Add the BTREE_FORDELETE and BTREE_AUXDELETE flags to the b-tree layer interface

and use them.  Add assert() statement to verify that they are correct.

FossilOrigin-Name: 85c467041c9378cae3038756da815e9117ee8c7d

1 files changed, 33 insertions, 13 deletions

diff --git a/src/btree.c b/src/btree.c
index 481418a3d..b08367cbd 100644
--- a/src/btree.c
+++ b/src/btree.c
@@ -4049,13 +4049,13 @@ int sqlite3BtreeSavepoint(Btree *p, int op, int iSavepoint){
 ** on the database already. If a write-cursor is requested, then
 ** the caller is assumed to have an open write transaction.
 **
-** If wrFlag==0, then the cursor can only be used for reading.
-** If wrFlag==1, then the cursor can be used for reading or for
-** writing if other conditions for writing are also met.  These
-** are the conditions that must be met in order for writing to
-** be allowed:
+** If the BTREE_WRCSR bit of wrFlag is clear, then the cursor can only
+** be used for reading.  If the BTREE_WRCSR bit is set, then the cursor
+** can be used for reading or for writing if other conditions for writing
+** are also met.  These are the conditions that must be met in order
+** for writing to be allowed:
 **
-** 1:  The cursor must have been opened with wrFlag==1
+** 1:  The cursor must have been opened with wrFlag containing BTREE_WRCSR
 **
 ** 2:  Other database connections that share the same pager cache
 **     but which are not in the READ_UNCOMMITTED state may not have
@@ -4067,6 +4067,16 @@ int sqlite3BtreeSavepoint(Btree *p, int op, int iSavepoint){
 **
 ** 4:  There must be an active transaction.
 **
+** The BTREE_FORDELETE bit of wrFlag may optionally be set if BTREE_WRCSR
+** is set.  If FORDELETE is set, that is a hint to the implementation that
+** this cursor will only be used to seek to and delete entries of an index
+** as part of a larger DELETE statement.  The FORDELETE hint is not used by
+** this implementation.  But in a hypothetical alternative storage engine 
+** in which index entries are automatically deleted when corresponding table
+** rows are deleted, the FORDELETE flag is a hint that all SEEK and DELETE
+** operations on this cursor can be no-ops and all READ operations can 
+** return a null row (2-bytes: 0x01 0x00).
+**
 ** No checking is done to make sure that page iTable really is the
 ** root page of a b-tree.  If it is not, then the cursor acquired
 ** will not work correctly.
@@ -8082,13 +8092,21 @@ end_insert:
 /*
 ** Delete the entry that the cursor is pointing to. 
 **
-** If the second parameter is zero, then the cursor is left pointing at an
-** arbitrary location after the delete. If it is non-zero, then the cursor 
-** is left in a state such that the next call to BtreeNext() or BtreePrev()
-** moves it to the same row as it would if the call to BtreeDelete() had
-** been omitted.
-*/
-int sqlite3BtreeDelete(BtCursor *pCur, int bPreserve){
+** If the BTREE_SAVEPOSITION bit of the flags parameter is zero, then
+** the cursor is left pointing at an arbitrary location after the delete.
+** But if that bit is set, then the cursor is left in a state such that
+** the next call to BtreeNext() or BtreePrev() moves it to the same row
+** as it would have been on if the call to BtreeDelete() had been omitted.
+**
+** The BTREE_AUXDELETE bit of flags indicates that is one of several deletes
+** associated with a single table entry and its indexes.  Only one of those
+** deletes is considered the "primary" delete.  The primary delete occurs
+** on a cursor that is not a BTREE_FORDELETE cursor.  All but one delete
+** operation on non-FORDELETE cursors is tagged with the AUXDELETE flag.
+** The BTREE_AUXDELETE bit is a hint that is not used by this implementation,
+** but which might be used by alternative storage engines.
+*/
+int sqlite3BtreeDelete(BtCursor *pCur, u8 flags){
   Btree *p = pCur->pBtree;
   BtShared *pBt = p->pBt;              
   int rc;                              /* Return code */
@@ -8098,6 +8116,7 @@ int sqlite3BtreeDelete(BtCursor *pCur, int bPreserve){
   int iCellDepth;                      /* Depth of node containing pCell */ 
   u16 szCell;                          /* Size of the cell being deleted */
   int bSkipnext = 0;                   /* Leaf cursor in SKIPNEXT state */
+  u8 bPreserve = flags & BTREE_SAVEPOSITION;  /* Keep cursor valid */
 
   assert( cursorOwnsBtShared(pCur) );
   assert( pBt->inTransaction==TRANS_WRITE );
@@ -8107,6 +8126,7 @@ int sqlite3BtreeDelete(BtCursor *pCur, int bPreserve){
   assert( !hasReadConflicts(p, pCur->pgnoRoot) );
   assert( pCur->aiIdx[pCur->iPage]<pCur->apPage[pCur->iPage]->nCell );
   assert( pCur->eState==CURSOR_VALID );
+  assert( (flags & ~(BTREE_SAVEPOSITION | BTREE_AUXDELETE))==0 );
 
   iCellDepth = pCur->iPage;
   iCellIdx = pCur->aiIdx[iCellDepth];