When flattening the right operand of a LEFT JOIN

(check-in [41c27bc0ff1d3135]), ensure that the OP_IfNullRow opcode does not
NULL-out a subquery result that was computed within OP_Once.  This fixes
the problem problem reported by
[forum:/forumpost/402f05296d|forum post 402f05296d].

FossilOrigin-Name: 8fe13f7a5e5eb798189acb25a608df7a94c2f5cc83463331a048b779c7890c82

1 files changed, 13 insertions, 7 deletions

diff --git a/src/expr.c b/src/expr.c
index 763e961b6..b688e5b40 100644
--- a/src/expr.c
+++ b/src/expr.c
@@ -4833,16 +4833,22 @@ expr_code_doover:
           break;
         }
       }
-      addrINR = sqlite3VdbeAddOp1(v, OP_IfNullRow, pExpr->iTable);
-      /* Temporarily disable factoring of constant expressions, since
-      ** even though expressions may appear to be constant, they are not
-      ** really constant because they originate from the right-hand side
-      ** of a LEFT JOIN. */
-      pParse->okConstFactor = 0;
+      addrINR = sqlite3VdbeAddOp3(v, OP_IfNullRow, pExpr->iTable, 0, target);
+      /* The OP_IfNullRow opcode above can overwrite the result register with
+      ** NULL.  So we have to ensure that the result register is not a value
+      ** that is suppose to be a constant.  Two defenses are needed:
+      **   (1)  Temporarily disable factoring of constant expressions
+      **   (2)  Make sure the computed value really is stored in register
+      **        "target" and not someplace else.
+      */
+      pParse->okConstFactor = 0;   /* note (1) above */
       inReg = sqlite3ExprCodeTarget(pParse, pExpr->pLeft, target);
       pParse->okConstFactor = okConstFactor;
+      if( inReg!=target ){         /* note (2) above */
+        sqlite3VdbeAddOp2(v, OP_SCopy, inReg, target);
+        inReg = target;
+      }
       sqlite3VdbeJumpHere(v, addrINR);
-      sqlite3VdbeChangeP3(v, addrINR, inReg);
       break;
     }