Fix NULLIF()'s handling of read-write expanded objects.

If passed a read-write expanded object pointer, the EEOP_NULLIF
code would hand that same pointer to the equality function
and then (unless equality was reported) also return the same
pointer as its value.  This is no good, because a function that
receives a read-write expanded object pointer is fully entitled
to scribble on or even delete the object, thus corrupting the
NULLIF output.  (This problem is likely unobservable with the
equality functions provided in core Postgres, but it's easy to
demonstrate with one coded in plpgsql.)

To fix, make sure the pointer passed to the equality function
is read-only.  We can still return the original read-write
pointer as the NULLIF result, allowing optimization of later
operations.

Per bug #18722 from Alexander Lakhin.  This has been wrong
since we invented expanded objects, so back-patch to all
supported branches.

Discussion: https://postgr.es/m/18722-fd9e645448cc78b4@postgresql.org

src/backend/executor/execExpr.c

@@ -1169,6 +1169,14 @@ ExecInitExprRec(Expr *node, ExprState *state,
 							 op->args, op->opfuncid, op->inputcollid,
 							 state);
 
+				/*
+				 * If first argument is of varlena type, we'll need to ensure
+				 * that the value passed to the comparison function is a
+				 * read-only pointer.
+				 */
+				scratch.d.func.make_ro =
+					(get_typlen(exprType((Node *) linitial(op->args))) == -1);
+
 				/*
 				 * Change opcode of call instruction to EEOP_NULLIF.
 				 *

src/backend/executor/execExprInterp.c

@@ -1273,12 +1273,24 @@ ExecInterpExpr(ExprState *state, ExprContext *econtext, bool *isnull)
 			 * The arguments are already evaluated into fcinfo->args.
 			 */
 			FunctionCallInfo fcinfo = op->d.func.fcinfo_data;
+			Datum		save_arg0 = fcinfo->args[0].value;
 
 			/* if either argument is NULL they can't be equal */
 			if (!fcinfo->args[0].isnull && !fcinfo->args[1].isnull)
 			{
 				Datum		result;
 
+				/*
+				 * If first argument is of varlena type, it might be an
+				 * expanded datum.  We need to ensure that the value passed to
+				 * the comparison function is a read-only pointer.  However,
+				 * if we end by returning the first argument, that will be the
+				 * original read-write pointer if it was read-write.
+				 */
+				if (op->d.func.make_ro)
+					fcinfo->args[0].value =
+						MakeExpandedObjectReadOnlyInternal(save_arg0);
+
 				fcinfo->isnull = false;
 				result = op->d.func.fn_addr(fcinfo);
 
@@ -1293,7 +1305,7 @@ ExecInterpExpr(ExprState *state, ExprContext *econtext, bool *isnull)
 			}
 
 			/* Arguments aren't equal, so return the first one */
-			*op->resvalue = fcinfo->args[0].value;
+			*op->resvalue = save_arg0;
 			*op->resnull = fcinfo->args[0].isnull;
 
 			EEO_NEXT();

src/backend/jit/llvm/llvmjit_expr.c

@@ -1550,6 +1550,9 @@ llvm_compile_expr(ExprState *state)
 
 					v_fcinfo = l_ptr_const(fcinfo, l_ptr(StructFunctionCallInfoData));
 
+					/* save original arg[0] */
+					v_arg0 = l_funcvalue(b, v_fcinfo, 0);
+
 					/* if either argument is NULL they can't be equal */
 					v_argnull0 = l_funcnull(b, v_fcinfo, 0);
 					v_argnull1 = l_funcnull(b, v_fcinfo, 1);
@@ -1566,7 +1569,6 @@ llvm_compile_expr(ExprState *state)
 
 					/* one (or both) of the arguments are null, return arg[0] */
 					LLVMPositionBuilderAtEnd(b, b_hasnull);
-					v_arg0 = l_funcvalue(b, v_fcinfo, 0);
 					LLVMBuildStore(b, v_argnull0, v_resnullp);
 					LLVMBuildStore(b, v_arg0, v_resvaluep);
 					LLVMBuildBr(b, opblocks[opno + 1]);
@@ -1574,12 +1576,35 @@ llvm_compile_expr(ExprState *state)
 					/* build block to invoke function and check result */
 					LLVMPositionBuilderAtEnd(b, b_nonull);
 
+					/*
+					 * If first argument is of varlena type, it might be an
+					 * expanded datum.  We need to ensure that the value
+					 * passed to the comparison function is a read-only
+					 * pointer.  However, if we end by returning the first
+					 * argument, that will be the original read-write pointer
+					 * if it was read-write.
+					 */
+					if (op->d.func.make_ro)
+					{
+						LLVMValueRef v_params[1];
+						LLVMValueRef v_arg0_ro;
+
+						v_params[0] = v_arg0;
+						v_arg0_ro =
+							l_call(b,
+								   llvm_pg_var_func_type("MakeExpandedObjectReadOnlyInternal"),
+								   llvm_pg_func(mod, "MakeExpandedObjectReadOnlyInternal"),
+								   v_params, lengthof(v_params), "");
+						LLVMBuildStore(b, v_arg0_ro,
+									   l_funcvaluep(b, v_fcinfo, 0));
+					}
+
 					v_retval = BuildV1Call(context, b, mod, fcinfo, &v_fcinfo_isnull);
 
 					/*
-					 * If result not null, and arguments are equal return null
-					 * (same result as if there'd been NULLs, hence reuse
-					 * b_hasnull).
+					 * If result not null and arguments are equal return null,
+					 * else return arg[0] (same result as if there'd been
+					 * NULLs, hence reuse b_hasnull).
 					 */
 					v_argsequal = LLVMBuildAnd(b,
 											   LLVMBuildICmp(b, LLVMIntEQ,

src/include/executor/execExpr.h

@@ -345,6 +345,7 @@ typedef struct ExprEvalStep
 			/* faster to access without additional indirection: */
 			PGFunction	fn_addr;	/* actual call address */
 			int			nargs;	/* number of arguments */
+			bool		make_ro;	/* make arg0 R/O (used only for NULLIF) */
 		}			func;
 
 		/* for EEOP_BOOL_*_STEP */

src/test/regress/expected/case.out

@@ -397,6 +397,14 @@ SELECT CASE make_ad(1,2)
  right
 (1 row)
 
+-- While we're here, also test handling of a NULLIF arg that is a read/write
+-- object (bug #18722)
+SELECT NULLIF(make_ad(1,2), array[2,3]::arrdomain);
+ nullif 
+--------
+ {1,2}
+(1 row)
+
 ROLLBACK;
 -- Test interaction of CASE with ArrayCoerceExpr (bug #15471)
 BEGIN;

src/test/regress/sql/case.sql

@@ -242,6 +242,11 @@ SELECT CASE make_ad(1,2)
   WHEN array[1,2]::arrdomain THEN 'right'
   END;
 
+-- While we're here, also test handling of a NULLIF arg that is a read/write
+-- object (bug #18722)
+
+SELECT NULLIF(make_ad(1,2), array[2,3]::arrdomain);
+
 ROLLBACK;
 
 -- Test interaction of CASE with ArrayCoerceExpr (bug #15471)