          sqlite3PagerSnapshotUnlock(pPager);
         }
       }
     }

src/pager.c

@@ -7653,6 +7653,38 @@ int sqlite3PagerSnapshotRecover(Pager *pPager){
   }
   return rc;
 }
+
+/*
+** The caller currently has a read transaction open on the database.
+** If this is not a WAL database, SQLITE_ERROR is returned. Otherwise,
+** this function takes a SHARED lock on the CHECKPOINTER slot and then
+** checks if the snapshot passed as the second argument is still 
+** available. If so, SQLITE_OK is returned.
+**
+** If the snapshot is not available, SQLITE_ERROR is returned. Or, if
+** the CHECKPOINTER lock cannot be obtained, SQLITE_BUSY. If any error
+** occurs (any value other than SQLITE_OK is returned), the CHECKPOINTER
+** lock is released before returning.
+*/
+int sqlite3PagerSnapshotCheck(Pager *pPager, sqlite3_snapshot *pSnapshot){
+  int rc;
+  if( pPager->pWal ){
+    rc = sqlite3WalSnapshotCheck(pPager->pWal, pSnapshot);
+  }else{
+    rc = SQLITE_ERROR;
+  }
+  return rc;
+}
+
+/*
+** Release a lock obtained by an earlier successful call to
+** sqlite3PagerSnapshotCheck().
+*/
+void sqlite3PagerSnapshotUnlock(Pager *pPager){
+  assert( pPager->pWal );
+  return sqlite3WalSnapshotUnlock(pPager->pWal);
+}
+
 #endif /* SQLITE_ENABLE_SNAPSHOT */
 #endif /* !SQLITE_OMIT_WAL */
 

src/pager.h

@@ -186,6 +186,8 @@ int sqlite3PagerSharedLock(Pager *pPager);
   int sqlite3PagerSnapshotGet(Pager *pPager, sqlite3_snapshot **ppSnapshot);
   int sqlite3PagerSnapshotOpen(Pager *pPager, sqlite3_snapshot *pSnapshot);
   int sqlite3PagerSnapshotRecover(Pager *pPager);
+  int sqlite3PagerSnapshotCheck(Pager *pPager, sqlite3_snapshot *pSnapshot);
+  void sqlite3PagerSnapshotUnlock(Pager *pPager);
 # endif
 #else
 # define sqlite3PagerUseWal(x,y) 0

src/sqlite.h.in

@@ -9035,22 +9035,33 @@ SQLITE_EXPERIMENTAL int sqlite3_snapshot_get(
 ** CAPI3REF: Start a read transaction on an historical snapshot
 ** METHOD: sqlite3_snapshot
 **
-** ^The [sqlite3_snapshot_open(D,S,P)] interface starts a
-** read transaction for schema S of
-** [database connection] D such that the read transaction
-** refers to historical [snapshot] P, rather than the most
-** recent change to the database.
-** ^The [sqlite3_snapshot_open()] interface returns SQLITE_OK on success
-** or an appropriate [error code] if it fails.
-**
-** ^In order to succeed, a call to [sqlite3_snapshot_open(D,S,P)] must be
-** the first operation following the [BEGIN] that takes the schema S
-** out of [autocommit mode].
-** ^In other words, schema S must not currently be in
-** a transaction for [sqlite3_snapshot_open(D,S,P)] to work, but the
-** database connection D must be out of [autocommit mode].
-** ^A [snapshot] will fail to open if it has been overwritten by a
-** [checkpoint].
+** ^The [sqlite3_snapshot_open(D,S,P)] interface either starts a new read 
+** transaction or upgrades an existing one for schema S of 
+** [database connection] D such that the read transaction refers to 
+** historical [snapshot] P, rather than the most recent change to the 
+** database. ^The [sqlite3_snapshot_open()] interface returns SQLITE_OK 
+** on success or an appropriate [error code] if it fails.
+**
+** ^In order to succeed, the database connection must not be in 
+** [autocommit mode] when [sqlite3_snapshot_open(D,S,P)] is called. If there
+** is already a read transaction open on schema S, then the database handle
+** must have no active statements (SELECT statements that have been passed
+** to sqlite3_step() but not sqlite3_reset() or sqlite3_finalize()). 
+** SQLITE_ERROR is returned if either of these conditions is violated, or
+** if schema S does not exist, or if the snapshot object is invalid.
+**
+** ^A call to sqlite3_snapshot_open() will fail to open if the specified
+** snapshot has been overwritten by a [checkpoint]. In this case 
+** SQLITE_BUSY_SNAPSHOT is returned.
+**
+** If there is already a read transaction open when this function is 
+** invoked, then the same read transaction remains open (on the same
+** database snapshot) if SQLITE_ERROR, SQLITE_BUSY or SQLITE_BUSY_SNAPSHOT
+** is returned. If another error code - for example SQLITE_PROTOCOL or an
+** SQLITE_IOERR error code - is returned, then the final state of the
+** read transaction is undefined. If SQLITE_OK is returned, then the 
+** read transaction is now open on database snapshot P.
+**
 ** ^(A call to [sqlite3_snapshot_open(D,S,P)] will fail if the
 ** database connection D does not know that the database file for
 ** schema S is in [WAL mode].  A database connection might not know

src/test1.c

@@ -2393,6 +2393,8 @@ static int SQLITE_TCLAPI test_snapshot_open(
   if( rc!=SQLITE_OK ){
     Tcl_SetObjResult(interp, Tcl_NewStringObj(sqlite3ErrName(rc), -1));
     return TCL_ERROR;
+  }else{
+    Tcl_ResetResult(interp);
   }
   return TCL_OK;
 }

src/wal.c

@@ -3769,6 +3769,43 @@ int sqlite3_snapshot_cmp(sqlite3_snapshot *p1, sqlite3_snapshot *p2){
   if( pHdr1->mxFrame>pHdr2->mxFrame ) return +1;
   return 0;
 }
+
+/*
+** The caller currently has a read transaction open on the database.
+** This function takes a SHARED lock on the CHECKPOINTER slot and then
+** checks if the snapshot passed as the second argument is still 
+** available. If so, SQLITE_OK is returned.
+**
+** If the snapshot is not available, SQLITE_ERROR is returned. Or, if
+** the CHECKPOINTER lock cannot be obtained, SQLITE_BUSY. If any error
+** occurs (any value other than SQLITE_OK is returned), the CHECKPOINTER
+** lock is released before returning.
+*/
+int sqlite3WalSnapshotCheck(Wal *pWal, sqlite3_snapshot *pSnapshot){
+  int rc;
+  rc = walLockShared(pWal, WAL_CKPT_LOCK);
+  if( rc==SQLITE_OK ){
+    WalIndexHdr *pNew = (WalIndexHdr*)pSnapshot;
+    if( memcmp(pNew->aSalt, pWal->hdr.aSalt, sizeof(pWal->hdr.aSalt))
+     || pNew->mxFrame<walCkptInfo(pWal)->nBackfillAttempted
+    ){
+      rc = SQLITE_BUSY_SNAPSHOT;
+      walUnlockShared(pWal, WAL_CKPT_LOCK);
+    }
+  }
+  return rc;
+}
+
+/*
+** Release a lock obtained by an earlier successful call to
+** sqlite3WalSnapshotCheck().
+*/
+void sqlite3WalSnapshotUnlock(Wal *pWal){
+  assert( pWal );
+  walUnlockShared(pWal, WAL_CKPT_LOCK);
+}
+
+
 #endif /* SQLITE_ENABLE_SNAPSHOT */
 
 #ifdef SQLITE_ENABLE_ZIPVFS

src/wal.h

@@ -132,6 +132,8 @@ int sqlite3WalHeapMemory(Wal *pWal);
 int sqlite3WalSnapshotGet(Wal *pWal, sqlite3_snapshot **ppSnapshot);
 void sqlite3WalSnapshotOpen(Wal *pWal, sqlite3_snapshot *pSnapshot);
 int sqlite3WalSnapshotRecover(Wal *pWal);
+int sqlite3WalSnapshotCheck(Wal *pWal, sqlite3_snapshot *pSnapshot);
+void sqlite3WalSnapshotUnlock(Wal *pWal);
 #endif
 
 #ifdef SQLITE_ENABLE_ZIPVFS

test/snapshot.test

@@ -217,9 +217,19 @@ foreach {tn tcl} {
         SELECT * FROM t2;
     }
   } {a b c d e f}
-  do_test $tn.3.2.2 {
-    list [catch {snapshot_open db main $snapshot } msg] $msg
+
+  # Update - it is no longer an error to have a read-transaction open, 
+  # provided there are no active SELECT statements.
+  do_test $tn.3.2.2a {
+    db eval "SELECT * FROM t2" {
+      set res [list [catch {snapshot_open db main $snapshot } msg] $msg]
+      break
+    }
+    set res
   } {1 SQLITE_ERROR}
+  do_test $tn.3.2.2b {
+    snapshot_open db main $snapshot
+  } {}
 
   do_test $tn.3.2.3 {
     execsql {
@@ -231,12 +241,17 @@ foreach {tn tcl} {
   } {1 SQLITE_ERROR}
   do_execsql_test $tn.3.2.4 COMMIT
 
-  do_test $tn.3.3.1 {
+  do_test $tn.3.3.1a {
     execsql { PRAGMA journal_mode = DELETE }
     execsql { BEGIN }
     list [catch {snapshot_open db main $snapshot } msg] $msg
   } {1 SQLITE_ERROR}
 
+  do_test $tn.3.3.1b {
+    execsql { COMMIT ; BEGIN ; SELECT * FROM t2 }
+    list [catch {snapshot_open db main $snapshot } msg] $msg
+  } {1 SQLITE_ERROR}
+
   do_test $tn.$tn.3.3.2 {
     snapshot_free $snapshot
     execsql COMMIT