Index: src/vdbeapi.c
==================================================================
--- src/vdbeapi.c
+++ src/vdbeapi.c
@@ -398,11 +398,14 @@
 #ifndef SQLITE_OMIT_WAL
   int i;
   for(i=0; i<db->nDb; i++){
     Btree *pBt = db->aDb[i].pBt;
     if( pBt ){
-      int nEntry = sqlite3PagerWalCallback(sqlite3BtreePager(pBt));
+      int nEntry;
+      sqlite3BtreeEnter(pBt);
+      nEntry = sqlite3PagerWalCallback(sqlite3BtreePager(pBt));
+      sqlite3BtreeLeave(pBt);
       if( db->xWalCallback && nEntry>0 && rc==SQLITE_OK ){
         rc = db->xWalCallback(db->pWalArg, db, db->aDb[i].zName, nEntry);
       }
     }
   }

Index: test/threadtest4.c
==================================================================
--- test/threadtest4.c
+++ test/threadtest4.c
@@ -14,17 +14,21 @@
 ** from multiple threads in shared-cache mode.
 **
 ** This test program runs on unix-like systems only.  It uses pthreads.
 ** To compile:
 **
-**     gcc -o tt4 -I. threadtest4.c sqlite3.c -ldl -lpthread
+**     gcc -g -Wall -I. threadtest4.c sqlite3.c -ldl -lpthread
 **
 ** To run:
 **
-**     ./tt4 10
+**     ./a.out 10
+**
+** The argument is the number of threads.  There are also options, such
+** as -wal and -multithread and -serialized.
 **
-** The argument is the number of threads.
+** Consider also compiling with clang instead of gcc and adding the
+** -fsanitize=thread option.
 */
 #include "sqlite3.h"
 #include <pthread.h>
 #include <sched.h>
 #include <stdio.h>
@@ -38,10 +42,11 @@
 ** thread.
 */
 typedef struct WorkerInfo WorkerInfo;
 struct WorkerInfo {
   int tid;                    /* Thread ID */
+  int nWorker;                /* Total number of workers */
   unsigned wkrFlags;          /* Flags */
   sqlite3 *mainDb;            /* Database connection of the main thread */
   sqlite3 *db;                /* Database connection of this thread */
   int nErr;                   /* Number of errors seen by this thread */
   int nTest;                  /* Number of tests run by this thread */
@@ -282,11 +287,11 @@
   int i;
   int rc;
   sqlite3_stmt *pStmt;
 
   printf("worker %d startup\n", p->tid);  fflush(stdout);
-  for(iOuter=1; iOuter<=4; iOuter++){
+  for(iOuter=1; iOuter<=p->nWorker; iOuter++){
     worker_open_connection(p, iOuter);
     for(i=0; i<4; i++){
       worker_add_content(p, i*100+1, (i+1)*100, (p->tid+iOuter)%3 + 1);
       worker_add_content(p, i*100+1, (i+1)*100, (p->tid+iOuter+1)%3 + 1);
       worker_add_content(p, i*100+1, (i+1)*100, (p->tid+iOuter+2)%3 + 1);
@@ -300,10 +305,21 @@
     }else if( sqlite3_column_int(pStmt, 0)!=400 ){
       worker_error(p, "Wrong result: %d", sqlite3_column_int(pStmt,0));
     }
     if( p->nErr ) break;
     sqlite3_finalize(pStmt);
+
+    if( ((iOuter+p->tid)%3)==0 ){
+      sqlite3_db_release_memory(p->db);
+      p->nTest++;
+    }
+
+    if( iOuter==p->tid ){
+      pthread_mutex_lock(p->pWrMutex);
+      run_sql(p, "VACUUM");
+      pthread_mutex_unlock(p->pWrMutex);
+    }
 
     worker_delete_all_content(p, (p->tid+iOuter)%2);
     worker_close_connection(p);
     p->db = 0;
   }
@@ -360,10 +376,12 @@
        "usage:  %s ?OPTIONS? N\n"
        "N is the number of threads and must be at least 2.\n"
        "Options:\n"
        "  --serialized\n"
        "  --multithread\n"
+       "  --wal\n"
+       "  --trace\n"
        ,argv[0]
     );
     exit(1);
   }
   if( !sqlite3_threadsafe() ){
@@ -404,10 +422,11 @@
   run_sql(p, "CREATE INDEX test3.t3xy ON t3(x,y)");
   aInfo = safe_malloc( sizeof(*aInfo)*nWorker );
   memset(aInfo, 0, sizeof(*aInfo)*nWorker);
   for(i=0; i<nWorker; i++){
     aInfo[i].tid = i+1;
+    aInfo[i].nWorker = nWorker;
     aInfo[i].wkrFlags = wkrFlags;
     aInfo[i].mainDb = db;
     aInfo[i].pWrMutex = &wrMutex;
     rc = pthread_create(&aInfo[i].id, 0, worker_thread, &aInfo[i]);
     if( rc!=0 ){