Hack to have multiple connections to a single file share a

single memory mapping of the databse file.

FossilOrigin-Name: ec37ad6d08362f4c9faad9b629c0fa23f5864ff6ad7f4cbed93a25d5f7b815d8

manifest

@@ -1,5 +1,5 @@
-C Fix\san\serror\sin\s[b22cdd67]\sthat\scan\scause\sa\snegative\sinfinity\sto\sbe\s(rarely)\nreported\sas\sa\spositive\sinfinity.
-D 2017-09-12T15:05:34.004
+C Hack\sto\shave\smultiple\sconnections\sto\sa\ssingle\sfile\sshare\sa\nsingle\smemory\smapping\sof\sthe\sdatabse\sfile.
+D 2017-09-12T18:03:12.014
 F Makefile.in c644bbe8ebe4aae82ad6783eae6b6beea4c727b99ff97568b847ced5e2ac7afb
 F Makefile.linux-gcc 7bc79876b875010e8c8f9502eb935ca92aa3c434
 F Makefile.msc 6a7a74bf60ad395098c0bd175ab054cd65ef85d7f034198d52bcc4d9e5fb4c6b
@@ -441,7 +441,7 @@ F src/os.c 93e0979b9b55df29c0c4923f73b48e9d3fe728f01dd8ed4f6a9d2f1d79779bc8
 F src/os.h 8e976e59eb4ca1c0fca6d35ee803e38951cb0343
 F src/os_common.h b2f4707a603e36811d9b1a13278bffd757857b85
 F src/os_setup.h 0dbaea40a7d36bf311613d31342e0b99e2536586
-F src/os_unix.c 489aa972ccc34f7b4770b891694b32101c59ddd4be4ef0ddd9a4da58c145c1a6
+F src/os_unix.c f36ad99b752d4abf1b8c1d1b8ea83fa6a5010c7a76cf6a0a76abb83641aa8948
 F src/os_win.c 964165b66cde03abc72fe948198b01be608436894732eadb94c8720d2467f223
 F src/os_win.h 7b073010f1451abe501be30d12f6bc599824944a
 F src/pager.c 967168bba88d2dc790ed9618bd4ba7bfe475b67b521ef6da305a6425c592928f
@@ -1653,7 +1653,10 @@ F vsixtest/vsixtest.tcl 6a9a6ab600c25a91a7acc6293828957a386a8a93
 F vsixtest/vsixtest.vcxproj.data 2ed517e100c66dc455b492e1a33350c1b20fbcdc
 F vsixtest/vsixtest.vcxproj.filters 37e51ffedcdb064aad6ff33b6148725226cd608e
 F vsixtest/vsixtest_TemporaryKey.pfx e5b1b036facdb453873e7084e1cae9102ccc67a0
-P b22cdd6734ecda2b2b9749668f353abc2660f192d6a40c5d266309d30e25695e
-R 39b328d722a14e00d87cd1b0274ba2b6
-U drh
-Z 7a920600be1c7fd6aaa5dec418f9fad6
+P 9780b23ca375de6a542516fbc03eb39d5a393ca577718fda231d0d0ccf3b1c7e
+R 4c111361f04c7540b8ad9592d8f1048c
+T *branch * shared-mapping-hack
+T *sym-shared-mapping-hack *
+T -sym-trunk *
+U dan
+Z 4c10a02d9e6c902f708178ec77270bb4

manifest.uuid

@@ -1 +1 @@
-9780b23ca375de6a542516fbc03eb39d5a393ca577718fda231d0d0ccf3b1c7e
+ec37ad6d08362f4c9faad9b629c0fa23f5864ff6ad7f4cbed93a25d5f7b815d8

src/os_unix.c

@@ -46,6 +46,8 @@
 #include "sqliteInt.h"
 #if SQLITE_OS_UNIX              /* This file is used on unix only */
 
+#define SQLITE_SHARED_MAPPING 1
+
 /*
 ** There are various methods for file locking used for concurrency
 ** control:
@@ -1115,6 +1117,10 @@ struct unixInodeInfo {
   sem_t *pSem;                    /* Named POSIX semaphore */
   char aSemName[MAX_PATHNAME+2];  /* Name of that semaphore */
 #endif
+#ifdef SQLITE_SHARED_MAPPING
+  sqlite3_int64 nSharedMapping;   /* Size of mapped region in bytes */
+  void *pSharedMapping;           /* Memory mapped region */
+#endif
 };
 
 /*
@@ -1249,6 +1255,13 @@ static void releaseInodeInfo(unixFile *pFile){
     pInode->nRef--;
     if( pInode->nRef==0 ){
       assert( pInode->pShmNode==0 );
+#ifdef SQLITE_SHARED_MAPPING
+      if( pInode->pSharedMapping ){
+        osMunmap(pInode->pSharedMapping, pInode->nSharedMapping);
+        pInode->pSharedMapping = 0;
+        pInode->nSharedMapping = 0;
+      }
+#endif
       closePendingFds(pFile);
       if( pInode->pPrev ){
         assert( pInode->pPrev->pNext==pInode );
@@ -2055,6 +2068,14 @@ static int nolockUnlock(sqlite3_file *NotUsed, int NotUsed2){
 ** Close the file.
 */
 static int nolockClose(sqlite3_file *id) {
+#ifdef SQLITE_SHARED_MAPPING
+  unixFile *pFd = (unixFile*)id;
+  if( pFd->pInode ){
+    unixEnterMutex();
+    releaseInodeInfo(pFd);
+    unixLeaveMutex();
+  }
+#endif
   return closeUnixFile(id);
 }
 
@@ -3874,6 +3895,9 @@ static int unixFileControl(sqlite3_file *id, int op, void *pArg){
       *(i64*)pArg = pFile->mmapSizeMax;
       if( newLimit>=0 && newLimit!=pFile->mmapSizeMax && pFile->nFetchOut==0 ){
         pFile->mmapSizeMax = newLimit;
+#ifdef SQLITE_SHARED_MAPPING
+        if( pFile->pInode==0 )
+#endif
         if( pFile->mmapSize>0 ){
           unixUnmapfile(pFile);
           rc = unixMapfile(pFile, -1);
@@ -4774,6 +4798,9 @@ static int unixShmUnmap(
 */
 static void unixUnmapfile(unixFile *pFd){
   assert( pFd->nFetchOut==0 );
+#ifdef SQLITE_SHARED_MAPPING
+  if( pFd->pInode ) return;
+#endif
   if( pFd->pMapRegion ){
     osMunmap(pFd->pMapRegion, pFd->mmapSizeActual);
     pFd->pMapRegion = 0;
@@ -4905,6 +4932,28 @@ static int unixMapfile(unixFile *pFd, i64 nMap){
     nMap = pFd->mmapSizeMax;
   }
 
+#ifdef SQLITE_SHARED_MAPPING
+  if( pFd->pInode ){
+    unixInodeInfo *pInode = pFd->pInode;
+    if( pFd->pMapRegion ) return SQLITE_OK;
+    unixEnterMutex();
+    if( pInode->pSharedMapping==0 ){
+      u8 *pNew = osMmap(0, nMap, PROT_READ, MAP_SHARED, pFd->h, 0);
+      if( pNew==MAP_FAILED ){
+        unixLogError(SQLITE_OK, "mmap", pFd->zPath);
+        pFd->mmapSizeMax = 0;
+      }else{
+        pInode->pSharedMapping = pNew;
+        pInode->nSharedMapping = nMap;
+      }
+    }
+    pFd->pMapRegion = pInode->pSharedMapping;
+    pFd->mmapSizeActual = pFd->mmapSize = pInode->nSharedMapping;
+    unixLeaveMutex();
+    return SQLITE_OK;
+  }
+#endif
+
   assert( nMap>0 || (pFd->mmapSize==0 && pFd->pMapRegion==0) );
   if( nMap!=pFd->mmapSize ){
     unixRemapfile(pFd, nMap);
@@ -5353,6 +5402,9 @@ static int fillInUnixFile(
   if( pLockingStyle == &posixIoMethods
 #if defined(__APPLE__) && SQLITE_ENABLE_LOCKING_STYLE
     || pLockingStyle == &nfsIoMethods
+#endif
+#ifdef SQLITE_SHARED_MAPPING
+    || pLockingStyle == &nolockIoMethods
 #endif
   ){
     unixEnterMutex();