Kluge slot_compile_deform() to ignore incorrect attnotnull markings.

Since we mustn't force an initdb in released branches, there is no
simple way to correct the markings of pg_subscription.subslotname
and pg_subscription_rel.srsublsn as attnotnull in existing pre-v13
installations.

Fortunately, released branches don't rely on attnotnull being correct
for much.  The planner looks at it in relation_excluded_by_constraints,
but it'd be difficult to get that to matter for a query on a system
catalog.  The only place where it's really problematic is in JIT's
slot_compile_deform(), which can produce incorrect code that crashes
if there are NULLs in an allegedly not-null column.

Hence, hack up slot_compile_deform() to be specifically aware of
these two incorrect markings and not trust them.

This applies to v11 and v12; the JIT code didn't exist before that,
and we've fixed the markings in v13.

Discussion: https://postgr.es/m/229396.1595191345@sss.pgh.pa.us

src/backend/jit/llvm/llvmjit_deform.c

@@ -22,11 +22,27 @@
 
 #include "access/htup_details.h"
 #include "access/tupdesc_details.h"
+#include "catalog/pg_subscription.h"
+#include "catalog/pg_subscription_rel.h"
 #include "executor/tuptable.h"
 #include "jit/llvmjit.h"
 #include "jit/llvmjit_emit.h"
 
 
+/*
+ * Through an embarrassing oversight, pre-v13 installations may have
+ * pg_subscription.subslotname and pg_subscription_rel.srsublsn marked as
+ * attnotnull, which they should not be.  To avoid possible crashes, use
+ * this macro instead of testing attnotnull directly.
+ */
+#define ATTNOTNULL(att) \
+	((att)->attnotnull && \
+	 !(((att)->attrelid == SubscriptionRelationId && \
+		(att)->attnum == Anum_pg_subscription_subslotname) || \
+	   ((att)->attrelid == SubscriptionRelRelationId && \
+		(att)->attnum == Anum_pg_subscription_rel_srsublsn)))
+
+
 /*
  * Create a function that deforms a tuple of type desc up to natts columns.
  */
@@ -113,7 +129,7 @@ slot_compile_deform(LLVMJitContext *context, TupleDesc desc, int natts)
 		 * it's guaranteed that all columns up to here exist at least in the
 		 * NULL bitmap.
 		 */
-		if (att->attnotnull)
+		if (ATTNOTNULL(att))
 			guaranteed_column_number = attnum;
 	}
 
@@ -376,7 +392,7 @@ slot_compile_deform(LLVMJitContext *context, TupleDesc desc, int natts)
 		 * into account, because in case they're present the heaptuple's natts
 		 * would have indicated that a slot_getmissingattrs() is needed.
 		 */
-		if (!att->attnotnull)
+		if (!ATTNOTNULL(att))
 		{
 			LLVMBasicBlockRef b_ifnotnull;
 			LLVMBasicBlockRef b_ifnull;
@@ -557,7 +573,7 @@ slot_compile_deform(LLVMJitContext *context, TupleDesc desc, int natts)
 			known_alignment = -1;
 			attguaranteedalign = false;
 		}
-		else if (att->attnotnull && attguaranteedalign && known_alignment >= 0)
+		else if (ATTNOTNULL(att) && attguaranteedalign && known_alignment >= 0)
 		{
 			/*
 			 * If the offset to the column was previously known a NOT NULL &
@@ -567,7 +583,7 @@ slot_compile_deform(LLVMJitContext *context, TupleDesc desc, int natts)
 			Assert(att->attlen > 0);
 			known_alignment += att->attlen;
 		}
-		else if (att->attnotnull && (att->attlen % alignto) == 0)
+		else if (ATTNOTNULL(att) && (att->attlen % alignto) == 0)
 		{
 			/*
 			 * After a NOT NULL fixed-width column with a length that is a

src/test/regress/expected/subscription.out

@@ -147,6 +147,13 @@ DROP SUBSCRIPTION testsub;
 ERROR:  DROP SUBSCRIPTION cannot run inside a transaction block
 COMMIT;
 ALTER SUBSCRIPTION testsub SET (slot_name = NONE);
+\dRs+
+                                              List of subscriptions
+  Name   |           Owner            | Enabled |     Publication     | Synchronous commit |       Conninfo       
+---------+----------------------------+---------+---------------------+--------------------+----------------------
+ testsub | regress_subscription_user2 | f       | {testpub2,testpub3} | local              | dbname=doesnotexist2
+(1 row)
+
 -- now it works
 BEGIN;
 DROP SUBSCRIPTION testsub;

src/test/regress/sql/subscription.sql

@@ -109,6 +109,8 @@ COMMIT;
 
 ALTER SUBSCRIPTION testsub SET (slot_name = NONE);
 
+\dRs+
+
 -- now it works
 BEGIN;
 DROP SUBSCRIPTION testsub;