Experimental "PRAGMA data_version" command for detecting when another process

has changed the database file.

FossilOrigin-Name: 43db1f44bce5a0ee50197b95ab0d844540b69d86

src/pager.c

@@ -646,6 +646,7 @@ struct Pager {
   u8 setMaster;               /* True if a m-j name has been written to jrnl */
   u8 doNotSpill;              /* Do not spill the cache when non-zero */
   u8 subjInMemory;            /* True to use in-memory sub-journals */
+  u8 bUseFetch;               /* True to use xFetch() */
   Pgno dbSize;                /* Number of pages in the database */
   Pgno dbOrigSize;            /* dbSize before the current transaction */
   Pgno dbFileSize;            /* Number of pages in the database file */
@@ -663,9 +664,9 @@ struct Pager {
   sqlite3_backup *pBackup;    /* Pointer to list of ongoing backup processes */
   PagerSavepoint *aSavepoint; /* Array of active savepoints */
   int nSavepoint;             /* Number of elements in aSavepoint[] */
+  u32 nReset;                 /* Number of calls to pager_reset() */
   char dbFileVers[16];        /* Changes whenever database file changes */
 
-  u8 bUseFetch;               /* True to use xFetch() */
   int nMmapOut;               /* Number of mmap pages currently outstanding */
   sqlite3_int64 szMmap;       /* Desired maximum mmap size */
   PgHdr *pMmapFreelist;       /* List of free mmap page headers (pDirty) */
@@ -1681,10 +1682,19 @@ static int writeMasterJournal(Pager *pPager, const char *zMaster){
 ** Discard the entire contents of the in-memory page-cache.
 */
 static void pager_reset(Pager *pPager){
+  pPager->nReset++;
   sqlite3BackupRestart(pPager->pBackup);
   sqlite3PcacheClear(pPager->pPCache);
 }
 
+/*
+** Return the pPager->nReset value
+*/
+u32 sqlite3PagerDataVersion(Pager *pPager){
+  assert( pPager->eState>PAGER_OPEN );
+  return pPager->nReset;
+}
+
 /*
 ** Free all structures in the Pager.aSavepoint[] array and set both
 ** Pager.aSavepoint and Pager.nSavepoint to zero. Close the sub-journal