Modify the unixShmLock() function to avoid iterating through the (possibly large) set of connections to the same database file.

FossilOrigin-Name: e0faddf0dfc3a40b6b94408296dd781dd0264ecc9f2129ce4405438433fb00e0

manifest

@@ -1,5 +1,5 @@
-C Add\sfts5\stest\sto\sconfirm\sthat\sfor\sa\stable\swith\scolumns\sa,\sb,\sc\sand\sd,\s"{a\sb}"\sand\s"-{c\sd}"\sare\shandled\ssimilarly.
+C Modify\sthe\sunixShmLock()\sfunction\sto\savoid\siterating\sthrough\sthe\s(possibly\slarge)\sset\sof\sconnections\sto\sthe\ssame\sdatabase\sfile.
-D 2020-08-28T11:19:49.438
+D 2020-08-28T19:27:15.533
 F .fossil-settings/empty-dirs dbb81e8fc0401ac46a1491ab34a7f2c7c0452f2f06b54ebb845d024ca8283ef1
 F .fossil-settings/ignore-glob 35175cdfcf539b2318cb04a9901442804be81cd677d8b889fcc9149c21f239ea
 F LICENSE.md df5091916dbb40e6e9686186587125e1b2ff51f022cc334e886c19a0e9982724
@@ -519,7 +519,7 @@ F src/os.c 80e4cf3e5da06be03ca641661e331ce60eeeeabf0d7354dbb1c0e166d0eedbbe
 F src/os.h 48388821692e87da174ea198bf96b1b2d9d83be5dfc908f673ee21fafbe0d432
 F src/os_common.h b2f4707a603e36811d9b1a13278bffd757857b85
 F src/os_setup.h 0dbaea40a7d36bf311613d31342e0b99e2536586
-F src/os_unix.c 13553fb5ffbe8c0e60f5d7f553667560b7dece9e31cdfcf8b57b33092a11f226
+F src/os_unix.c 0120726d5ceb10f0a932dbcca3cd2c4c0f110c7f8eab3a788b34cc7accdad6cc
 F src/os_win.c a2149ff0a85c1c3f9cc102a46c673ce87e992396ba3411bfb53db66813b32f1d
 F src/os_win.h 7b073010f1451abe501be30d12f6bc599824944a
 F src/pager.c 3700a1c55427a3d4168ad1f1b8a8b0cb9ace1d107e4506e30a8f1e66d8a1195e
@@ -1879,7 +1879,10 @@ F vsixtest/vsixtest.tcl 6a9a6ab600c25a91a7acc6293828957a386a8a93
 F vsixtest/vsixtest.vcxproj.data 2ed517e100c66dc455b492e1a33350c1b20fbcdc
 F vsixtest/vsixtest.vcxproj.filters 37e51ffedcdb064aad6ff33b6148725226cd608e
 F vsixtest/vsixtest_TemporaryKey.pfx e5b1b036facdb453873e7084e1cae9102ccc67a0
-P 3f7bbb840de0a9b1ca89288805cb151aea6fcb82efda9ba39f51abf1b17c070b
+P 1a04920998368e56276fd0b100be8343609c6ff8a731cf8e26a0490f9c6dabdf
-R 68183e6ade81d2ecd369f241c0d2a0e4
+R 2ca214335aa99f522db85aff8974dca1
+T *branch * unixshmlock-opt
+T *sym-unixshmlock-opt *
+T -sym-trunk *
 U dan
-Z 8ead3559fc701c9e301c92555bb4b25a
+Z 2322c300ab82408a3c9b557087654b84

manifest.uuid

@@ -1 +1 @@
-1a04920998368e56276fd0b100be8343609c6ff8a731cf8e26a0490f9c6dabdf
+e0faddf0dfc3a40b6b94408296dd781dd0264ecc9f2129ce4405438433fb00e0

src/os_unix.c

@@ -4255,6 +4255,7 @@ struct unixShmNode {
   char **apRegion;           /* Array of mapped shared-memory regions */
   int nRef;                  /* Number of unixShm objects pointing to this */
   unixShm *pFirst;           /* All unixShm objects pointing to this */
+  int aLock[SQLITE_SHM_NLOCK];  /* # shared locks on slot, -1==excl lock */
 #ifdef SQLITE_DEBUG
   u8 exclMask;               /* Mask of exclusive locks held */
   u8 sharedMask;             /* Mask of shared locks held */
@@ -4795,6 +4796,38 @@ shmpage_out:
   return rc;
 }
 
+/*
+** Check that the pShmNode->aLock[] array comports with the locking bitmasks
+** held by each client. Return true if it does, or false otherwise. This
+** is to be used in an assert(). e.g.
+**
+**     assert( assertLockingArrayOk(pShmNode) );
+*/
+#ifdef SQLITE_DEBUG
+static int assertLockingArrayOk(unixShmNode *pShmNode){
+  unixShm *pX;
+  int aLock[SQLITE_SHM_NLOCK];
+  assert( sqlite3_mutex_held(pShmNode->pShmMutex) );
+
+  memset(aLock, 0, sizeof(aLock));
+  for(pX=pShmNode->pFirst; pX; pX=pX->pNext){
+    int i;
+    for(i=0; i<SQLITE_SHM_NLOCK; i++){
+      if( pX->exclMask & (1<<i) ){
+        assert( aLock[i]==0 );
+        aLock[i] = -1;
+      }else if( pX->sharedMask & (1<<i) ){
+        assert( aLock[i]>=0 );
+        aLock[i]++;
+      }
+    }
+  }
+
+  assert( 0==memcmp(pShmNode->aLock, aLock, sizeof(aLock)) );
+  return (memcmp(pShmNode->aLock, aLock, sizeof(aLock))==0);
+}
+#endif
+
 /*
 ** Change the lock state for a shared-memory segment.
 **
@@ -4811,10 +4844,10 @@ static int unixShmLock(
 ){
   unixFile *pDbFd = (unixFile*)fd;      /* Connection holding shared memory */
   unixShm *p = pDbFd->pShm;             /* The shared memory being locked */
-  unixShm *pX;                          /* For looping over all siblings */
   unixShmNode *pShmNode = p->pShmNode;  /* The underlying file iNode */
   int rc = SQLITE_OK;                   /* Result code */
   u16 mask;                             /* Mask of locks to take or release */
+  int *aLock = pShmNode->aLock;
 
   assert( pShmNode==pDbFd->pInode->pShmNode );
   assert( pShmNode->pInode==pDbFd->pInode );
@@ -4853,21 +4886,25 @@ static int unixShmLock(
   mask = (1<<(ofst+n)) - (1<<ofst);
   assert( n>1 || mask==(1<<ofst) );
   sqlite3_mutex_enter(pShmNode->pShmMutex);
+  assert( assertLockingArrayOk(pShmNode) );
   if( flags & SQLITE_SHM_UNLOCK ){
-    u16 allMask = 0; /* Mask of locks held by siblings */
+    int ii;
+    int bUnlock = 1;
 
-    /* See if any siblings hold this same lock */
+    for(ii=ofst; ii<ofst+n; ii++){
-    for(pX=pShmNode->pFirst; pX; pX=pX->pNext){
+      if( aLock[ii]>((p->sharedMask & (1<<ii)) ? 1 : 0) ){
-      if( pX==p ) continue;
+        bUnlock = 0;
-      assert( (pX->exclMask & (p->exclMask|p->sharedMask))==0 );
+      }
-      allMask |= pX->sharedMask;
     }
 
-    /* Unlock the system-level locks */
+    if( bUnlock ){
-    if( (mask & allMask)==0 ){
       rc = unixShmSystemLock(pDbFd, F_UNLCK, ofst+UNIX_SHM_BASE, n);
-    }else{
+      if( rc==SQLITE_OK ){
-      rc = SQLITE_OK;
+        memset(&aLock[ofst], 0, sizeof(int)*n);
+      }
+    }else if( p->sharedMask & (1<<ofst) ){
+      assert( n==1 && aLock[ofst]>1 );
+      aLock[ofst]--;
     }
 
     /* Undo the local locks */
@@ -4876,55 +4913,47 @@ static int unixShmLock(
       p->sharedMask &= ~mask;
     } 
   }else if( flags & SQLITE_SHM_SHARED ){
-    u16 allShared = 0;  /* Union of locks held by connections other than "p" */
+    assert( n==1 );
-
+    assert( (p->exclMask & (1<<ofst))==0 );
-    /* Find out which shared locks are already held by sibling connections.
+    if( (p->sharedMask & mask)==0 ){
-    ** If any sibling already holds an exclusive lock, go ahead and return
+      if( aLock[ofst]<0 ){
-    ** SQLITE_BUSY.
-    */
-    for(pX=pShmNode->pFirst; pX; pX=pX->pNext){
-      if( (pX->exclMask & mask)!=0 ){
         rc = SQLITE_BUSY;
-        break;
+      }else if( aLock[ofst]==0 ){
-      }
-      allShared |= pX->sharedMask;
-    }
-
-    /* Get shared locks at the system level, if necessary */
-    if( rc==SQLITE_OK ){
-      if( (allShared & mask)==0 ){
         rc = unixShmSystemLock(pDbFd, F_RDLCK, ofst+UNIX_SHM_BASE, n);
-      }else{
-        rc = SQLITE_OK;
       }
-    }
 
-    /* Get the local shared locks */
+      /* Get the local shared locks */
-    if( rc==SQLITE_OK ){
+      if( rc==SQLITE_OK ){
-      p->sharedMask |= mask;
+        p->sharedMask |= mask;
+        aLock[ofst]++;
+      }
     }
   }else{
     /* Make sure no sibling connections hold locks that will block this
-    ** lock.  If any do, return SQLITE_BUSY right away.
+    ** lock.  If any do, return SQLITE_BUSY right away.  */
-    */
+    int ii;
-    for(pX=pShmNode->pFirst; pX; pX=pX->pNext){
+    for(ii=ofst; ii<ofst+n; ii++){
-      if( (pX->exclMask & mask)!=0 || (pX->sharedMask & mask)!=0 ){
+      assert( (p->sharedMask & mask)==0 );
+      if( (p->exclMask & (1<<ii))==0 && aLock[ii] ){
         rc = SQLITE_BUSY;
         break;
       }
     }
-  
+
-    /* Get the exclusive locks at the system level.  Then if successful
+    /* Get the exclusive locks at the system level. Then if successful
-    ** also mark the local connection as being locked.
+    ** also update the in-memory values. */
-    */
     if( rc==SQLITE_OK ){
       rc = unixShmSystemLock(pDbFd, F_WRLCK, ofst+UNIX_SHM_BASE, n);
       if( rc==SQLITE_OK ){
         assert( (p->sharedMask & mask)==0 );
         p->exclMask |= mask;
+        for(ii=ofst; ii<ofst+n; ii++){
+          aLock[ii] = -1;
+        }
       }
     }
   }
+  assert( assertLockingArrayOk(pShmNode) );
   sqlite3_mutex_leave(pShmNode->pShmMutex);
   OSTRACE(("SHM-LOCK shmid-%d, pid-%d got %03x,%03x\n",
            p->id, osGetpid(0), p->sharedMask, p->exclMask));