Index: src/sqlite.h.in
==================================================================
--- src/sqlite.h.in
+++ src/sqlite.h.in
@@ -506,11 +506,10 @@
 #define SQLITE_BUSY_SNAPSHOT           (SQLITE_BUSY   |  (2<<8))
 #define SQLITE_CANTOPEN_NOTEMPDIR      (SQLITE_CANTOPEN | (1<<8))
 #define SQLITE_CANTOPEN_ISDIR          (SQLITE_CANTOPEN | (2<<8))
 #define SQLITE_CANTOPEN_FULLPATH       (SQLITE_CANTOPEN | (3<<8))
 #define SQLITE_CANTOPEN_CONVPATH       (SQLITE_CANTOPEN | (4<<8))
-#define SQLITE_CANTOPEN_DIRTYWAL       (SQLITE_CANTOPEN | (5<<8))
 #define SQLITE_CORRUPT_VTAB            (SQLITE_CORRUPT | (1<<8))
 #define SQLITE_READONLY_RECOVERY       (SQLITE_READONLY | (1<<8))
 #define SQLITE_READONLY_CANTLOCK       (SQLITE_READONLY | (2<<8))
 #define SQLITE_READONLY_ROLLBACK       (SQLITE_READONLY | (3<<8))
 #define SQLITE_READONLY_DBMOVED        (SQLITE_READONLY | (4<<8))
@@ -1059,10 +1058,24 @@
 ** [SQLITE_FCNTL_BEGIN_ATOMIC_WRITE] to be rolled back.
 ** ^This file control takes the file descriptor out of batch write mode
 ** so that all subsequent write operations are independent.
 ** ^SQLite will never invoke SQLITE_FCNTL_ROLLBACK_ATOMIC_WRITE without
 ** a prior successful call to [SQLITE_FCNTL_BEGIN_ATOMIC_WRITE].
+**
+** <li>[[SQLITE_FCNTL_READ_SHM]]
+** The [SQLITE_FCNTL_READ_SHM] opcode may be used to read data from 
+** the shared-memory region used by wal mode if the previous call
+** to the xShmMap method returned SQLITE_READONLY_CANTLOCK. The argument
+** must point to an instance of a structure declared as:
+** <blockquote><pre>
+**   struct ReadShmArg { void *pBuf; int nBuf };
+** </pre></blockquote>)^
+** Before returning, this operation allocates a buffer large enough for
+** the entire shared-memory using sqlite3_malloc() and populates it with
+** a copy thereof. ReadShmArg.pBuf is set to point to the buffer, and
+** ReadShmArg to its size in bytes. It is the responsibility of the caller
+** to eventually free the buffer using sqlite3_free().
 ** </ul>
 */
 #define SQLITE_FCNTL_LOCKSTATE               1
 #define SQLITE_FCNTL_GET_LOCKPROXYFILE       2
 #define SQLITE_FCNTL_SET_LOCKPROXYFILE       3

Index: src/wal.c
==================================================================
--- src/wal.c
+++ src/wal.c
@@ -2343,11 +2343,12 @@
   if( !useWal ){
     rc = walIndexReadHdr(pWal, pChanged);
     if( rc==SQLITE_READONLY_CANTLOCK ){
       /* This is a readonly_shm connection and there are no other connections
       ** to the database. So the *-shm file may not be accessed using mmap.
-      ** Take WAL_READ_LOCK(0) before proceding.  */
+      ** Try to open an "unlocked" transaction - one that loads the *-shm
+      ** file into memory using read() - instead.  */
       assert( pWal->nWiData>0 && pWal->apWiData[0]==0 );
       assert( pWal->readOnly & WAL_SHM_RDONLY );
       return walBeginUnlocked(pWal, pChanged);
     }
     if( rc==SQLITE_BUSY ){