pg_noreturn to replace pg_attribute_noreturn()

We want to support a "noreturn" decoration on more compilers besides
just GCC-compatible ones, but for that we need to move the decoration
in front of the function declaration instead of either behind it or
wherever, which is the current style afforded by GCC-style attributes.
Also rename the macro to "pg_noreturn" to be similar to the C11
standard "noreturn".

pg_noreturn is now supported on all compilers that support C11 (using
_Noreturn), as well as GCC-compatible ones (using __attribute__, as
before), as well as MSVC (using __declspec).  (When PostgreSQL
requires C11, the latter two variants can be dropped.)

Now, all supported compilers effectively support pg_noreturn, so the
extra code for !HAVE_PG_ATTRIBUTE_NORETURN can be dropped.

This also fixes a possible problem if third-party code includes
stdnoreturn.h, because then the current definition of

    #define pg_attribute_noreturn() __attribute__((noreturn))

would cause an error.

Note that the C standard does not support a noreturn attribute on
function pointer types.  So we have to drop these here.  There are
only two instances at this time, so it's not a big loss.  In one case,
we can make up for it by adding the pg_noreturn to a wrapper function
and adding a pg_unreachable(), in the other case, the latter was
already done before.

Reviewed-by: Dagfinn Ilmari Mannsåker <ilmari@ilmari.org>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://www.postgresql.org/message-id/flat/pxr5b3z7jmkpenssra5zroxi7qzzp6eswuggokw64axmdixpnk@zbwxuq7gbbcw

3 files changed, 4 insertions, 5 deletions

diff --git a/src/test/modules/libpq_pipeline/libpq_pipeline.c b/src/test/modules/libpq_pipeline/libpq_pipeline.c
index 7ff18e91e66..ac9ac95135f 100644
--- a/src/test/modules/libpq_pipeline/libpq_pipeline.c
+++ b/src/test/modules/libpq_pipeline/libpq_pipeline.c
@@ -24,7 +24,7 @@
 
 
 static void exit_nicely(PGconn *conn);
-static void pg_attribute_noreturn() pg_fatal_impl(int line, const char *fmt,...)
+pg_noreturn static void pg_fatal_impl(int line, const char *fmt,...)
 			pg_attribute_printf(2, 3);
 static bool process_result(PGconn *conn, PGresult *res, int results,
 						   int numsent);
@@ -71,8 +71,7 @@ exit_nicely(PGconn *conn)
  * Print an error to stderr and terminate the program.
  */
 #define pg_fatal(...) pg_fatal_impl(__LINE__, __VA_ARGS__)
-static void
-pg_attribute_noreturn()
+pg_noreturn static void
 pg_fatal_impl(int line, const char *fmt,...)
 {
 	va_list		args;
diff --git a/src/test/modules/test_shm_mq/test_shm_mq.h b/src/test/modules/test_shm_mq/test_shm_mq.h
index 9ad9f63b44e..5346557d473 100644
--- a/src/test/modules/test_shm_mq/test_shm_mq.h
+++ b/src/test/modules/test_shm_mq/test_shm_mq.h
@@ -40,6 +40,6 @@ extern void test_shm_mq_setup(int64 queue_size, int32 nworkers,
 							  shm_mq_handle **input);
 
 /* Main entrypoint for a worker. */
-extern PGDLLEXPORT void test_shm_mq_main(Datum) pg_attribute_noreturn();
+pg_noreturn extern PGDLLEXPORT void test_shm_mq_main(Datum);
 
 #endif
diff --git a/src/test/modules/worker_spi/worker_spi.c b/src/test/modules/worker_spi/worker_spi.c
index 5b87d4f7038..9c53d896b6a 100644
--- a/src/test/modules/worker_spi/worker_spi.c
+++ b/src/test/modules/worker_spi/worker_spi.c
@@ -44,7 +44,7 @@ PG_MODULE_MAGIC;
 
 PG_FUNCTION_INFO_V1(worker_spi_launch);
 
-PGDLLEXPORT void worker_spi_main(Datum main_arg) pg_attribute_noreturn();
+PGDLLEXPORT pg_noreturn void worker_spi_main(Datum main_arg);
 
 /* GUC variables */
 static int	worker_spi_naptime = 10;