Put BtCursor objects on a singly-linked list instead of a doubly-linked list.

Add the BTCF_Multiple flag.  Only invoke saveAllCursors() when using a cursor
that has BTCF_Multiple set.

FossilOrigin-Name: 429ccef2b36fc46e92914eb54afd5f800b1a40ff

manifest

@@ -1,5 +1,5 @@
-C Convert\sseveral\s5-year-old\sand\solder\sNEVER()\smacros\sinto\sassert().
-D 2015-06-30T01:25:52.068
+C Put\sBtCursor\sobjects\son\sa\ssingly-linked\slist\sinstead\sof\sa\sdoubly-linked\slist.\nAdd\sthe\sBTCF_Multiple\sflag.\s\sOnly\sinvoke\ssaveAllCursors()\swhen\susing\sa\scursor\nthat\shas\sBTCF_Multiple\sset.
+D 2015-06-30T02:47:36.537
 F Makefile.arm-wince-mingw32ce-gcc d6df77f1f48d690bd73162294bbba7f59507c72f
 F Makefile.in 285a0a234ed7610d431d91671c136098c2bd86a9
 F Makefile.linux-gcc 91d710bdc4998cb015f39edf3cb314ec4f4d7e23
@@ -269,9 +269,9 @@ F src/auth.c b56c78ebe40a2110fd361379f7e8162d23f92240
 F src/backup.c ff743689c4d6c5cb55ad42ed9d174b2b3e71f1e3
 F src/bitvec.c d1f21d7d91690747881f03940584f4cc548c9d3d
 F src/btmutex.c 45a968cc85afed9b5e6cf55bf1f42f8d18107f79
-F src/btree.c 5d5d48461b3838988bd1f5b197c145df66786805
+F src/btree.c 08ae64b0387e72b8201981a2d7b0f58b92fdff29
 F src/btree.h 969adc948e89e449220ff0ff724c94bb2a52e9f1
-F src/btreeInt.h 8ca7124af9ee2ce27747a4e5500c27a254dea8eb
+F src/btreeInt.h e4eabc722b1ae017ac9c266a75769d4c6a9afde6
 F src/build.c b3f15255d5b16e42dafeaa638fd4f8a47c94ed70
 F src/callback.c 7b44ce59674338ad48b0e84e7b72f935ea4f68b0
 F src/complete.c a5cf5b4b56390cfb7b8636e8f7ddef90258dd575
@@ -1364,7 +1364,7 @@ F tool/vdbe_profile.tcl 67746953071a9f8f2f668b73fe899074e2c6d8c1
 F tool/warnings-clang.sh f6aa929dc20ef1f856af04a730772f59283631d4
 F tool/warnings.sh 48bd54594752d5be3337f12c72f28d2080cb630b
 F tool/win/sqlite.vsix deb315d026cc8400325c5863eef847784a219a2f
-P 53fe685d56368af78238c8981cc41704929f7660
-R 0ca8cb603f507b8c27049a9ebf015fe3
+P ed54c14ea8a72d69d69e0c0d7f6936f54efc04c2
+R f3f8b025f26bf8ce4de670023e83b4e0
 U drh
-Z e0e6efeb2c6c43faed874d1e7ab03448
+Z aa2cb7c94aea3021edb11ed06abf028f

manifest.uuid

@@ -1 +1 @@
-ed54c14ea8a72d69d69e0c0d7f6936f54efc04c2
+429ccef2b36fc46e92914eb54afd5f800b1a40ff

src/btree.c

@@ -655,6 +655,15 @@ static int SQLITE_NOINLINE saveCursorsOnList(BtCursor*,Pgno,BtCursor*);
 ** routine is called just before cursor pExcept is used to modify the
 ** table, for example in BtreeDelete() or BtreeInsert().
 **
+** If there are two or more cursors on the same btree, then all such 
+** cursors should have their BTCF_Multiple flag set.  The btreeCursor()
+** routine enforces that rule.  This routine only needs to be called in
+** the uncommon case when pExpect has the BTCF_Multiple flag set.
+**
+** If pExpect!=NULL and if no other cursors are found on the same root-page,
+** then the BTCF_Multiple flag on pExpect is cleared, to avoid another
+** pointless call to this routine.
+**
 ** Implementation note:  This routine merely checks to see if any cursors
 ** need to be saved.  It calls out to saveCursorsOnList() in the (unusual)
 ** event that cursors are in need to being saved.
@@ -666,7 +675,9 @@ static int saveAllCursors(BtShared *pBt, Pgno iRoot, BtCursor *pExcept){
   for(p=pBt->pCursor; p; p=p->pNext){
     if( p!=pExcept && (0==iRoot || p->pgnoRoot==iRoot) ) break;
   }
-  return p ? saveCursorsOnList(p, iRoot, pExcept) : SQLITE_OK;
+  if( p ) return saveCursorsOnList(p, iRoot, pExcept);
+  if( pExcept ) pExcept->curFlags &= ~BTCF_Multiple;
+  return SQLITE_OK;
 }
 
 /* This helper routine to saveAllCursors does the actual work of saving
@@ -4015,6 +4026,7 @@ static int btreeCursor(
   BtCursor *pCur                         /* Space for new cursor */
 ){
   BtShared *pBt = p->pBt;                /* Shared b-tree handle */
+  BtCursor *pX;                          /* Looping over other all cursors */
 
   assert( sqlite3BtreeHoldsMutex(p) );
   assert( wrFlag==0 || wrFlag==1 );
@@ -4051,10 +4063,15 @@ static int btreeCursor(
   assert( wrFlag==0 || wrFlag==BTCF_WriteFlag );
   pCur->curFlags = wrFlag;
   pCur->curPagerFlags = wrFlag ? 0 : PAGER_GET_READONLY;
-  pCur->pNext = pBt->pCursor;
-  if( pCur->pNext ){
-    pCur->pNext->pPrev = pCur;
+  /* If there are two or more cursors on the same btree, then all such
+  ** cursors *must* have the BTCF_Multiple flag set. */
+  for(pX=pBt->pCursor; pX; pX=pX->pNext){
+    if( pX->pgnoRoot==(Pgno)iTable ){
+      pX->curFlags |= BTCF_Multiple;
+      pCur->curFlags |= BTCF_Multiple;
+    }
   }
+  pCur->pNext = pBt->pCursor;
   pBt->pCursor = pCur;
   pCur->eState = CURSOR_INVALID;
   return SQLITE_OK;
@@ -4112,13 +4129,18 @@ int sqlite3BtreeCloseCursor(BtCursor *pCur){
     BtShared *pBt = pCur->pBt;
     sqlite3BtreeEnter(pBtree);
     sqlite3BtreeClearCursor(pCur);
-    if( pCur->pPrev ){
-      pCur->pPrev->pNext = pCur->pNext;
-    }else{
+    assert( pBt->pCursor!=0 );
+    if( pBt->pCursor==pCur ){
       pBt->pCursor = pCur->pNext;
-    }
-    if( pCur->pNext ){
-      pCur->pNext->pPrev = pCur->pPrev;
+    }else{
+      BtCursor *pPrev = pBt->pCursor;
+      do{
+        if( pPrev->pNext==pCur ){
+          pPrev->pNext = pCur->pNext;
+          break;
+        }
+        pPrev = pPrev->pNext;
+      }while( ALWAYS(pPrev) );
     }
     for(i=0; i<=pCur->iPage; i++){
       releasePage(pCur->apPage[i]);
@@ -7895,8 +7917,10 @@ int sqlite3BtreeInsert(
   ** doing any work. To avoid thwarting these optimizations, it is important
   ** not to clear the cursor here.
   */
-  rc = saveAllCursors(pBt, pCur->pgnoRoot, pCur);
-  if( rc ) return rc;
+  if( pCur->curFlags & BTCF_Multiple ){
+    rc = saveAllCursors(pBt, pCur->pgnoRoot, pCur);
+    if( rc ) return rc;
+  }
 
   if( pCur->pKeyInfo==0 ){
     assert( pKey==0 );
@@ -8042,8 +8066,10 @@ int sqlite3BtreeDelete(BtCursor *pCur){
   ** deleted writable. Then free any overflow pages associated with the 
   ** entry and finally remove the cell itself from within the page.  
   */
-  rc = saveAllCursors(pBt, pCur->pgnoRoot, pCur);
-  if( rc ) return rc;
+  if( pCur->curFlags & BTCF_Multiple ){
+    rc = saveAllCursors(pBt, pCur->pgnoRoot, pCur);
+    if( rc ) return rc;
+  }
 
   /* If this is a delete operation to remove a row from a table b-tree,
   ** invalidate any incrblob cursors open on the row being deleted.  */

src/btreeInt.h

@@ -507,7 +507,7 @@ struct CellInfo {
 struct BtCursor {
   Btree *pBtree;            /* The Btree to which this cursor belongs */
   BtShared *pBt;            /* The BtShared this cursor points to */
-  BtCursor *pNext, *pPrev;  /* Forms a linked list of all cursors */
+  BtCursor *pNext;          /* Forms a linked list of all cursors */
   Pgno *aOverflow;          /* Cache of overflow page locations */
   CellInfo info;            /* A parse of the cell we are pointing at */
   i64 nKey;                 /* Size of pKey, or last integer key */
@@ -526,6 +526,7 @@ struct BtCursor {
   i8 iPage;                 /* Index of current page in apPage */
   u8 curIntKey;             /* Value of apPage[0]->intKey */
   struct KeyInfo *pKeyInfo; /* Argument passed to comparison function */
+  void *padding1;           /* Make object size a multiple of 16 */
   u16 aiIdx[BTCURSOR_MAX_DEPTH];        /* Current index in apPage[i] */
   MemPage *apPage[BTCURSOR_MAX_DEPTH];  /* Pages from root to current page */
 };
@@ -538,6 +539,7 @@ struct BtCursor {
 #define BTCF_ValidOvfl    0x04   /* True if aOverflow is valid */
 #define BTCF_AtLast       0x08   /* Cursor is pointing ot the last entry */
 #define BTCF_Incrblob     0x10   /* True if an incremental I/O handle */
+#define BTCF_Multiple     0x20   /* Maybe another cursor on the same btree */
 
 /*
 ** Potential values for BtCursor.eState.