Clarify the purpose of a test for a race-condition in walIndexReadHdr().

FossilOrigin-Name: c041c6a9786bc9ebb82527f7a2c96d255aec927f

diff --git a/manifest b/manifest
index 7b1ca4d5407c52584dd7d0947cf95e0488ccad1b..b77b7279562f375ac0427a0dc046c54b509d420f 100644 (file)
--- a/manifest
+++ b/manifest
@@ -1,5 +1,5 @@
-C Remove\sa\scondition\sfrom\ssqlite3WalRead()\sthat\sis\sunreachable\sas\sof\sthe\schanges\sto\sclear\sentries\sout\sof\sthe\swal-index\shash\stables\son\stransaction\sor\ssavepoint\srollback.
-D 2010-06-05T18:12:24
+C Clarify\sthe\spurpose\sof\sa\stest\sfor\sa\srace-condition\sin\swalIndexReadHdr().
+D 2010-06-05T18:34:26
 F Makefile.arm-wince-mingw32ce-gcc fcd5e9cd67fe88836360bb4f9ef4cb7f8e2fb5a0
 F Makefile.in a5cad1f8f3e021356bfcc6c77dc16f6f1952bbc3
 F Makefile.linux-gcc d53183f4aa6a9192d249731c90dbdffbd2c68654
@@ -224,7 +224,7 @@ F src/vdbeblob.c 5327132a42a91e8b7acfb60b9d2c3b1c5c863e0e
 F src/vdbemem.c 2a82f455f6ca6f78b59fb312f96054c04ae0ead1
 F src/vdbetrace.c 864cef96919323482ebd9986f2132435115e9cc2
 F src/vtab.c a0f8a40274e4261696ef57aa806de2776ab72cda
-F src/wal.c 22a58cdfc167d29353480f1a587d844997120518
+F src/wal.c a97ac7ee623fd92140bb0b37bc210b9cccb283f5
 F src/wal.h 4ace25262452d17e7d3ec970c89ee17794004008
 F src/walker.c 3112bb3afe1d85dc52317cb1d752055e9a781f8f
 F src/where.c 75fee9e255b62f773fcadd1d1f25b6f63ac7a356
@@ -817,7 +817,7 @@ F tool/speedtest2.tcl ee2149167303ba8e95af97873c575c3e0fab58ff
 F tool/speedtest8.c 2902c46588c40b55661e471d7a86e4dd71a18224
 F tool/speedtest8inst1.c 293327bc76823f473684d589a8160bde1f52c14e
 F tool/vdbe-compress.tcl d70ea6d8a19e3571d7ab8c9b75cba86d1173ff0f
-P 3fe0cc784ac586358c08f87fba458dfbb5eec6f2
-R 2aa1b1d87a9dc475383f9885b8e6d33d
+P 394204735a842b04b677cca20485b1578e475d4c
+R 27fd1a32d66f249d5fa488d9e53b7946
 U dan
-Z e4cbf876bae12325e294a44d5e590e7b
+Z ac39f481b39b4a8a76aecf6dead9965f

diff --git a/manifest.uuid b/manifest.uuid
index b1cde350afe132dd955aaef4697d3172bfe13611..c58bba614c0fe38bdaf63156c246245f5432daf1 100644 (file)
--- a/manifest.uuid
+++ b/manifest.uuid
@@ -1 +1 @@
-394204735a842b04b677cca20485b1578e475d4c
\ No newline at end of file
+c041c6a9786bc9ebb82527f7a2c96d255aec927f
\ No newline at end of file

diff --git a/src/wal.c b/src/wal.c
index 15a9ecc16adc1f7fb23c40f7925e3549e7c8bcbc..45a2d75e725079636b9cd297dcf3f84946cac29a 100644 (file)
--- a/src/wal.c
+++ b/src/wal.c
@@ -1745,6 +1745,7 @@ static int walTryBeginRead(Wal *pWal, int *pChanged, int useWal, int cnt){
     ** and can be safely ignored.
     */
     rc = walLockShared(pWal, WAL_READ_LOCK(0));
+    sqlite3OsShmBarrier(pWal->pDbFd);
     if( rc==SQLITE_OK ){
       if( memcmp(pHdr, &pWal->hdr, sizeof(WalIndexHdr)) ){
         /* It is not safe to allow the reader to continue here if frames
@@ -1817,9 +1818,29 @@ static int walTryBeginRead(Wal *pWal, int *pChanged, int useWal, int cnt){
     if( rc ){
       return rc==SQLITE_BUSY ? WAL_RETRY : rc;
     }
+    /* Now that the read-lock has been obtained, check that neither the
+    ** value in the aReadMark[] array or the contents of the wal-index
+    ** header have changed.
+    **
+    ** It is necessary to check that the wal-index header did not change
+    ** between the time it was read and when the shared-lock was obtained
+    ** on WAL_READ_LOCK(mxI) was obtained to account for the possibility
+    ** that the log file may have been wrapped by a writer, or that frames
+    ** that occur later in the log than pWal->hdr.mxFrame may have been
+    ** copied into the database by a checkpointer. If either of these things
+    ** happened, then reading the database with the current value of
+    ** pWal->hdr.mxFrame risks reading a corrupted snapshot. So, retry
+    ** instead.
+    **
+    ** This does not guarantee that the copy wal-index header is up to
+    ** date before proceeding. This would not be possible without somehow
+    ** blocking writers. It only guarantees that a damaging 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.
+    */
+    sqlite3OsShmBarrier(pWal->pDbFd);
     if( pInfo->aReadMark[mxI]!=mxReadMark
-     || pHdr[0].mxFrame!=pWal->hdr.mxFrame
-     || (sqlite3OsShmBarrier(pWal->pDbFd), pHdr[1].mxFrame!=pWal->hdr.mxFrame)
+     || memcmp(pHdr, &pWal->hdr, sizeof(WalIndexHdr))
     ){
       walUnlockShared(pWal, WAL_READ_LOCK(mxI));
       return WAL_RETRY;