diff --git a/manifest b/manifest
index 3d7f93a31e..755c31ef40 100644
--- a/manifest
+++ b/manifest
@@ -1,5 +1,5 @@
-C Merge\slatest\strunk\schanges\sinto\sthis\sbranch.
-D 2020-05-06T15:07:25.513
+C Add\stests\sfor\srunning\srecovery\swhen\sopening\sa\swrite\stransaction\sor\sperforming\sa\scheckpoint\swith\sblocking\slocks\senabled.\sFix\ssome\sfailing\sassert()\sstatements.
+D 2020-05-06T17:18:57.298
 F .fossil-settings/empty-dirs dbb81e8fc0401ac46a1491ab34a7f2c7c0452f2f06b54ebb845d024ca8283ef1
 F .fossil-settings/ignore-glob 35175cdfcf539b2318cb04a9901442804be81cd677d8b889fcc9149c21f239ea
 F LICENSE.md df5091916dbb40e6e9686186587125e1b2ff51f022cc334e886c19a0e9982724
@@ -517,7 +517,7 @@ F src/os.c 669cc3839cc35d20f81faf0be1ab6d4581cea35e9d8f3a9d48a98d6571f7c285
 F src/os.h 48388821692e87da174ea198bf96b1b2d9d83be5dfc908f673ee21fafbe0d432
 F src/os_common.h b2f4707a603e36811d9b1a13278bffd757857b85
 F src/os_setup.h 0dbaea40a7d36bf311613d31342e0b99e2536586
-F src/os_unix.c daabdb3c33ef4a6b49ff6809968af2f8c0d3c5811bc654d74375f015180235aa
+F src/os_unix.c 0e1acfbb8a75f2a4acede6078b072238df63981b172a9cd16f8b090d68291c15
 F src/os_win.c 035a813cbd17f355bdcad7ab894af214a9c13a1db8aeac902365350b98cd45a7
 F src/os_win.h 7b073010f1451abe501be30d12f6bc599824944a
 F src/pager.c be4d6d58df5c5d02b84547fd253a80f035ea2ee53931a5393d96af0ba3f04edb
@@ -617,7 +617,7 @@ F src/vdbetrace.c fa3bf238002f0bbbdfb66cc8afb0cea284ff9f148d6439bc1f6f2b4c3b7143
 F src/vdbevtab.c 8094dfc28dad82d60a1c832020a1b201a5381dc185c14638affc6d4e9d54c653
 F src/vtab.c 7b704a90515a239c6cdba6a66b1bb3a385e62326cceb5ecb05ec7a091d6b8515
 F src/vxworks.h d2988f4e5a61a4dfe82c6524dd3d6e4f2ce3cdb9
-F src/wal.c 4099109dcf2b302e0d151e6fa0678d9bbc3eb923b8bd13a3cd07db0e7df5016b
+F src/wal.c 9cf3f288cd4e48b49725e03037b60246a154ca8775892c75c319db8f5652ae8a
 F src/wal.h 484890eaaefb89c0f2043ae34c20bf367383f201f63da631ecae424797a7f60d
 F src/walker.c 7c429c694abd12413a5c17aec9f47cfe9eba6807e6b0a32df883e8e3a14835ed
 F src/where.c 9546c82056e8cdb27291f98cf1adca5d271240b399bb97b32f77fc2bea6146c9
@@ -1689,6 +1689,7 @@ F test/walprotocol2.test 7d3b6b4bf0b12f8007121b1e6ef714bc99101fb3b48e46371df1db8
 F test/walro.test cb438d05ba0d191f10b688e39c4f0cd5b71569a1d1f4440e5bdf3c6880e08c20
 F test/walro2.test 0e79dd15cbdb4f482c01ea248373669c732414a726b357d04846a816afafb768
 F test/walrofault.test c70cb6e308c443867701856cce92ad8288cd99488fa52afab77cca6cfd51af68
+F test/walsetlk.test 9da1ee8a18b0416a5ce1914331c1303bcb6cb7bac905eb660f788f023718d8f4
 F test/walshared.test 0befc811dcf0b287efae21612304d15576e35417
 F test/walslow.test c05c68d4dc2700a982f89133ce103a1a84cc285f
 F test/walthread.test 14b20fcfa6ae152f5d8e12f5dc8a8a724b7ef189f5d8ef1e2ceab79f2af51747
@@ -1863,7 +1864,7 @@ F vsixtest/vsixtest.tcl 6a9a6ab600c25a91a7acc6293828957a386a8a93
 F vsixtest/vsixtest.vcxproj.data 2ed517e100c66dc455b492e1a33350c1b20fbcdc
 F vsixtest/vsixtest.vcxproj.filters 37e51ffedcdb064aad6ff33b6148725226cd608e
 F vsixtest/vsixtest_TemporaryKey.pfx e5b1b036facdb453873e7084e1cae9102ccc67a0
-P d6f819a9e6b35f3fd558bd93255a6a24ad690a0fa15a82b009ca9c641db983c6 9a84d8eb44b12ddea921baa73ac3bffa8c8d23a852605dc1dbde37907360bd79
-R de537778f79587f163af3363949d5ea7
+P a3727dba10b476f414f0a049b35f2a5c169a408eccefa6833821560684e07f8b
+R e1d7b36a881e7c026e918bb1e2d9268d
 U dan
-Z 87b208fa9f7bcdf2aaff63753ffc5546
+Z 92e7a8a964d43aa5533a6dd765b70a70
diff --git a/manifest.uuid b/manifest.uuid
index 9593fe8777..78c3a449bb 100644
--- a/manifest.uuid
+++ b/manifest.uuid
@@ -1 +1 @@
-a3727dba10b476f414f0a049b35f2a5c169a408eccefa6833821560684e07f8b
\ No newline at end of file
+d096ea3fe20f8af90f9e3f798fd0a69527b32ac8e208ade58b9d0af0e831f41b
\ No newline at end of file
diff --git a/src/os_unix.c b/src/os_unix.c
index e1ed575b01..72c446ea77 100644
--- a/src/os_unix.c
+++ b/src/os_unix.c
@@ -4832,7 +4832,6 @@ static int unixShmLock(
 #ifdef SQLITE_ENABLE_SETLK_TIMEOUT
   assert( (flags & SQLITE_SHM_UNLOCK) || pDbFd->iBusyTimeout==0 || (
          (ofst!=2)                                   /* not RECOVER */
-      && (n==1)                                      /* Single lock only */
       && (ofst!=1 || (p->exclMask|p->sharedMask)==0)
       && (ofst!=0 || (p->exclMask|p->sharedMask)<3)
       && (ofst<3  || (p->exclMask|p->sharedMask)<(1<<ofst))
diff --git a/src/wal.c b/src/wal.c
index a70a716244..7abff64078 100644
--- a/src/wal.c
+++ b/src/wal.c
@@ -2175,28 +2175,34 @@ static int walIndexReadHdr(Wal *pWal, int *pChanged){
   /* If the first attempt failed, it might have been due to a race
   ** with a writer.  So get a WRITE lock and try again.
   */
-  assert( badHdr==0 || pWal->writeLock==0 );
   if( badHdr ){
     if( pWal->bShmUnreliable==0 && (pWal->readOnly & WAL_SHM_RDONLY) ){
       if( SQLITE_OK==(rc = walLockShared(pWal, WAL_WRITE_LOCK)) ){
         walUnlockShared(pWal, WAL_WRITE_LOCK);
         rc = SQLITE_READONLY_RECOVERY;
       }
-    }else if( SQLITE_OK==(rc = walLockExclusive(pWal, WAL_WRITE_LOCK, 1)) ){
-      pWal->writeLock = 1;
-      if( SQLITE_OK==(rc = walIndexPage(pWal, 0, &page0)) ){
-        badHdr = walIndexTryHdr(pWal, pChanged);
-        if( badHdr ){
-          /* If the wal-index header is still malformed even while holding
-          ** a WRITE lock, it can only mean that the header is corrupted and
-          ** needs to be reconstructed.  So run recovery to do exactly that.
-          */
-          rc = walIndexRecover(pWal);
-          *pChanged = 1;
+    }else{
+      int bWriteLock = pWal->writeLock;
+      if( bWriteLock 
+       || SQLITE_OK==(rc = walLockExclusive(pWal, WAL_WRITE_LOCK, 1)) 
+      ){
+        pWal->writeLock = 1;
+        if( SQLITE_OK==(rc = walIndexPage(pWal, 0, &page0)) ){
+          badHdr = walIndexTryHdr(pWal, pChanged);
+          if( badHdr ){
+            /* If the wal-index header is still malformed even while holding
+            ** a WRITE lock, it can only mean that the header is corrupted and
+            ** needs to be reconstructed.  So run recovery to do exactly that.
+            */
+            rc = walIndexRecover(pWal);
+            *pChanged = 1;
+          }
+        }
+        if( bWriteLock==0 ){
+          pWal->writeLock = 0;
+          walUnlockExclusive(pWal, WAL_WRITE_LOCK, 1);
         }
       }
-      pWal->writeLock = 0;
-      walUnlockExclusive(pWal, WAL_WRITE_LOCK, 1);
     }
   }
 
@@ -2808,6 +2814,7 @@ int sqlite3WalBeginReadTransaction(Wal *pWal, int *pChanged){
   int rc;                         /* Return code */
   int cnt = 0;                    /* Number of TryBeginRead attempts */
 
+  assert( pWal->ckptLock==0 );
 
 #ifdef SQLITE_ENABLE_SNAPSHOT
   int bChanged = 0;
@@ -2832,6 +2839,7 @@ int sqlite3WalBeginReadTransaction(Wal *pWal, int *pChanged){
     if( rc!=SQLITE_OK ){
       return rc;
     }
+    pWal->ckptLock = 1;
   }
 #endif
 
@@ -2895,8 +2903,10 @@ int sqlite3WalBeginReadTransaction(Wal *pWal, int *pChanged){
   }
 
   /* Release the shared CKPT lock obtained above. */
-  if( pSnapshot ){
+  if( pWal->ckptLock ){
+    assert( pSnapshot );
     walUnlockShared(pWal, WAL_CKPT_LOCK);
+    pWal->ckptLock = 0;
   }
 #endif
   return rc;
@@ -3698,7 +3708,9 @@ int sqlite3WalCheckpoint(
 
   /* Read the wal-index header. */
   if( rc==SQLITE_OK ){
+    walDisableBlocking(pWal);
     rc = walIndexReadHdr(pWal, &isChanged);
+    walEnableBlocking(db, pWal);
     if( isChanged && pWal->pDbFd->pMethods->iVersion>=3 ){
       sqlite3OsUnfetch(pWal->pDbFd, 0, 0);
     }
diff --git a/test/walsetlk.test b/test/walsetlk.test
new file mode 100644
index 0000000000..ff7ec18b3f
--- /dev/null
+++ b/test/walsetlk.test
@@ -0,0 +1,75 @@
+# 2020 May 06
+#
+# 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.
+#
+#***********************************************************************
+#
+
+set testdir [file dirname $argv0]
+source $testdir/tester.tcl
+set testprefix walsetlk
+
+ifcapable !wal {finish_test ; return }
+db timeout 1000
+
+#-------------------------------------------------------------------------
+# 1.*: Test that nothing goes wrong if recovery is forced while opening
+#      a write transaction or performing a checkpoint with blocking locks.
+#
+
+do_execsql_test 1.0 {
+  CREATE TABLE t1(x, y);
+  PRAGMA journal_mode = wal;
+  INSERT INTO t1 VALUES(1, 2);
+  INSERT INTO t1 VALUES(3, 4);
+  INSERT INTO t1 VALUES(5, 6);
+  INSERT INTO t1 VALUES(7, 8);
+} {wal}
+
+sqlite3 db2 test.db
+db2 timeout 1000
+
+do_execsql_test -db db2 1.1 {
+  SELECT * FROM t1
+} {1 2 3 4 5 6 7 8}
+
+set fd [open test.db-shm r+]
+puts $fd "blahblahblahblah"
+flush $fd
+
+do_execsql_test 1.2 {
+  BEGIN;
+    INSERT INTO t1 VALUES(9, 10);
+}
+
+do_execsql_test -db db2 1.3 {
+  SELECT * FROM t1
+} {1 2 3 4 5 6 7 8}
+
+do_test 1.3 {
+  list [catch {db2 eval { BEGIN EXCLUSIVE }} msg] $msg
+} {1 {database is locked}}
+
+do_execsql_test 1.4 { COMMIT }
+do_execsql_test -db db2 1.5 {
+  SELECT * FROM t1
+} {1 2 3 4 5 6 7 8 9 10}
+
+puts $fd "blahblahblahblah"
+flush $fd
+
+do_execsql_test -db db2 1.6 {
+  PRAGMA wal_checkpoint = TRUNCATE
+} {0 0 0}
+
+do_test 1.7 {
+  file size test.db-wal
+} 0
+
+finish_test
+