Index: src/btree.c
==================================================================
--- src/btree.c
+++ src/btree.c
@@ -1755,35 +1755,40 @@
 #if !defined(SQLITE_OMIT_SHARED_CACHE) && !defined(SQLITE_OMIT_DISKIO)
   /*
   ** If this Btree is a candidate for shared cache, try to find an
   ** existing BtShared object that we can share with
   */
-  if( isMemdb==0 && isTempDb==0 ){
+  if( isTempDb==0 && (isMemdb==0 || (vfsFlags&SQLITE_OPEN_URI)!=0) ){
     if( vfsFlags & SQLITE_OPEN_SHAREDCACHE ){
       int nFullPathname = pVfs->mxPathname+1;
       char *zFullPathname = sqlite3Malloc(nFullPathname);
       MUTEX_LOGIC( sqlite3_mutex *mutexShared; )
       p->sharable = 1;
       if( !zFullPathname ){
         sqlite3_free(p);
         return SQLITE_NOMEM;
       }
-      rc = sqlite3OsFullPathname(pVfs, zFilename, nFullPathname, zFullPathname);
-      if( rc ){
-        sqlite3_free(zFullPathname);
-        sqlite3_free(p);
-        return rc;
+      if( isMemdb ){
+        memcpy(zFullPathname, zFilename, sqlite3Strlen30(zFilename)+1);
+      }else{
+        rc = sqlite3OsFullPathname(pVfs, zFilename,
+                                   nFullPathname, zFullPathname);
+        if( rc ){
+          sqlite3_free(zFullPathname);
+          sqlite3_free(p);
+          return rc;
+        }
       }
 #if SQLITE_THREADSAFE
       mutexOpen = sqlite3MutexAlloc(SQLITE_MUTEX_STATIC_OPEN);
       sqlite3_mutex_enter(mutexOpen);
       mutexShared = sqlite3MutexAlloc(SQLITE_MUTEX_STATIC_MASTER);
       sqlite3_mutex_enter(mutexShared);
 #endif
       for(pBt=GLOBAL(BtShared*,sqlite3SharedCacheList); pBt; pBt=pBt->pNext){
         assert( pBt->nRef>0 );
-        if( 0==strcmp(zFullPathname, sqlite3PagerFilename(pBt->pPager))
+        if( 0==strcmp(zFullPathname, sqlite3PagerFilename(pBt->pPager, 0))
                  && sqlite3PagerVfs(pBt->pPager)==pVfs ){
           int iDb;
           for(iDb=db->nDb-1; iDb>=0; iDb--){
             Btree *pExisting = db->aDb[iDb].pBt;
             if( pExisting && pExisting->pBt==pBt ){
@@ -8044,18 +8049,19 @@
   return sqlite3StrAccumFinish(&sCheck.errMsg);
 }
 #endif /* SQLITE_OMIT_INTEGRITY_CHECK */
 
 /*
-** Return the full pathname of the underlying database file.
+** Return the full pathname of the underlying database file.  Return
+** an empty string if the database is in-memory or a TEMP database.
 **
 ** The pager filename is invariant as long as the pager is
 ** open so it is safe to access without the BtShared mutex.
 */
 const char *sqlite3BtreeGetFilename(Btree *p){
   assert( p->pBt->pPager!=0 );
-  return sqlite3PagerFilename(p->pBt->pPager);
+  return sqlite3PagerFilename(p->pBt->pPager, 1);
 }
 
 /*
 ** Return the pathname of the journal file for this database. The return
 ** value of this routine is the same regardless of whether the journal file

Index: src/main.c
==================================================================
--- src/main.c
+++ src/main.c
@@ -2053,10 +2053,11 @@
     zFile = sqlite3_malloc(nUri+2);
     if( !zFile ) return SQLITE_NOMEM;
     memcpy(zFile, zUri, nUri);
     zFile[nUri] = '\0';
     zFile[nUri+1] = '\0';
+    flags &= ~SQLITE_OPEN_URI;
   }
 
   *ppVfs = sqlite3_vfs_find(zVfs);
   if( *ppVfs==0 ){
     *pzErrMsg = sqlite3_mprintf("no such vfs: %s", zVfs);

Index: src/pager.c
==================================================================
--- src/pager.c
+++ src/pager.c
@@ -4358,11 +4358,16 @@
   *ppPager = 0;
 
 #ifndef SQLITE_OMIT_MEMORYDB
   if( flags & PAGER_MEMORY ){
     memDb = 1;
-    zFilename = 0;
+    if( zFilename && zFilename[0] ){
+      zPathname = sqlite3DbStrDup(0, zFilename);
+      if( zPathname==0  ) return SQLITE_NOMEM;
+      nPathname = sqlite3Strlen30(zPathname);
+      zFilename = 0;
+    }
   }
 #endif
 
   /* Compute and store the full pathname in an allocated buffer pointed
   ** to by zPathname, length nPathname. Or, if this is a temporary file,
@@ -6294,13 +6299,20 @@
   return rc;
 }
 
 /*
 ** Return the full pathname of the database file.
+**
+** Except, if the pager is in-memory only, then return an empty string if
+** nullIfMemDb is true.  This routine is called with nullIfMemDb==1 when
+** used to report the filename to the user, for compatibility with legacy
+** behavior.  But when the Btree needs to know the filename for matching to
+** shared cache, it uses nullIfMemDb==0 so that in-memory databases can
+** participate in shared-cache.
 */
-const char *sqlite3PagerFilename(Pager *pPager){
-  return pPager->zFilename;
+const char *sqlite3PagerFilename(Pager *pPager, int nullIfMemDb){
+  return (nullIfMemDb && pPager->memDb) ? "" : pPager->zFilename;
 }
 
 /*
 ** Return the VFS structure for the pager.
 */

Index: src/pager.h
==================================================================
--- src/pager.h
+++ src/pager.h
@@ -149,11 +149,11 @@
 
 /* Functions used to query pager state and configuration. */
 u8 sqlite3PagerIsreadonly(Pager*);
 int sqlite3PagerRefcount(Pager*);
 int sqlite3PagerMemUsed(Pager*);
-const char *sqlite3PagerFilename(Pager*);
+const char *sqlite3PagerFilename(Pager*, int);
 const sqlite3_vfs *sqlite3PagerVfs(Pager*);
 sqlite3_file *sqlite3PagerFile(Pager*);
 const char *sqlite3PagerJournalname(Pager*);
 int sqlite3PagerNosync(Pager*);
 void *sqlite3PagerTempSpace(Pager*);

Index: src/test_btree.c
==================================================================
--- src/test_btree.c
+++ src/test_btree.c
@@ -31,11 +31,11 @@
 #ifndef SQLITE_OMIT_SHARED_CACHE
   extern BtShared *sqlite3SharedCacheList;
   BtShared *pBt;
   Tcl_Obj *pRet = Tcl_NewObj();
   for(pBt=GLOBAL(BtShared*,sqlite3SharedCacheList); pBt; pBt=pBt->pNext){
-    const char *zFile = sqlite3PagerFilename(pBt->pPager);
+    const char *zFile = sqlite3PagerFilename(pBt->pPager, 1);
     Tcl_ListObjAppendElement(interp, pRet, Tcl_NewStringObj(zFile, -1));
     Tcl_ListObjAppendElement(interp, pRet, Tcl_NewIntObj(pBt->nRef));
   }
   Tcl_SetObjResult(interp, pRet);
 #endif

Index: src/vdbe.c
==================================================================
--- src/vdbe.c
+++ src/vdbe.c
@@ -5509,11 +5509,11 @@
   eOld = sqlite3PagerGetJournalMode(pPager);
   if( eNew==PAGER_JOURNALMODE_QUERY ) eNew = eOld;
   if( !sqlite3PagerOkToChangeJournalMode(pPager) ) eNew = eOld;
 
 #ifndef SQLITE_OMIT_WAL
-  zFilename = sqlite3PagerFilename(pPager);
+  zFilename = sqlite3PagerFilename(pPager, 1);
 
   /* Do not allow a transition to journal_mode=WAL for a database
   ** in temporary storage or if the VFS does not support shared memory 
   */
   if( eNew==PAGER_JOURNALMODE_WAL

Index: test/shared.test
==================================================================
--- test/shared.test
+++ test/shared.test
@@ -1054,9 +1054,49 @@
   execsql { DROP TABLE t1 } db2
 } {}
 db close
 db2 close
 
-}
+# Shared cache on a :memory: database.  This only works for URI filenames.
+#
+do_test shared-$av-16.1 {
+  sqlite3 db1 file::memory: -uri 1
+  sqlite3 db2 file::memory: -uri 1
+  db1 eval {
+    CREATE TABLE t1(x); INSERT INTO t1 VALUES(1),(2),(3);
+  }
+  db2 eval {
+    SELECT x FROM t1 ORDER BY x;
+  }
+} {1 2 3}
+do_test shared-$av-16.2 {
+  db2 eval {
+    INSERT INTO t1 VALUES(99);
+    DELETE FROM t1 WHERE x=2;
+  }
+  db1 eval {
+    SELECT x FROM t1 ORDER BY x;
+  }
+} {1 3 99}
+
+# Verify that there is no cache sharing ordinary (non-URI) filenames are
+# used.
+#
+do_test shared-$av-16.3 {
+  db1 close
+  db2 close
+  sqlite3 db1 :memory:
+  sqlite3 db2 :memory:
+  db1 eval {
+    CREATE TABLE t1(x); INSERT INTO t1 VALUES(4),(5),(6);
+  }
+  catchsql {
+    SELECT * FROM t1;
+  } db2
+} {1 {no such table: t1}}
+db1 close
+db2 close
+
+}  ;# end of autovacuum on/off loop
 
 sqlite3_enable_shared_cache $::enable_shared_cache
 finish_test