Avoid Assert failure when processing empty statement in aborted xact.

exec_parse_message() wants to create a cached plan in all cases,
including for empty input.  The empty-input path does not have
a test for being in an aborted transaction, making it possible
that plancache.c will fail due to trying to do database lookups
even though there's no real work to do.

One solution would be to throw an aborted-transaction error in
this path too, but it's not entirely clear whether the lack of
such an error was intentional or whether some clients might be
relying on non-error behavior.  Instead, let's hack plancache.c
so that it treats empty statements with the same logic it
already had for transaction control commands, ensuring that it
can soldier through even in an already-aborted transaction.

Per bug #17983 from Alexander Lakhin.  Back-patch to all
supported branches.

Discussion: https://postgr.es/m/17983-da4569fcb878672e@postgresql.org

3 files changed, 22 insertions, 1 deletions

diff --git a/src/backend/utils/cache/plancache.c b/src/backend/utils/cache/plancache.c
index abc30628920..25e951e9c10 100644
--- a/src/backend/utils/cache/plancache.c
+++ b/src/backend/utils/cache/plancache.c
@@ -78,9 +78,11 @@
 /*
  * We must skip "overhead" operations that involve database access when the
  * cached plan's subject statement is a transaction control command.
+ * For the convenience of postgres.c, treat empty statements as control
+ * commands too.
  */
 #define IsTransactionStmtPlan(plansource)  \
-	((plansource)->raw_parse_tree && \
+	((plansource)->raw_parse_tree == NULL || \
 	 IsA((plansource)->raw_parse_tree->stmt, TransactionStmt))
 
 /*
diff --git a/src/test/regress/expected/psql.out b/src/test/regress/expected/psql.out
index a73140543ba..60f922050e7 100644
--- a/src/test/regress/expected/psql.out
+++ b/src/test/regress/expected/psql.out
@@ -215,6 +215,17 @@ SELECT 3 AS x, 'Hello', 4 AS y, true AS "dirty\name" \gdesc \g
  3 | Hello    | 4 | t
 (1 row)
 
+-- test for server bug #17983 with empty statement in aborted transaction
+set search_path = default;
+begin;
+bogus;
+ERROR:  syntax error at or near "bogus"
+LINE 1: bogus;
+        ^
+;
+\gdesc
+The command has no result, or the result has no columns.
+rollback;
 -- \gexec
 create temporary table gexec_test(a int, b text, c date, d float);
 select format('create index on gexec_test(%I)', attname)
diff --git a/src/test/regress/sql/psql.sql b/src/test/regress/sql/psql.sql
index f43ed2731b1..531223e3cfd 100644
--- a/src/test/regress/sql/psql.sql
+++ b/src/test/regress/sql/psql.sql
@@ -112,6 +112,14 @@ SELECT 1 AS x, 'Hello', 2 AS y, true AS "dirty\name"
 -- all on one line
 SELECT 3 AS x, 'Hello', 4 AS y, true AS "dirty\name" \gdesc \g
 
+-- test for server bug #17983 with empty statement in aborted transaction
+set search_path = default;
+begin;
+bogus;
+;
+\gdesc
+rollback;
+
 -- \gexec
 
 create temporary table gexec_test(a int, b text, c date, d float);