Clarify an assert in sqlite3WalExclusiveMode().

FossilOrigin-Name: 255850699ddbf4aad8cc3223aefbada35daa0703

manifest

@@ -1,5 +1,5 @@
-C Remove\san\sunnecessary\sbranch\sfrom\swal.c.
+C Clarify\san\sassert\sin\ssqlite3WalExclusiveMode().
-D 2010-06-04T17:16:53
+D 2010-06-04T18:38:00
 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 ea912c55b33ae62ae375efb80ca7ae6ef919943c
+F src/wal.c e237581eeb99a9c766419e824c766ac67f37adb9
 F src/wal.h 4ace25262452d17e7d3ec970c89ee17794004008
 F src/walker.c 3112bb3afe1d85dc52317cb1d752055e9a781f8f
 F src/where.c 75fee9e255b62f773fcadd1d1f25b6f63ac7a356
@@ -762,7 +762,7 @@ F test/vtab_alter.test 9e374885248f69e251bdaacf480b04a197f125e5
 F test/vtab_err.test 0d4d8eb4def1d053ac7c5050df3024fd47a3fbd8
 F test/vtab_shared.test 0eff9ce4f19facbe0a3e693f6c14b80711a4222d
 F test/wal.test bfec61450b47cdf09f7d2269f9e9967683b8b0fc
-F test/wal2.test c90d20363f17373cbf5bdfcee3571b43e8fa597b
+F test/wal2.test 743d9b86041e57ba986dd7e3891c67725f9e2b2b
 F test/wal3.test 08626ca06bc87f34179258515c2d674b56b7e150
 F test/wal_common.tcl 3e953ae60919281688ea73e4d0aa0e1bc94becd9
 F test/walbak.test e7650a26eb4b8abeca9b145b1af1e63026dde432
@@ -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 ca327e32cfe1633f2c9d3f058e411f108aaa2b3c
+P 8e54786c9a0c5c399f228f56c73271f84d75694b
-R 02edcf55cffb3be8e9337fa14f5f9042
+R 723b0db73791affd894a1e0706a506aa
 U dan
-Z 3024f5e351971331772efea8971b5efc
+Z 1b6d1f85256b6057d423f1cf677d1ba1

manifest.uuid

@@ -1 +1 @@
-8e54786c9a0c5c399f228f56c73271f84d75694b
+255850699ddbf4aad8cc3223aefbada35daa0703

src/wal.c

@@ -2448,14 +2448,20 @@ int sqlite3WalCallback(Wal *pWal){
 int sqlite3WalExclusiveMode(Wal *pWal, int op){
   int rc;
   assert( pWal->writeLock==0 );
-  /* pWal->readLock is usually set, but might be -1 if there was a prior OOM */
+
+  /* pWal->readLock is usually set, but might be -1 if there was a 
+  ** prior error while attempting to acquire are read-lock. This cannot 
+  ** happen if the connection is actually in exclusive mode (as no xShmLock
+  ** locks are taken in this case). Nor should the pager attempt to
+  ** upgrade to exclusive-mode following such an error.
+  */
   assert( pWal->readLock>=0 || pWal->lockError );
+  assert( pWal->readLock>=0 || (op<=0 && pWal->exclusiveMode==0) );
+
   if( op==0 ){
     if( pWal->exclusiveMode ){
       pWal->exclusiveMode = 0;
-      if( pWal->readLock>=0 
+      if( walLockShared(pWal, WAL_READ_LOCK(pWal->readLock))!=SQLITE_OK ){
-       && walLockShared(pWal, WAL_READ_LOCK(pWal->readLock))!=SQLITE_OK
-      ){
         pWal->exclusiveMode = 1;
       }
       rc = pWal->exclusiveMode==0;

test/wal2.test

@@ -421,6 +421,12 @@ tvfs delete
 #   wal2-6.4.*: Check that xShmLock calls are omitted in exclusive locking
 #               mode.
 #
+#   wal2-6.5.*: 
+#
+#   wal2-6.6.*: Check that if the xShmLock() to reaquire a WAL read-lock when
+#               exiting exclusive mode fails (i.e. SQLITE_IOERR), then the
+#               connection silently remains in exclusive mode.
+#
 do_test wal2-6.1.1 {
   file delete -force test.db test.db-wal test.db-journal
   sqlite3 db test.db
@@ -718,6 +724,50 @@ do_test wal2-6.5.3 {
 } {}
 db close
 
+proc lock_control {method filename handle spec} {
+  foreach {start n op type} $spec break
+  if {$op == "lock"} { return SQLITE_IOERR }
+  return SQLITE_OK
+}
+do_test wal2-6.6.1 {
+  testvfs T
+  T script lock_control
+  T filter {}
+  sqlite3 db test.db -vfs T
+  execsql { PRAGMA locking_mode = exclusive }
+  execsql { INSERT INTO t2 VALUES('V', 'VI') }
+} {}
+do_test wal2-6.6.2 {
+  execsql { PRAGMA locking_mode = normal }
+  T filter xShmLock
+  execsql { INSERT INTO t2 VALUES('VII', 'VIII') }
+} {}
+do_test wal2-6.6.3 {
+  # At this point the connection should still be in exclusive-mode, even
+  # though it tried to exit exclusive-mode when committing the INSERT
+  # statement above. To exit exclusive mode, SQLite has to take a read-lock 
+  # on the WAL file using xShmLock(). Since that call failed, it remains
+  # in exclusive mode.
+  #
+  sqlite3 db2 test.db -vfs T
+  catchsql { SELECT * FROM t2 } db2
+} {1 {database is locked}}
+do_test wal2-6.6.2 {
+  db2 close
+  T filter {}
+  execsql { INSERT INTO t2 VALUES('IX', 'X') }
+} {}
+do_test wal2-6.6.3 {
+  # This time, we have successfully exited exclusive mode. So the second
+  # connection can read the database.
+  sqlite3 db2 test.db -vfs T
+  catchsql { SELECT * FROM t2 } db2
+} {0 {I II III IV V VI VII VIII IX X}}
+
+db close
+db2 close
+T delete
+
 #-------------------------------------------------------------------------
 # Test a theory about the checksum algorithm. Theory was false and this
 # test did not provoke a bug.