Avoid unnecessary page cache allocations when move a page while autovacuuming

an in-memory database, since the allocation might fail making it impossible to
rollback the transaction.

FossilOrigin-Name: 9a429349ccc2fa9acd28365a86578f602e87dafb

manifest

@@ -1,5 +1,8 @@
-C Improve\scomments\sand\sother\sthings\sin\sfts3_write.c.
-D 2009-11-20T05:05:19
+-----BEGIN PGP SIGNED MESSAGE-----
+Hash: SHA1
+
+C Avoid\sunnecessary\spage\scache\sallocations\swhen\smove\sa\spage\swhile\sautovacuuming\s\nan\sin-memory\sdatabase,\ssince\sthe\sallocation\smight\sfail\smaking\sit\simpossible\sto\nrollback\sthe\stransaction.
+D 2009-11-20T13:18:14
 F Makefile.arm-wince-mingw32ce-gcc fcd5e9cd67fe88836360bb4f9ef4cb7f8e2fb5a0
 F Makefile.in 7f6c6aa7feeeb5e26e01b344161d9aa1b5d64177
 F Makefile.linux-gcc d53183f4aa6a9192d249731c90dbdffbd2c68654
@@ -150,12 +153,12 @@ F src/os_common.h 240c88b163b02c21a9f21f87d49678a0aa21ff30
 F src/os_os2.c 75a8c7b9a00a2cf1a65f9fa4afbc27d46634bb2f
 F src/os_unix.c bdd6ca0932dcb51c344081aff430bcc71c14db7f
 F src/os_win.c 5ffab20249a61e0625f869efe157fa009747039b
-F src/pager.c 47343e340df799199fd1f1a184f0df8861519e1e
+F src/pager.c 4adc8baf93d50aa8e567173d27a37ff103106dab
 F src/pager.h 1b32faf2e578ac3e7bcf9c9d11217128261c5c54
 F src/parse.y f785d814562a14dc19202f61abb4372845f64752
 F src/pcache.c 3b079306376e0e04c0d3df40c0a4b750a1839310
 F src/pcache.h c683390d50f856d4cd8e24342ae62027d1bb6050
-F src/pcache1.c db4a996e456eec2380732a1036e23e3f551f6573
+F src/pcache1.c 2bb2261190b42a348038f5b1c285c8cef415fcc8
 F src/pragma.c 6936d7df5e04b9f996f8f320d15e65b6944b2caa
 F src/prepare.c ad90970bba3aead154266d8bb6faf9fbb5233b94
 F src/printf.c 03fabdd6112a0e23f78f8ac9a1396947ade0273b
@@ -772,7 +775,14 @@ F tool/speedtest2.tcl ee2149167303ba8e95af97873c575c3e0fab58ff
 F tool/speedtest8.c 2902c46588c40b55661e471d7a86e4dd71a18224
 F tool/speedtest8inst1.c 293327bc76823f473684d589a8160bde1f52c14e
 F tool/vdbe-compress.tcl d70ea6d8a19e3571d7ab8c9b75cba86d1173ff0f
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manifest.uuid

@@ -1 +1 @@
-1cf0e3cc14bad22867e740736c2886dc1c4a48dc
+9a429349ccc2fa9acd28365a86578f602e87dafb

src/pager.c

@@ -5154,7 +5154,14 @@ int sqlite3PagerMovepage(Pager *pPager, DbPage *pPg, Pgno pgno, int isCommit){
   assert( !pPgOld || pPgOld->nRef==1 );
   if( pPgOld ){
     pPg->flags |= (pPgOld->flags&PGHDR_NEED_SYNC);
-    sqlite3PcacheDrop(pPgOld);
+    if( MEMDB ){
+      /* Do not discard pages from an in-memory database since we might
+      ** need to rollback later.  Just move the page out of the way. */
+      assert( pPager->dbSizeValid );
+      sqlite3PcacheMove(pPgOld, pPager->dbSize+1);
+    }else{
+      sqlite3PcacheDrop(pPgOld);
+    }
   }
 
   origPgno = pPg->pgno;
@@ -5199,18 +5206,12 @@ int sqlite3PagerMovepage(Pager *pPager, DbPage *pPg, Pgno pgno, int isCommit){
 
   /*
   ** For an in-memory database, make sure the original page continues
-  ** to exist, in case the transaction needs to roll back.  We allocate
-  ** the page now, instead of at rollback, because we can better deal
-  ** with an out-of-memory error now.  Ticket #3761.
+  ** to exist, in case the transaction needs to roll back.  Use pPgOld
+  ** as the original page since it has already been allocated.
   */
   if( MEMDB ){
-    DbPage *pNew;
-    rc = sqlite3PagerAcquire(pPager, origPgno, &pNew, 1);
-    if( rc!=SQLITE_OK ){
-      sqlite3PcacheMove(pPg, origPgno);
-      return rc;
-    }
-    sqlite3PagerUnref(pNew);
+    sqlite3PcacheMove(pPgOld, origPgno);
+    sqlite3PagerUnref(pPgOld);
   }
 
   return SQLITE_OK;

src/pcache1.c

@@ -665,15 +665,7 @@ static void pcache1Rekey(
   pPage->iKey = iNew;
   pPage->pNext = pCache->apHash[h];
   pCache->apHash[h] = pPage;
-
-  /* The xRekey() interface is only used to move pages earlier in the
-  ** database file (in order to move all free pages to the end of the
-  ** file where they can be truncated off.)  Hence, it is not possible
-  ** for the new page number to be greater than the largest previously
-  ** fetched page.  But we retain the following test in case xRekey()
-  ** begins to be used in different ways in the future.
-  */
-  if( NEVER(iNew>pCache->iMaxKey) ){
+  if( iNew>pCache->iMaxKey ){
     pCache->iMaxKey = iNew;
   }