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, 3 insertions, 3 deletions

diff --git a/src/backend/postmaster/autovacuum.c b/src/backend/postmaster/autovacuum.c
index 800815dfbcc..71c34027c88 100644
--- a/src/backend/postmaster/autovacuum.c
+++ b/src/backend/postmaster/autovacuum.c
@@ -317,7 +317,7 @@ int			AutovacuumLauncherPid = 0;
 
 static Oid	do_start_worker(void);
 static void ProcessAutoVacLauncherInterrupts(void);
-static void AutoVacLauncherShutdown(void) pg_attribute_noreturn();
+pg_noreturn static void AutoVacLauncherShutdown(void);
 static void launcher_determine_sleep(bool canlaunch, bool recursing,
 									 struct timeval *nap);
 static void launch_worker(TimestampTz now);
diff --git a/src/backend/postmaster/launch_backend.c b/src/backend/postmaster/launch_backend.c
index ecd04655c2a..77fb877dbad 100644
--- a/src/backend/postmaster/launch_backend.c
+++ b/src/backend/postmaster/launch_backend.c
@@ -171,7 +171,7 @@ static pid_t internal_forkexec(const char *child_kind, int child_slot,
 typedef struct
 {
 	const char *name;
-	void		(*main_fn) (const void *startup_data, size_t startup_data_len) pg_attribute_noreturn();
+	void		(*main_fn) (const void *startup_data, size_t startup_data_len);
 	bool		shmem_attach;
 } child_process_kind;
 
diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index 57155c00e01..d13846298bd 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -425,7 +425,7 @@ static void LogChildExit(int lev, const char *procname,
 static void PostmasterStateMachine(void);
 static void UpdatePMState(PMState newState);
 
-static void ExitPostmaster(int status) pg_attribute_noreturn();
+pg_noreturn static void ExitPostmaster(int status);
 static int	ServerLoop(void);
 static int	BackendStartup(ClientSocket *client_sock);
 static void report_fork_failure_to_client(ClientSocket *client_sock, int errnum);