diff --git a/manifest b/manifest
index 70811a6f76..ef1d96bd61 100644
--- a/manifest
+++ b/manifest
@@ -1,5 +1,5 @@
-C add\sa\ssanity\scheck\sto\smake\ssure\sthe\sconfigure\sscript\sand\sVERSION\sfile\sare\salways\sin\ssync\s(CVS\s6420)
-D 2009-04-01T07:38:31
+C Add\ssome\sassert()\sstatements\sto\squerySharedCacheTableLock().\s(CVS\s6421)
+D 2009-04-01T09:41:54
 F Makefile.arm-wince-mingw32ce-gcc fcd5e9cd67fe88836360bb4f9ef4cb7f8e2fb5a0
 F Makefile.in 583e87706abc3026960ed759aff6371faf84c211
 F Makefile.linux-gcc d53183f4aa6a9192d249731c90dbdffbd2c68654
@@ -103,7 +103,7 @@ F src/auth.c c8b2ab5c8bad4bd90ed7c294694f48269162c627
 F src/backup.c 0082d0e5a63f04e88faee0dff0a7d63d3e92a78d
 F src/bitvec.c 44f7059ac1f874d364b34af31b9617e52223ba75
 F src/btmutex.c 341502bc496dc0840dcb00cde65680fb0e85c3ab
-F src/btree.c 304c18b7e3d6e921764cede594c36e755200ce08
+F src/btree.c afb15be98c01b07528072dd6e18b7f644859ada0
 F src/btree.h e302c5747494067cd4f5763000fbe7bca767d816
 F src/btreeInt.h df64030d632f8c8ac217ed52e8b6b3eacacb33a5
 F src/build.c 72357fd75ef036d0afbf1756edab6d62c56fcf4b
@@ -532,6 +532,7 @@ F test/shared.test 3b448dc0f7a9356e641894ed81c27599f39d809d
 F test/shared2.test 8973e41e008acbbd8d1b191b91a23cd472f058e8
 F test/shared3.test 9c880afc081d797da514ef64bccf36f3fce2f09c
 F test/shared4.test d0fadacb50bb6981b2fb9dc6d1da30fa1edddf83
+F test/shared6.test 9c155088d28bcaa5d87c52b6406ef5d0e7086e8e
 F test/shared_err.test 91e26ec4f3fbe07951967955585137e2f18993de
 F test/shortread1.test bb591ef20f0fd9ed26d0d12e80eee6d7ac8897a3
 F test/sidedelete.test f0ad71abe6233e3b153100f3b8d679b19a488329
@@ -713,7 +714,7 @@ F tool/speedtest16.c c8a9c793df96db7e4933f0852abb7a03d48f2e81
 F tool/speedtest2.tcl ee2149167303ba8e95af97873c575c3e0fab58ff
 F tool/speedtest8.c 2902c46588c40b55661e471d7a86e4dd71a18224
 F tool/speedtest8inst1.c 293327bc76823f473684d589a8160bde1f52c14e
-P ce683cbd329eab45abb6f97f677f560b0fd1cdb6
-R 2718bfb20c645bac2c120cb851c22c61
-U vapier
-Z ee7f3525d69da1abde020f897f014028
+P 1b9da6d73f603e66d078463c3571dda86ceedfb3
+R 14b12ad716bad78f9281344d4310ecc6
+U danielk1977
+Z 50c3252197be65f8aa74e6bb679653bc
diff --git a/manifest.uuid b/manifest.uuid
index a6a970bd41..9b8025b568 100644
--- a/manifest.uuid
+++ b/manifest.uuid
@@ -1 +1 @@
-1b9da6d73f603e66d078463c3571dda86ceedfb3
\ No newline at end of file
+3e3b5e861aeff2e4ef568c422236fdf7fa22bed3
\ No newline at end of file
diff --git a/src/btree.c b/src/btree.c
index c09d778530..0d3106a264 100644
--- 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 1.582 2009/03/30 18:50:05 danielk1977 Exp $
+** $Id: btree.c,v 1.583 2009/04/01 09:41:54 danielk1977 Exp $
 **
 ** This file implements a external (disk-based) database using BTrees.
 ** See the header comment on "btreeInt.h" for additional information.
@@ -103,6 +103,13 @@ static int querySharedCacheTableLock(Btree *p, Pgno iTab, u8 eLock){
   assert( eLock==READ_LOCK || eLock==WRITE_LOCK );
   assert( p->db!=0 );
   
+  /* If requesting a write-lock, then the Btree must have an open write
+  ** transaction on this file. And, obviously, for this to be so there 
+  ** must be an open write transaction on the file itself.
+  */
+  assert( eLock==READ_LOCK || (p==pBt->pWriter && p->inTrans==TRANS_WRITE) );
+  assert( eLock==READ_LOCK || pBt->inTransaction==TRANS_WRITE );
+  
   /* This is a no-op if the shared-cache is not enabled */
   if( !p->sharable ){
     return SQLITE_OK;
@@ -138,8 +145,18 @@ static int querySharedCacheTableLock(Btree *p, Pgno iTab, u8 eLock){
     iTab==MASTER_ROOT
   ){
     for(pIter=pBt->pLock; pIter; pIter=pIter->pNext){
-      if( pIter->pBtree!=p && pIter->iTable==iTab && 
-          (pIter->eLock!=eLock || eLock!=READ_LOCK) ){
+      /* The condition (pIter->eLock!=eLock) in the following if(...) 
+      ** statement is a simplification of:
+      **
+      **   (eLock==WRITE_LOCK || pIter->eLock==WRITE_LOCK)
+      **
+      ** since we know that if eLock==WRITE_LOCK, then no other connection
+      ** may hold a WRITE_LOCK on any table in this file (since there can
+      ** only be a single writer).
+      */
+      assert( pIter->eLock==READ_LOCK || pIter->eLock==WRITE_LOCK );
+      assert( eLock==READ_LOCK || pIter->pBtree==p || pIter->eLock==READ_LOCK);
+      if( pIter->pBtree!=p && pIter->iTable==iTab && pIter->eLock!=eLock ){
         sqlite3ConnectionBlocked(p->db, pIter->pBtree->db);
         if( eLock==WRITE_LOCK ){
           assert( p==pBt->pWriter );
diff --git a/test/shared6.test b/test/shared6.test
new file mode 100644
index 0000000000..716c057217
--- /dev/null
+++ b/test/shared6.test
@@ -0,0 +1,138 @@
+# 2009 April 01
+#
+# 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.
+#
+#***********************************************************************
+#
+# $Id: shared6.test,v 1.1 2009/04/01 09:41:54 danielk1977 Exp $
+
+set testdir [file dirname $argv0]
+source $testdir/tester.tcl
+ifcapable !shared_cache { finish_test ; return }
+
+do_test shared7-1.1.1 {
+  execsql {
+    CREATE TABLE t1(a, b);
+    CREATE TABLE t2(c, d);
+    CREATE TABLE t3(e, f);
+  }
+} {}
+do_test shared7-1.1.2 {
+  set ::enable_shared_cache [sqlite3_enable_shared_cache 1]
+  sqlite3_enable_shared_cache
+} {1}
+
+do_test shared7-1.1.3 {
+  sqlite3 db1 test.db
+  sqlite3 db2 test.db
+} {}
+
+# Exclusive shared-cache locks. Test the following:
+#
+#   1.2.1: If [db1] has an exclusive lock, [db2] cannot read.
+#   1.2.2: If [db1] has an exclusive lock, [db1] can read.
+#   1.2.3: If [db1] has a non-exclusive write-lock, [db2] can read.
+# 
+do_test shared7-1.2.1 {
+  execsql { SELECT * FROM t1 } db2    ;# Cache a compiled statement
+  execsql { BEGIN EXCLUSIVE } db1
+  catchsql { SELECT * FROM t1 } db2   ;# Execute the cached compiled statement
+} {1 {database table is locked}}
+do_test shared7-1.2.2 {
+  execsql { SELECT * FROM t1 } db1
+} {}
+do_test shared7-1.2.3 {
+  execsql {
+    COMMIT;
+    BEGIN;
+    INSERT INTO t2 VALUES(3, 4);
+  } db1
+  execsql { SELECT * FROM t1 } db2
+} {}
+do_test shared7-1.2.X {
+  execsql { COMMIT } db1
+} {}
+
+# Regular shared-cache locks. Verify the following:
+#
+#   1.3.1: If [db1] has a write-lock on t1, [db1] can read from t1.
+#   1.3.2: If [db1] has a write-lock on t1, [db2] can read from t2.
+#   1.3.3: If [db1] has a write-lock on t1, [db2] cannot read from t1.
+#   1.3.4: If [db1] has a write-lock on t1, [db2] cannot write to t1.
+#   1.3.5: If [db1] has a read-lock on t1, [db2] can read from t1.
+#   1.3.6: If [db1] has a read-lock on t1, [db2] cannot write to t1.
+#
+do_test shared7-1.3.1 {
+  execsql {
+    BEGIN;
+    INSERT INTO t1 VALUES(1, 2);
+  } db1
+  execsql { SELECT * FROM t1 } db1
+} {1 2}
+do_test shared7-1.3.2 {
+  execsql { SELECT * FROM t2 } db2
+} {3 4}
+do_test shared7-1.3.3 {
+  catchsql { SELECT * FROM t1 } db2
+} {1 {database table is locked: t1}}
+do_test shared7-1.3.4 {
+  catchsql { INSERT INTO t2 VALUES(1, 2) } db2
+} {1 {database table is locked}}
+do_test shared7-1.3.5 {
+  execsql {
+    COMMIT;
+    BEGIN;
+    SELECT * FROM t1;
+  } db1
+  execsql { SELECT * FROM t1 } db2
+} {1 2}
+do_test shared7-1.3.5 {
+  catchsql { INSERT INTO t1 VALUES(5, 6) } db2
+} {1 {database table is locked: t1}}
+do_test shared7-1.3.X {
+  execsql { COMMIT } db1
+} {}
+
+# Read-uncommitted mode.
+#
+# For these tests, connection [db2] is in read-uncommitted mode.
+#
+#   1.4.1: If [db1] has a write-lock on t1, [db2] can still read from t1.
+#   1.4.2: If [db1] has a write-lock on the db schema (sqlite_master table), 
+#          [db2] cannot read from the schema.
+#   1.4.3: If [db1] has a read-lock on t1, [db2] cannot write to t1.
+#
+do_test shared7-1.4.1 {
+  execsql { PRAGMA read_uncommitted = 1 } db2
+  execsql {
+    BEGIN;
+    INSERT INTO t1 VALUES(5, 6);
+  } db1
+  execsql { SELECT * FROM t1 } db2
+} {1 2 5 6}
+do_test shared7-1.4.2 {
+  execsql { CREATE TABLE t4(a, b) } db1
+  catchsql { SELECT * FROM t1 } db2
+} {1 {database table is locked}}
+do_test shared7-1.4.3 {
+  execsql {
+    COMMIT;
+    BEGIN;
+    SELECT * FROM t1;
+  } db1
+  catchsql { INSERT INTO t1 VALUES(7, 8) } db2
+} {1 {database table is locked: t1}}
+
+do_test shared7-1.X {
+  db1 close
+  db2 close
+} {}
+
+sqlite3_enable_shared_cache $::enable_shared_cache
+finish_test
+