Index: src/main.c
==================================================================
--- src/main.c
+++ src/main.c
@@ -4207,15 +4207,33 @@
   if( db->autoCommit==0 ){
     int iDb;
     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);
         }
       }
     }
   }
 

Index: src/pager.c
==================================================================
--- src/pager.c
+++ src/pager.c
@@ -7651,10 +7651,42 @@
   }else{
     rc = SQLITE_ERROR;
   }
   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 */
 
 #ifdef SQLITE_ENABLE_ZIPVFS
 /*

Index: src/pager.h
==================================================================
--- src/pager.h
+++ src/pager.h
@@ -184,10 +184,12 @@
 # endif
 # ifdef SQLITE_ENABLE_SNAPSHOT
   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
 #endif
 

Index: src/sqlite.h.in
==================================================================
--- src/sqlite.h.in
+++ src/sqlite.h.in
@@ -9033,26 +9033,37 @@
 
 /*
 ** 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.
+** ^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.
 **
-** ^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].
+** 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
 ** that the database file is in [WAL mode] if there has been no prior
 ** I/O on that database connection, or if the database entered [WAL mode] 

Index: src/test1.c
==================================================================
--- src/test1.c
+++ src/test1.c
@@ -2391,10 +2391,12 @@
 
   rc = sqlite3_snapshot_open(db, zName, pSnapshot);
   if( rc!=SQLITE_OK ){
     Tcl_SetObjResult(interp, Tcl_NewStringObj(sqlite3ErrName(rc), -1));
     return TCL_ERROR;
+  }else{
+    Tcl_ResetResult(interp);
   }
   return TCL_OK;
 }
 #endif /* SQLITE_ENABLE_SNAPSHOT */
 

Index: src/wal.c
==================================================================
--- src/wal.c
+++ src/wal.c
@@ -3767,10 +3767,47 @@
   if( pHdr1->aSalt[0]>pHdr2->aSalt[0] ) return +1;
   if( pHdr1->mxFrame<pHdr2->mxFrame ) return -1;
   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
 /*
 ** If the argument is not NULL, it points to a Wal object that holds a

Index: src/wal.h
==================================================================
--- src/wal.h
+++ src/wal.h
@@ -130,10 +130,12 @@
 
 #ifdef SQLITE_ENABLE_SNAPSHOT
 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
 /* If the WAL file is not empty, return the number of bytes of content
 ** stored in each frame (i.e. the db page-size when the WAL was created).

Index: test/snapshot.test
==================================================================
--- test/snapshot.test
+++ test/snapshot.test
@@ -215,13 +215,23 @@
     execsql {
       BEGIN;
         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 {
       COMMIT;
       BEGIN;
@@ -229,15 +239,20 @@
     }
     list [catch {snapshot_open db main $snapshot } msg] $msg
   } {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
   } {}

ADDED   test/snapshot_up.test
Index: test/snapshot_up.test
==================================================================
--- /dev/null
+++ test/snapshot_up.test
@@ -0,0 +1,184 @@
+# 2018 August 6
+#
+# 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.
+#
+#***********************************************************************
+#
+# Tests for calling sqlite3_snapshot_open() when there is already
+# a read transaction open on the database.
+#
+
+set testdir [file dirname $argv0]
+source $testdir/tester.tcl
+ifcapable !snapshot {finish_test; return}
+set testprefix snapshot_up
+
+# This test does not work with the inmemory_journal permutation. The reason
+# is that each connection opened as part of this permutation executes
+# "PRAGMA journal_mode=memory", which fails if the database is in wal mode
+# and there are one or more existing connections.
+if {[permutation]=="inmemory_journal"} {
+  finish_test
+  return
+}
+
+do_execsql_test 1.0 {
+  CREATE TABLE t1(a, b, c);
+  PRAGMA journal_mode = wal;
+  INSERT INTO t1 VALUES(1, 2, 3);
+  INSERT INTO t1 VALUES(4, 5, 6);
+  INSERT INTO t1 VALUES(7, 8, 9);
+} {wal}
+
+do_test 1.1 {
+  execsql BEGIN
+  set ::snap1 [sqlite3_snapshot_get db main]
+  execsql COMMIT
+  execsql { INSERT INTO t1 VALUES(10, 11, 12); }
+  execsql BEGIN
+  set ::snap2 [sqlite3_snapshot_get db main]
+  execsql COMMIT
+  execsql { INSERT INTO t1 VALUES(13, 14, 15); }
+  execsql BEGIN
+  set ::snap3 [sqlite3_snapshot_get db main]
+  execsql COMMIT
+} {}
+
+do_execsql_test 1.2 {
+  BEGIN;
+    SELECT * FROM t1
+} {1 2 3 4 5 6 7 8 9 10 11 12 13 14 15}
+
+do_test 1.3 {
+  sqlite3_snapshot_open db main $::snap1
+  execsql { SELECT * FROM t1 }
+} {1 2 3 4 5 6 7 8 9}
+
+do_test 1.4 {
+  sqlite3_snapshot_open db main $::snap2
+  execsql { SELECT * FROM t1 }
+} {1 2 3 4 5 6 7 8 9 10 11 12}
+
+do_test 1.5 {
+  sqlite3 db2 test.db
+  execsql { PRAGMA wal_checkpoint } db2
+} {0 5 4}
+
+do_execsql_test 1.6 {
+  SELECT * FROM t1
+} {1 2 3 4 5 6 7 8 9 10 11 12}
+
+do_test 1.7 {
+  list [catch { sqlite3_snapshot_open db main $::snap1 } msg] $msg
+} {1 SQLITE_BUSY_SNAPSHOT}
+
+do_execsql_test 1.8 {
+  SELECT * FROM t1
+} {1 2 3 4 5 6 7 8 9 10 11 12}
+
+do_test 1.9 {
+  execsql { COMMIT ; BEGIN }
+  list [catch { sqlite3_snapshot_open db main $::snap1 } msg] $msg
+} {1 SQLITE_BUSY_SNAPSHOT}
+
+do_test 1.10 {
+  execsql { COMMIT }
+  execsql {
+    PRAGMA wal_checkpoint;
+    DELETE FROM t1 WHERE a = 1;
+  } db2
+  execsql BEGIN
+  set ::snap4 [sqlite3_snapshot_get db main]
+  execsql COMMIT
+  execsql {
+    DELETE FROM t1 WHERE a = 4;
+  } db2
+} {}
+
+do_test 1.11 {
+  execsql { 
+    BEGIN;
+      SELECT * FROM t1
+  }
+} {7 8 9 10 11 12 13 14 15}
+do_test 1.12 {
+  sqlite3_snapshot_open db main $::snap4
+  execsql { SELECT * FROM t1 }
+} {4 5 6 7 8 9 10 11 12 13 14 15}
+
+do_test 1.13 {
+  list [catch { sqlite3_snapshot_open db main $::snap3 } msg] $msg
+} {1 SQLITE_BUSY_SNAPSHOT}
+do_test 1.14 {
+  execsql { SELECT * FROM t1 }
+} {4 5 6 7 8 9 10 11 12 13 14 15}
+
+db close
+db2 close
+sqlite3 db test.db
+do_execsql_test 1.15 {
+  BEGIN;
+    SELECT * FROM t1
+} {7 8 9 10 11 12 13 14 15}
+do_test 1.16 {
+  list [catch { sqlite3_snapshot_open db main $::snap4 } msg] $msg
+} {1 SQLITE_BUSY_SNAPSHOT}
+do_execsql_test 1.17 { COMMIT }
+
+sqlite3_snapshot_free $::snap1
+sqlite3_snapshot_free $::snap2
+sqlite3_snapshot_free $::snap3
+sqlite3_snapshot_free $::snap4
+
+#-------------------------------------------------------------------------
+catch { db close }
+sqlite3 db test.db
+sqlite3 db2 test.db
+sqlite3 db3 test.db
+
+proc xBusy {args} { return 1 }
+db3 busy xBusy
+
+do_test 2.1 {
+  execsql { INSERT INTO t1 VALUES(16, 17, 18) } db2
+  execsql BEGIN
+  set ::snap1 [sqlite3_snapshot_get db main]
+  execsql COMMIT
+  execsql { INSERT INTO t1 VALUES(19, 20, 21) } db2
+  execsql BEGIN
+  set ::snap2 [sqlite3_snapshot_get db main]
+  execsql COMMIT
+  set {} {}
+} {}
+
+do_execsql_test -db db2 2.2 {
+  BEGIN;
+    INSERT INTO t1 VALUES(19, 20, 21);
+}
+
+do_test 2.3 {
+  execsql BEGIN
+  sqlite3_snapshot_open db main $::snap1
+  execsql { SELECT * FROM t1 }
+} {7 8 9 10 11 12 13 14 15 16 17 18}
+
+proc xBusy {args} { 
+  set ::res [list [catch { sqlite3_snapshot_open db main $::snap2 } msg] $msg]
+  return 1
+}
+db3 busy xBusy
+do_test 2.4 {
+  execsql {PRAGMA wal_checkpoint = restart} db3
+  set ::res
+} {1 SQLITE_BUSY}
+
+sqlite3_snapshot_free $::snap1
+sqlite3_snapshot_free $::snap2
+
+finish_test
+