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

FossilOrigin-Name: 34b6ac3d76dbc6819778ec2a0f81cbcdcc0cd1a6303381d97f1c479e4ecdd132

diff --git a/manifest b/manifest
index e74bcacf3c8fd268aacac4d8723436420408a940..83d6936a7d0d0b50cddb8611cca58aab8d7be8fc 100644 (file)
--- a/manifest
+++ b/manifest
@@ -1,5 +1,5 @@
-C In\sthe\sCLI,\sfor\scolumnar\soutput\sformats,\stry\sto\saccount\sfor\sthe\spresence\sof\nzero-width\sand\sdouble-width\scharacters\sin\sthe\soutput\sand\sadjust\scolumn\swidths\naccordingly.
-D 2024-09-25T09:39:11.501
+C When\spossible,\savoid\staking\swal\sfile\sread-lock\s0\sin\ssqlite3_snapshot_get().
+D 2024-09-26T18:02:17.495
 F .fossil-settings/empty-dirs dbb81e8fc0401ac46a1491ab34a7f2c7c0452f2f06b54ebb845d024ca8283ef1
 F .fossil-settings/ignore-glob 35175cdfcf539b2318cb04a9901442804be81cd677d8b889fcc9149c21f239ea
 F LICENSE.md df5091916dbb40e6e9686186587125e1b2ff51f022cc334e886c19a0e9982724
@@ -730,7 +730,7 @@ F src/insert.c f8d1a0f8ee258411009c6b7f2d93170e351bd19f5ad89d57e1180644297cbe70
 F src/json.c 68a98c020c22127f2d65f08855f7fc7460ff352a6ce0b543d8931dde83319c22
 F src/legacy.c d7874bc885906868cd51e6c2156698f2754f02d9eee1bae2d687323c3ca8e5aa
 F src/loadext.c 7432c944ff197046d67a1207790a1b13eec4548c85a9457eb0896bb3641dfb36
-F src/main.c 4db6e3bde55ba0b24ccc83600c2b6ea11429f61ce7b3a2e7e3b42e1b45366c3e
+F src/main.c 3bb768eeeee1cceaed391327362b8be8b5ce61a12fe24d5b555ce9c6b4a883de
 F src/malloc.c 410e570b30c26cc36e3372577df50f7a96ee3eed5b2b161c6b6b48773c650c5e
 F src/mem0.c 6a55ebe57c46ca1a7d98da93aaa07f99f1059645
 F src/mem1.c 3bb59158c38e05f6270e761a9f435bf19827a264c13d1631c58b84bdc96d73b2
@@ -849,7 +849,7 @@ F src/vdbetrace.c fe0bc29ebd4e02c8bc5c1945f1d2e6be5927ec12c06d89b03ef2a4def34bf8
 F src/vdbevtab.c fc46b9cbd759dc013f0b3724549cc0d71379183c667df3a5988f7e2f1bd485f3
 F src/vtab.c 316cd48e9320660db3047cd306cd056e4361180cebb4d0f10a39244e10c11422
 F src/vxworks.h d2988f4e5a61a4dfe82c6524dd3d6e4f2ce3cdb9
-F src/wal.c ef68130ba330ee18c1cb22da36a881c82e3a3b109badbdc6a9b9acaf788a6688
+F src/wal.c a0d42bfdef935e1389737152394d08e59e7c48697f40a9fc2e0552cb19dc731f
 F src/wal.h ba252daaa94f889f4b2c17c027e823d9be47ce39da1d3799886bbd51f0490452
 F src/walker.c d5006d6b005e4ea7302ad390957a8d41ed83faa177e412f89bc5600a7462a014
 F src/where.c 461d41017d900d4248a268df96d2d30506c4dcc2257f4167c4f46072003ce2cf
@@ -1647,7 +1647,7 @@ F test/skipscan5.test 0672103fd2c8f96bd114133f356192b35ece45c794fe3677e1d9e5e310
 F test/skipscan6.test bddbb35dd335e2d21b7791a61e3b2e1f3255dc307ce80aa6fe19cc298e6feb13
 F test/snapshot.test a504f2e7009f512ef66c719f0ea1c55a556bdaf1e1312c80a04d46fc1a3e9632
 F test/snapshot2.test 8d6ff5dd9cc503f6e12d408a30409c3f9c653507b24408d9cd7195931c89bc54
-F test/snapshot3.test 8744313270c55f6e18574283553d3c5c5fe4c5970585663613a0e75c151e599b
+F test/snapshot3.test 41350216abc6c7da37113ad462259c070786e5ad70bdc8709daaed148b1b3a2c
 F test/snapshot4.test d4e9347ef2fcabc491fc893506c7bbaf334da3be111d6eb4f3a97cc623b78322
 F test/snapshot_fault.test 129234ceb9b26a0e1000e8563a16e790f5c1412354e70749cbd78c3d5d07d60a
 F test/snapshot_up.test 77dc7853bfb2b4fa249f76e1714cfa1e596826165d9ef22c06ac3a0b7b778d9a
@@ -2213,9 +2213,11 @@ F vsixtest/vsixtest.tcl 6195aba1f12a5e10efc2b8c0009532167be5e301abe5b31385638080
 F vsixtest/vsixtest.vcxproj.data 2ed517e100c66dc455b492e1a33350c1b20fbcdc
 F vsixtest/vsixtest.vcxproj.filters 37e51ffedcdb064aad6ff33b6148725226cd608e
 F vsixtest/vsixtest_TemporaryKey.pfx e5b1b036facdb453873e7084e1cae9102ccc67a0
-P 42bb941584a1ac922ee6b0b6ecadce71c9259555563cf49913a6f820f3f9b887 b217e3004b58af0e777726bdd652b999ad41815261299ef4ce8f8d2f6b0afe8d
-R 5fcf6775e686fdd76943c66cce860cb0
-T +closed b217e3004b58af0e777726bdd652b999ad41815261299ef4ce8f8d2f6b0afe8d
-U drh
-Z af457fc67e3efab459ef7260d9a5da46
+P 9592b9ba3ad7a842cdd4c4010da278485a6fdec7e811bda01ebe640162a8c3b6
+R 8553e4183ac33940fe95918021cfe535
+T *branch * snapshot_get-locking
+T *sym-snapshot_get-locking *
+T -sym-trunk *
+U dan
+Z 2b5e37766517d26ed8f4d4495c7d929b
 # Remove this line to create a well-formed Fossil manifest.

diff --git a/manifest.uuid b/manifest.uuid
index 5d05a8c7ec528f03bd7b493653796b31cb18399f..986575a33e4de6349737f7bdfc3438cd2c57ae26 100644 (file)
--- a/manifest.uuid
+++ b/manifest.uuid
@@ -1 +1 @@
-9592b9ba3ad7a842cdd4c4010da278485a6fdec7e811bda01ebe640162a8c3b6
+34b6ac3d76dbc6819778ec2a0f81cbcdcc0cd1a6303381d97f1c479e4ecdd132

diff --git a/src/main.c b/src/main.c
index 6ab09c55602be76dc88ce4caec34a89b859f92e8..fceb7957b9cf3f564848d0625278d44051b2822f 100644 (file)
--- 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 89106038b89da6e056595c19c3081bb4f446b334..f4aa663fd3043eef5220300c0a5c5f4364f6044b 100644 (file)
--- 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;
+  }
 }
 
 /*

diff --git a/test/snapshot3.test b/test/snapshot3.test
index 4dca202b1cfc4a2f03f4608d254b8c67881ab01a..470d463a6618cd2bbc6c1448f34535fbf44f06a5 100644 (file)
--- a/test/snapshot3.test
+++ b/test/snapshot3.test
@@ -96,4 +96,43 @@ do_test 1.8 {
   list [catch { sqlite3_snapshot_open_blob db3 main $snap } msg] $msg
 } {1 SQLITE_ERROR_SNAPSHOT}
 
+#-------------------------------------------------------------------------
+reset_db
+do_execsql_test 2.0 {
+  PRAGMA journal_mode = wal;
+  CREATE TABLE t1(a, b);
+  INSERT INTO t1 VALUES(1, 2);
+  INSERT INTO t1 VALUES(3, 4);
+} {wal}
+
+sqlite3 db2 test.db
+sqlite3 db3 test.db
+do_execsql_test -db db2 2.0.1 {
+  SELECT * FROM t1
+} {1 2 3 4}
+do_execsql_test -db db3 2.0.2 {
+  SELECT * FROM t1
+} {1 2 3 4}
+
+do_execsql_test -db db2 2.2 {
+  PRAGMA wal_checkpoint;
+} {0 4 4}
+
+do_test 2.1 {
+  db eval { BEGIN }
+  set snap [sqlite3_snapshot_get db main]
+  set {} {}
+} {}
+
+do_execsql_test -db db2 2.3 {
+  INSERT INTO t1 VALUES(5, 6);
+} {}
+
+do_test 2.2 {
+  execsql { BEGIN } db3
+  sqlite3_snapshot_open db3 main $snap
+} {}
+
+sqlite3_snapshot_free $snap
+
 finish_test