Enhanced comments on table locking logic as it relates to preparing new

statements.  Added assert() and testcase() but no other changes to code. (CVS 6319)

FossilOrigin-Name: 4a12f5b818b769d7518c942ff3dedf453dde698e

diff --git a/manifest b/manifest
index b1238d99197f214efc1c8850098cf661d0e16eda..cbc7d9d58b92fb99b017b48ba9865cfef308b971 100644 (file)
--- a/manifest
+++ b/manifest
@@ -1,5 +1,5 @@
-C Add\stest\sfile\stest/count.test\sfor\stesting\s"SELECT\scount(*)"\sstatements.\sIt\sis\snot\sproperly\spopulated\syet.\s(CVS\s6318)
-D 2009-02-24T10:48:28
+C Enhanced\scomments\son\stable\slocking\slogic\sas\sit\srelates\sto\spreparing\snew\nstatements.\s\sAdded\sassert()\sand\stestcase()\sbut\sno\sother\schanges\sto\scode.\s(CVS\s6319)
+D 2009-02-24T16:18:05
 F Makefile.arm-wince-mingw32ce-gcc fcd5e9cd67fe88836360bb4f9ef4cb7f8e2fb5a0
 F Makefile.in d64baddbf55cdf33ff030e14da837324711a4ef7
 F Makefile.linux-gcc d53183f4aa6a9192d249731c90dbdffbd2c68654
@@ -103,7 +103,7 @@ F src/auth.c c8b2ab5c8bad4bd90ed7c294694f48269162c627
 F src/backup.c 2d3f31148d7b086c5c72d9edcd04fc2751b0aa6e
 F src/bitvec.c 44f7059ac1f874d364b34af31b9617e52223ba75
 F src/btmutex.c 63c5cc4ad5715690767ffcb741e185d7bc35ec1a
-F src/btree.c e0178d6fb69c8f332f3fba96cfc0b08275ad5e76
+F src/btree.c 8b8697ab5e8ca9aad12221bb9abfee1aed7a5cd1
 F src/btree.h 96a019c9f28da38e79940512d7800e419cd8c702
 F src/btreeInt.h 0a4884e6152d7cae9c741e91b830064c19fd2c05
 F src/build.c a1db48aec62c95049d783f231195812ff97ae268
@@ -149,7 +149,7 @@ F src/pcache.c fcf7738c83c4d3e9d45836b2334c8a368cc41274
 F src/pcache.h 9b927ccc5a538e31b4c3bc7eec4f976db42a1324
 F src/pcache1.c dabb8ab14827e090321f17150ce96fda172974e8
 F src/pragma.c 22ed04836aab8ce134c53be1ca896f3ad20fabdb
-F src/prepare.c d0bbb4b1a8b9c1db6d13788929839bb63764680e
+F src/prepare.c 1ede93a00f1835a10b3d3aad968e8f8ecd2c98dd
 F src/printf.c 9866a9a9c4a90f6d4147407f373df3fd5d5f9b6f
 F src/random.c 676b9d7ac820fe81e6fb2394ac8c10cff7f38628
 F src/resolve.c dea005135845acbbfae1f2968c0deb6b2e3d580c
@@ -702,7 +702,7 @@ F tool/speedtest16.c c8a9c793df96db7e4933f0852abb7a03d48f2e81
 F tool/speedtest2.tcl ee2149167303ba8e95af97873c575c3e0fab58ff
 F tool/speedtest8.c 2902c46588c40b55661e471d7a86e4dd71a18224
 F tool/speedtest8inst1.c 293327bc76823f473684d589a8160bde1f52c14e
-P 0e7c369c23a8767b4d3e5cdd47c14716992fb71a
-R 71f287412eb030dfd9b14d26e52b17fb
-U danielk1977
-Z e2d5b34166a5d213f9445c572c8e416e
+P a195d74ff9ce836447dba4da7edcc6f1cdae5574
+R 7311b1fa9b7b5c3d12ba433576f5510b
+U drh
+Z 99368cf9c56a6d99659a698dff53e252

diff --git a/manifest.uuid b/manifest.uuid
index b237de885fa79996d42a67ed565d598c38eb7199..e181bcc7c07b6f4814810c5b5d2198b1f7361253 100644 (file)
--- a/manifest.uuid
+++ b/manifest.uuid
@@ -1 +1 @@
-a195d74ff9ce836447dba4da7edcc6f1cdae5574
\ No newline at end of file
+4a12f5b818b769d7518c942ff3dedf453dde698e
\ No newline at end of file

diff --git a/src/btree.c b/src/btree.c
index 07169bed3d4b90821f93ce4289e94e8c7c6e3c66..9843acb0ab56a24d49eadc74d9aac6c881f30278 100644 (file)
--- a/src/btree.c
+++ b/src/btree.c
@@ -9,7 +9,7 @@
 **    May you share freely, never taking more than you give.
 **
 *************************************************************************
-** $Id: btree.c,v 1.567 2009/02/24 10:01:52 danielk1977 Exp $
+** $Id: btree.c,v 1.568 2009/02/24 16:18:05 drh Exp $
 **
 ** This file implements a external (disk-based) database using BTrees.
 ** See the header comment on "btreeInt.h" for additional information.
@@ -113,8 +113,9 @@ static int queryTableLock(Btree *p, Pgno iTab, u8 eLock){
   }
 
   /* This (along with lockTable()) is where the ReadUncommitted flag is
-  ** dealt with. If the caller is querying for a read-lock and the flag is
-  ** set, it is unconditionally granted - even if there are write-locks
+  ** dealt with. If the caller is querying for a read-lock on any table
+  ** other than the sqlite_master table (table 1) and if the ReadUncommitted
+  ** flag is set, then the lock granted even if there are write-locks
   ** on the table. If a write-lock is requested, the ReadUncommitted flag
   ** is not considered.
   **
@@ -122,9 +123,9 @@ static int queryTableLock(Btree *p, Pgno iTab, u8 eLock){
   ** ReadUncommitted flag is set, no entry is added to the locks list 
   ** (BtShared.pLock).
   **
-  ** To summarize: If the ReadUncommitted flag is set, then read cursors do
-  ** not create or respect table locks. The locking procedure for a 
-  ** write-cursor does not change.
+  ** To summarize: If the ReadUncommitted flag is set, then read cursors
+  ** on non-schema tables do not create or respect table locks. The locking
+  ** procedure for a write-cursor does not change.
   */
   if( 
     0==(p->db->flags&SQLITE_ReadUncommitted) || 
@@ -167,8 +168,9 @@ static int lockTable(Btree *p, Pgno iTable, u8 eLock){
 
   assert( SQLITE_OK==queryTableLock(p, iTable, eLock) );
 
-  /* If the read-uncommitted flag is set and a read-lock is requested,
-  ** return early without adding an entry to the BtShared.pLock list. See
+  /* If the read-uncommitted flag is set and a read-lock is requested on
+  ** a non-schema table, then the lock is always granted.  Return early
+  ** without adding an entry to the BtShared.pLock list. See
   ** comment in function queryTableLock() for more info on handling 
   ** the ReadUncommitted flag.
   */

diff --git a/src/prepare.c b/src/prepare.c
index e2dbaf52faddd142f3bff646fd004f975c582b54..fbd005cfb3ac06718366c563ea4ae0097fb7e65f 100644 (file)
--- a/src/prepare.c
+++ b/src/prepare.c
@@ -13,7 +13,7 @@
 ** interface, and routines that contribute to loading the database schema
 ** from disk.
 **
-** $Id: prepare.c,v 1.106 2009/02/19 14:39:25 danielk1977 Exp $
+** $Id: prepare.c,v 1.107 2009/02/24 16:18:05 drh Exp $
 */
 #include "sqliteInt.h"
 
@@ -540,17 +540,39 @@ static int sqlite3Prepare(
   assert( !db->mallocFailed );
   assert( sqlite3_mutex_held(db->mutex) );
 
-  /* If any attached database schemas are locked, do not proceed with
-  ** compilation. Instead return SQLITE_LOCKED immediately.
+  /* Check to verify that it is possible to get a read lock on all
+  ** database schemas.  The inability to get a read lock indicates that
+  ** some other database connection is holding a write-lock, which in
+  ** turn means that the other connection has made uncommitted changes
+  ** to the schema.
+  **
+  ** Were we to proceed and prepare the statement against the uncommitted
+  ** schema changes and if those schema changes are subsequently rolled
+  ** back and different changes are made in their place, then when this
+  ** prepared statement goes to run the schema cookie would fail to detect
+  ** the schema change.  Disaster would follow.
+  **
+  ** This thread is currently holding mutexes on all Btrees (because
+  ** of the sqlite3BtreeEnterAll() in sqlite3LockAndPrepare()) so it
+  ** is not possible for another thread to start a new schema change
+  ** while this routine is running.  Hence, we do not need to hold 
+  ** locks on the schema, we just need to make sure nobody else is 
+  ** holding them.
+  **
+  ** Note that setting READ_UNCOMMITTED overrides most lock detection,
+  ** but it does *not* override schema lock detection, so this all still
+  ** works even if READ_UNCOMMITTED is set.
   */
   for(i=0; i<db->nDb; i++) {
     Btree *pBt = db->aDb[i].pBt;
     if( pBt ){
+      assert( sqlite3BtreeHoldsMutex(pBt) );
       rc = sqlite3BtreeSchemaLocked(pBt);
       if( rc ){
         const char *zDb = db->aDb[i].zName;
         sqlite3Error(db, SQLITE_LOCKED, "database schema is locked: %s", zDb);
         (void)sqlite3SafetyOff(db);
+        testcase( db->flags & SQLITE_ReadUncommitted );
         return sqlite3ApiExit(db, SQLITE_LOCKED);
       }
     }