Instead of extra locks, use F_GETLK to ensure that readonly_shm clients cannot

connect to a wal-mode database if there are no writers.

FossilOrigin-Name: 5492f457dc7cc5c416de4b4e61e84bd2f10b4e6ce54011b7a60feb47f629c923

manifest

@@ -1,5 +1,5 @@
-C Use\sextra\slocks\sto\sprevent\sa\sreadonly_shm=1\sprocess\sfrom\sconnecting\sto\sa\nWAL-mode\sdatabase\sif\sthere\sare\sno\swriters.
+C Instead\sof\sextra\slocks,\suse\sF_GETLK\sto\sensure\sthat\sreadonly_shm\sclients\scannot\nconnect\sto\sa\swal-mode\sdatabase\sif\sthere\sare\sno\swriters.
-D 2017-10-25T23:28:13.771
+D 2017-10-26T17:05:22.656
 F Makefile.in e016061b23e60ac9ec27c65cb577292b6bde0307ca55abd874ab3487b3b1beb2
 F Makefile.linux-gcc 7bc79876b875010e8c8f9502eb935ca92aa3c434
 F Makefile.msc 37740aba9c4bb359c627eadccf1cfd7be4f5f847078723777ea7763969e533b1
@@ -447,7 +447,7 @@ F src/os.c 22d31db3ca5a96a408fbf1ceeaaebcaf64c87024d2ff9fe1cf2ddbec3e75c104
 F src/os.h 48388821692e87da174ea198bf96b1b2d9d83be5dfc908f673ee21fafbe0d432
 F src/os_common.h b2f4707a603e36811d9b1a13278bffd757857b85
 F src/os_setup.h 0dbaea40a7d36bf311613d31342e0b99e2536586
-F src/os_unix.c bec568f6f89e57b987f5ca38ae90087687d8e18669cd37eceb48ca6b56a8fbfb
+F src/os_unix.c 8103f60342c65d501b4e58b381796648d6584b4814ffee79cd3a6e0c12fb6545
 F src/os_win.c 6892c3ff23b7886577e47f13d827ca220c0831bae3ce00eea8c258352692f8c6
 F src/os_win.h 7b073010f1451abe501be30d12f6bc599824944a
 F src/pager.c 07cf850241667874fcce9d7d924c814305e499b26c804322e2261247b5921903
@@ -1666,10 +1666,7 @@ F vsixtest/vsixtest.tcl 6a9a6ab600c25a91a7acc6293828957a386a8a93
 F vsixtest/vsixtest.vcxproj.data 2ed517e100c66dc455b492e1a33350c1b20fbcdc
 F vsixtest/vsixtest.vcxproj.filters 37e51ffedcdb064aad6ff33b6148725226cd608e
 F vsixtest/vsixtest_TemporaryKey.pfx e5b1b036facdb453873e7084e1cae9102ccc67a0
-P d4f893e1ae53a0445939ea2920af87d21dd36270494381028b2eaebe5c188f18
+P 35d979082b4ab36d6a8975f8f15a50e69f46b72a173164d2b353377b9f758bd8
-R 7c17d54c5cff309a6f13f49dd620b485
+R 87551e9df4f45e3704d812374bd0d192
-T *branch * readonly-wal-recovery
+U dan
-T *sym-readonly-wal-recovery *
+Z a113fb74c5589c985f30184211e03e42
-T -sym-trunk *
-U drh
-Z 26f9af4fe6668c97b92039b43092c035

manifest.uuid

@@ -1 +1 @@
-35d979082b4ab36d6a8975f8f15a50e69f46b72a173164d2b353377b9f758bd8
+5492f457dc7cc5c416de4b4e61e84bd2f10b4e6ce54011b7a60feb47f629c923

src/os_unix.c

@@ -4145,7 +4145,6 @@ struct unixShm {
 */
 #define UNIX_SHM_BASE   ((22+SQLITE_SHM_NLOCK)*4)         /* first lock byte */
 #define UNIX_SHM_DMS    (UNIX_SHM_BASE+SQLITE_SHM_NLOCK)  /* deadman switch */
-#define UNIX_SHM_N_DMS  1000000000                        /* Size of the DMS */
 
 /*
 ** Apply posix advisory locks for all bytes from ofst through ofst+n-1.
@@ -4167,6 +4166,12 @@ static int unixShmSystemLock(
   pShmNode = pFile->pInode->pShmNode;
   assert( sqlite3_mutex_held(pShmNode->mutex) || pShmNode->nRef==0 );
 
+  /* Shared locks never span more than one byte */
+  assert( n==1 || lockType!=F_RDLCK );
+
+  /* Locks are within range */
+  assert( n>=1 && n<=SQLITE_SHM_NLOCK );
+
   if( pShmNode->h>=0 ){
     /* Initialize the locking parameters */
     memset(&f, 0, sizeof(f));
@@ -4181,15 +4186,7 @@ static int unixShmSystemLock(
 
   /* Update the global lock state and do debug tracing */
 #ifdef SQLITE_DEBUG
-  if( ofst<UNIX_SHM_DMS ){
+  { u16 mask;
-    u16 mask;
-
-    /* Shared locks never span more than one byte */
-    assert( n==1 || lockType!=F_RDLCK );
-
-    /* Locks are within range */
-    assert( n>=1 && n<=SQLITE_SHM_NLOCK );
-
   OSTRACE(("SHM-LOCK "));
   mask = ofst>31 ? 0xffff : (1<<(ofst+n)) - (1<<ofst);
   if( rc==SQLITE_OK ){
@@ -4392,38 +4389,30 @@ static int unixOpenSharedMemory(unixFile *pDbFd){
       */
       robustFchown(pShmNode->h, sStat.st_uid, sStat.st_gid);
   
-      /* Do not allow a read-only process to connect if there are no
+      /* Check to see if another process is holding the dead-man switch.
-      ** writers, because a read-only process is unable to recover the
+      ** For a readonly_shm client, if no other process holds the DMS lock,
-      ** shm file following a system crash.
+      ** the file cannot be opened and SQLITE_CANTOPEN_DIRTYWAL is returned.
-      */
+      ** Or, for a read-write connection, if no other process holds a
+      ** DMS lock the file is truncated to zero bytes in size.  */
       rc = SQLITE_OK;
       if( pShmNode->isReadonly ){
-        if( !unixShmSystemLock(pDbFd, F_RDLCK, UNIX_SHM_DMS, UNIX_SHM_N_DMS) ){
+        struct flock lock;
+        lock.l_whence = SEEK_SET;
+        lock.l_start = UNIX_SHM_DMS;
+        lock.l_len = 1;
+        lock.l_type = F_WRLCK;
+        if( osFcntl(pShmNode->h, F_GETLK, &lockInfo)!=0 ) {
+          rc = SQLITE_IOERR_LOCK;
+        }else if( lock.l_type==F_UNLCK ){
           rc = SQLITE_CANTOPEN_DIRTYWAL;
         }
-      }
+      }else if( unixShmSystemLock(pDbFd, F_WRLCK, UNIX_SHM_DMS, 1)==SQLITE_OK ){
-
-      /* If we are able to grab the dead-man switch, that means this is the
-      ** first (write-enable) process to connect to the database.  In that
-      ** case, truncate the shm file because the contents found on disk might
-      ** be invalid leftovers from a system crash.  The shm will be rebuilt
-      */
-      if( unixShmSystemLock(pDbFd, F_WRLCK, UNIX_SHM_DMS, 1)==SQLITE_OK ){
         if( robust_ftruncate(pShmNode->h, 0) ){
           rc = unixLogError(SQLITE_IOERR_SHMOPEN, "ftruncate", zShmFilename);
         }
       }
-
-      /* Acquires locks to tell other processes that a this process is
-      ** running and therefore the shm is valid they do not need to run
-      ** recovery.
-      */
       if( rc==SQLITE_OK ){
-        unsigned r;
         rc = unixShmSystemLock(pDbFd, F_RDLCK, UNIX_SHM_DMS, 1);
-        sqlite3_randomness(sizeof(r), &r);
-        r = 1 + (r%(UNIX_SHM_N_DMS-1));
-        (void)unixShmSystemLock(pDbFd, F_WRLCK, UNIX_SHM_DMS+r, 1);
       }
       if( rc ) goto shm_open_err;
     }