Revert hooks for session start and end, take two

The location of the session end hook has been chosen so as it is
possible to allow modules to do their own transactions, however any
trying to any any subsystem which went through before_shmem_exit()
would cause issues, limiting the pluggability of the hook.

Per discussion with Tom Lane and Andres Freund.

Discussion: https://postgr.es/m/18722.1569906636@sss.pgh.pa.us

Author: michaelpq

src/backend/tcop/postgres.c

@@ -171,9 +171,6 @@ static ProcSignalReason RecoveryConflictReason;
 static MemoryContext row_description_context = NULL;
 static StringInfoData row_description_buf;
 
-/* Hook for plugins to get control at start of session */
-session_start_hook_type session_start_hook = NULL;
-
 /* ----------------------------------------------------------------
  *		decls for routines only used in this file
  * ----------------------------------------------------------------
@@ -3971,9 +3968,6 @@ PostgresMain(int argc, char *argv[],
 	if (!IsUnderPostmaster)
 		PgStartTime = GetCurrentTimestamp();
 
-	if (session_start_hook)
-		(*session_start_hook) ();
-
 	/*
 	 * POSTGRES main processing loop begins here
 	 *

src/backend/utils/init/postinit.c

@@ -78,8 +78,6 @@ static bool ThereIsAtLeastOneRole(void);
 static void process_startup_options(Port *port, bool am_superuser);
 static void process_settings(Oid databaseid, Oid roleid);
 
-/* Hook for plugins to get control at end of session */
-session_end_hook_type session_end_hook = NULL;
 
 /*** InitPostgres support ***/
 
@@ -1197,10 +1195,6 @@ ShutdownPostgres(int code, Datum arg)
 	 * them explicitly.
 	 */
 	LockReleaseAll(USER_LOCKMETHOD, true);
-
-	/* Hook at session end */
-	if (session_end_hook)
-		(*session_end_hook) ();
 }
 
 

src/include/tcop/tcopprot.h

@@ -30,13 +30,6 @@ extern PGDLLIMPORT const char *debug_query_string;
 extern int	max_stack_depth;
 extern int	PostAuthDelay;
 
-/* Hook for plugins to get control at start and end of session */
-typedef void (*session_start_hook_type) (void);
-typedef void (*session_end_hook_type) (void);
-
-extern PGDLLIMPORT session_start_hook_type session_start_hook;
-extern PGDLLIMPORT session_end_hook_type session_end_hook;
-
 /* GUC-configurable parameters */
 
 typedef enum

src/test/modules/Makefile

@@ -21,7 +21,6 @@ SUBDIRS = \
 		  test_predtest \
 		  test_rbtree \
 		  test_rls_hooks \
-		  test_session_hooks \
 		  test_shm_mq \
 		  unsafe_tests \
 		  worker_spi