Fix a heap-corruption causing race condition in os_unix.c that could occur

when one thread wal opening a database file while another is unlocking the
same file.

FossilOrigin-Name: 2447e0fd9830b4de9feeb178a0bff67a9065e3072120d4453bcdd3950d681adb

manifest

@@ -1,5 +1,5 @@
-C Fix\san\sincorrect\scomment\son\sthe\sunix-nolock\sVFS\sobject.\s\sNo\sfunctional\ncode\schanges.
-D 2018-08-13T11:32:07.332
+C Fix\sa\sheap-corruption\scausing\srace\scondition\sin\sos_unix.c\sthat\scould\soccur\nwhen\sone\sthread\swal\sopening\sa\sdatabase\sfile\swhile\sanother\sis\sunlocking\sthe\nsame\sfile.
+D 2018-08-13T12:58:48.124
 F .fossil-settings/empty-dirs dbb81e8fc0401ac46a1491ab34a7f2c7c0452f2f06b54ebb845d024ca8283ef1
 F .fossil-settings/ignore-glob 35175cdfcf539b2318cb04a9901442804be81cd677d8b889fcc9149c21f239ea
 F Makefile.in 0a3a6c81e6fcb969ff9106e882f0a08547014ba463cb6beca4c4efaecc924ee6
@@ -482,7 +482,7 @@ F src/os.c 8aeb0b0f40f8f5b0da03fe49706695adaf42d2f516ab95abc72e86c245e119de
 F src/os.h 48388821692e87da174ea198bf96b1b2d9d83be5dfc908f673ee21fafbe0d432
 F src/os_common.h b2f4707a603e36811d9b1a13278bffd757857b85
 F src/os_setup.h 0dbaea40a7d36bf311613d31342e0b99e2536586
-F src/os_unix.c 4723f4d963c032b5162dd6de79c314c2f67bccfeea2e458aaefa0f3049877f2e
+F src/os_unix.c c8487c06ce047779d992d2f9147a372049b697a15b4fb9bbce16fab82fe78b2c
 F src/os_win.c 070cdbb400097c6cda54aa005356095afdc2f3ee691d17192c54724ef146a971
 F src/os_win.h 7b073010f1451abe501be30d12f6bc599824944a
 F src/pager.c 76d29b8a960dcb8b67210f095899d91e4a90673a6674ea58cfd1115b705a7fb9
@@ -1754,7 +1754,7 @@ F vsixtest/vsixtest.tcl 6a9a6ab600c25a91a7acc6293828957a386a8a93
 F vsixtest/vsixtest.vcxproj.data 2ed517e100c66dc455b492e1a33350c1b20fbcdc
 F vsixtest/vsixtest.vcxproj.filters 37e51ffedcdb064aad6ff33b6148725226cd608e
 F vsixtest/vsixtest_TemporaryKey.pfx e5b1b036facdb453873e7084e1cae9102ccc67a0
-P 4195a3f8b5d2c2ec63771890c5aa7b5e2de60b9fa2273652730239b8577ae418
-R dc8601ab971a0e9f35b0830adc99e5ec
-U drh
-Z 8549fb1e42f4bc7f9d102f40d2b9a827
+P 90f7c193b42f0d8120a8e429bdea5e1cec5d3f45b901db8fc5a5c2ca3e69cba8
+R c32540ea6b445bf71ef28da5133ef44f
+U dan
+Z e5c617af6463d8e8b140183e45df28af

manifest.uuid

@@ -1 +1 @@
-90f7c193b42f0d8120a8e429bdea5e1cec5d3f45b901db8fc5a5c2ca3e69cba8
+2447e0fd9830b4de9feeb178a0bff67a9065e3072120d4453bcdd3950d681adb

src/os_unix.c

@@ -714,6 +714,9 @@ static void unixLeaveMutex(void){
 static int unixMutexHeld(void) {
   return sqlite3_mutex_held(unixBigLock);
 }
+static int unixMutexNotheld(void) {
+  return sqlite3_mutex_notheld(unixBigLock);
+}
 #endif
 
 
@@ -1249,7 +1252,7 @@ static void storeLastErrno(unixFile *pFile, int error){
 /*
 ** Close all file descriptors accumuated in the unixInodeInfo->pUnused list.
 */ 
-static void closePendingFds(unixFile *pFile){
+static void closePendingFdsUnsafe(unixFile *pFile){
   unixInodeInfo *pInode = pFile->pInode;
   UnixUnusedFd *p;
   UnixUnusedFd *pNext;
@@ -1262,6 +1265,12 @@ static void closePendingFds(unixFile *pFile){
   pInode->pUnused = 0;
 }
 
+static void closePendingFds(unixFile *pFile){
+  unixEnterMutex();
+  closePendingFdsUnsafe(pFile);
+  unixLeaveMutex();
+}
+
 /*
 ** Release a unixInodeInfo structure previously allocated by findInodeInfo().
 **
@@ -1275,7 +1284,7 @@ static void releaseInodeInfo(unixFile *pFile){
     pInode->nRef--;
     if( pInode->nRef==0 ){
       assert( pInode->pShmNode==0 );
-      closePendingFds(pFile);
+      closePendingFdsUnsafe(pFile);
       if( pInode->pPrev ){
         assert( pInode->pPrev->pNext==pInode );
         pInode->pPrev->pNext = pInode->pNext;
@@ -1458,6 +1467,7 @@ static int unixCheckReservedLock(sqlite3_file *id, int *pResOut){
 
   assert( pFile );
   assert( pFile->eFileLock<=SHARED_LOCK );
+  assert( unixMutexNotheld() );
   sqlite3_mutex_enter(pFile->pInode->pLockMutex);
 
   /* Check if a thread in this process holds such a lock */
@@ -1670,6 +1680,7 @@ static int unixLock(sqlite3_file *id, int eFileLock){
   /* This mutex is needed because pFile->pInode is shared across threads
   */
   pInode = pFile->pInode;
+  assert( unixMutexNotheld() );
   sqlite3_mutex_enter(pInode->pLockMutex);
 
   /* If some thread using this PID has a lock via a different unixFile*
@@ -1862,6 +1873,7 @@ static int posixUnlock(sqlite3_file *id, int eFileLock, int handleNFSUnlock){
     return SQLITE_OK;
   }
   pInode = pFile->pInode;
+  assert( unixMutexNotheld() );
   sqlite3_mutex_enter(pInode->pLockMutex);
   assert( pInode->nShared!=0 );
   if( pFile->eFileLock>SHARED_LOCK ){
@@ -2793,6 +2805,7 @@ static int afpCheckReservedLock(sqlite3_file *id, int *pResOut){
     *pResOut = 1;
     return SQLITE_OK;
   }
+  assert( unixMutexNotheld() );
   sqlite3_mutex_enter(pFile->pInode->pLockMutex);
   /* Check if a thread in this process holds such a lock */
   if( pFile->pInode->eFileLock>SHARED_LOCK ){
@@ -2881,6 +2894,7 @@ static int afpLock(sqlite3_file *id, int eFileLock){
   /* This mutex is needed because pFile->pInode is shared across threads
   */
   pInode = pFile->pInode;
+  assert( unixMutexNotheld() );
   sqlite3_mutex_enter(pInode->pLockMutex);
 
   /* If some thread using this PID has a lock via a different unixFile*
@@ -3050,6 +3064,7 @@ static int afpUnlock(sqlite3_file *id, int eFileLock) {
     return SQLITE_OK;
   }
   pInode = pFile->pInode;
+  assert( unixMutexNotheld() );
   sqlite3_mutex_enter(pInode->pLockMutex);
   assert( pInode->nShared!=0 );
   if( pFile->eFileLock>SHARED_LOCK ){