Improved comments. Slightly tighter implementation, but no big changes.

FossilOrigin-Name: a1b3f28569f2a8d82b2931527fdfe191b421f3e1ea18ee30e04211e1ad645993

4 files changed, 24 insertions, 8 deletions

diff --git a/src/sqlite.h.in b/src/sqlite.h.in
index 7579b9926..cd524a046 100644
--- a/src/sqlite.h.in
+++ b/src/sqlite.h.in
@@ -4850,8 +4850,13 @@ SQLITE_DEPRECATED int sqlite3_memory_alarm(void(*)(void*,sqlite3_int64,int),
 ** ^Within the [xUpdate] method of a [virtual table], the
 ** sqlite3_value_nochange(X) interface returns true if and only if
 ** the column corresponding to X is unchanged by the UPDATE operation
-** and the [xColumn] method had previously queried [sqlite3_vtab_nochange()]
-** for that column and returned a NULL as a result.
+** that the xUpdate method call was invoked to implement and if
+** and the prior [xColumn] method call that was invoked to extracted
+** the value for that column returned without setting a result (probably
+** because it queried [sqlite3_vtab_nochange()] and found that the column
+** was unchanging).  If sqlite3_value_nochange(X) is invoked anywhere other
+** than within an [xUpdate] method call for an UPDATE statement, then
+** the return value is arbitrary and meaningless.
 **
 ** Please pay particular attention to the fact that the pointer returned
 ** from [sqlite3_value_blob()], [sqlite3_value_text()], or
diff --git a/src/sqliteInt.h b/src/sqliteInt.h
index 8e7913320..b7b402b8e 100644
--- a/src/sqliteInt.h
+++ b/src/sqliteInt.h
@@ -3117,6 +3117,7 @@ struct AuthContext {
 #define OPFLAG_PERMUTE       0x01    /* OP_Compare: use the permutation */
 #define OPFLAG_SAVEPOSITION  0x02    /* OP_Delete/Insert: save cursor pos */
 #define OPFLAG_AUXDELETE     0x04    /* OP_Delete: index in a DELETE op */
+#define OPFLAG_NOCHNG_MAGIC  0x6d    /* OP_MakeRecord: serialtype 10 is ok */
 
 /*
  * Each trigger present in the database schema is stored as an instance of
diff --git a/src/update.c b/src/update.c
index ee920d2a5..32c1a8371 100644
--- a/src/update.c
+++ b/src/update.c
@@ -828,7 +828,7 @@ static void updateVirtualTable(
       sqlite3ExprCode(pParse, pChanges->a[aXRef[i]].pExpr, regArg+2+i);
     }else{
       sqlite3VdbeAddOp3(v, OP_VColumn, iCsr, i, regArg+2+i);
-      sqlite3VdbeChangeP5(v, 1);
+      sqlite3VdbeChangeP5(v, 1); /* Enable sqlite3_vtab_nochange() */
     }
   }
   if( HasRowid(pTab) ){
@@ -863,7 +863,11 @@ static void updateVirtualTable(
     /* Create a record from the argument register contents and insert it into
     ** the ephemeral table. */
     sqlite3VdbeAddOp3(v, OP_MakeRecord, regArg, nArg, regRec);
-    sqlite3VdbeChangeP5(v, 2);
+#ifdef SQLITE_DEBUG
+    /* Signal an assert() within OP_MakeRecord that it is allowed to
+    ** accept no-change records with serial_type 10 */
+    sqlite3VdbeChangeP5(v, OPFLAG_NOCHNG_MAGIC);
+#endif
     sqlite3VdbeAddOp2(v, OP_NewRowid, ephemTab, regRowid);
     sqlite3VdbeAddOp3(v, OP_Insert, ephemTab, regRec, regRowid);
   }
diff --git a/src/vdbe.c b/src/vdbe.c
index dc058b65f..d878ca8e3 100644
--- a/src/vdbe.c
+++ b/src/vdbe.c
@@ -2792,12 +2792,17 @@ case OP_MakeRecord: {
   pRec = pLast;
   do{
     assert( memIsValid(pRec) );
-    pRec->uTemp = serial_type = sqlite3VdbeSerialType(pRec, file_format, &len);
+    serial_type = sqlite3VdbeSerialType(pRec, file_format, &len);
     if( pRec->flags & MEM_Zero ){
       if( serial_type==0 ){
-        assert( pOp->p5==2 || CORRUPT_DB );
-        /* serial_type 10 used internally only */
-        pRec->uTemp = 10;
+        /* Values with MEM_Null and MEM_Zero are created by xColumn virtual
+        ** table methods that never invoke sqlite3_result_xxxxx() while
+        ** computing an unchanging column value in an UPDATE statement.
+        ** Give such values a special internal-use-only serial-type of 10
+        ** so that they can be passed through to xUpdate and have
+        ** a true sqlite3_value_nochange(). */
+        assert( pOp->p5==OPFLAG_NOCHNG_MAGIC || CORRUPT_DB );
+        serial_type = 10;
       }else if( nData ){
         if( sqlite3VdbeMemExpandBlob(pRec) ) goto no_mem;
       }else{
@@ -2809,6 +2814,7 @@ case OP_MakeRecord: {
     testcase( serial_type==127 );
     testcase( serial_type==128 );
     nHdr += serial_type<=127 ? 1 : sqlite3VarintLen(serial_type);
+    pRec->uTemp = serial_type;
     if( pRec==pData0 ) break;
     pRec--;
   }while(1);