Don't use bgw_main even to specify in-core bgworker entrypoints.

On EXEC_BACKEND builds, this can fail if ASLR is in use.

Backpatch to 9.5.  On master, completely remove the bgw_main field
completely, since there is no situation in which it is safe for an
EXEC_BACKEND build.  On 9.6 and 9.5, leave the field intact to avoid
breaking things for third-party code that doesn't care about working
under EXEC_BACKEND.  Prior to 9.5, there are no in-core bgworker
entrypoints.

Petr Jelinek, reviewed by me.

Discussion: http://postgr.es/m/09d8ad33-4287-a09b-a77f-77f8761adb5e@2ndquadrant.com

src/backend/access/transam/parallel.c

@@ -106,7 +106,6 @@ static dlist_head pcxt_list = DLIST_STATIC_INIT(pcxt_list);
 static void HandleParallelMessage(ParallelContext *pcxt, int i, StringInfo msg);
 static void ParallelErrorContext(void *arg);
 static void ParallelExtensionTrampoline(dsm_segment *seg, shm_toc *toc);
-static void ParallelWorkerMain(Datum main_arg);
 
 
 /*
@@ -401,7 +400,9 @@ LaunchParallelWorkers(ParallelContext *pcxt)
 		BGWORKER_SHMEM_ACCESS | BGWORKER_BACKEND_DATABASE_CONNECTION;
 	worker.bgw_start_time = BgWorkerStart_ConsistentState;
 	worker.bgw_restart_time = BGW_NEVER_RESTART;
-	worker.bgw_main = ParallelWorkerMain;
+	worker.bgw_main = NULL;
+	sprintf(worker.bgw_library_name, "postgres");
+	sprintf(worker.bgw_function_name, "ParallelWorkerMain");
 	worker.bgw_main_arg = UInt32GetDatum(dsm_segment_handle(pcxt->seg));
 	worker.bgw_notify_pid = MyProcPid;
 	memset(&worker.bgw_extra, 0, BGW_EXTRALEN);
@@ -834,7 +835,7 @@ AtEOXact_Parallel(bool isCommit)
 /*
  * Main entrypoint for parallel workers.
  */
-static void
+void
 ParallelWorkerMain(Datum main_arg)
 {
 	dsm_segment *seg;

src/backend/postmaster/bgworker.c

@@ -16,6 +16,7 @@
 
 #include "miscadmin.h"
 #include "libpq/pqsignal.h"
+#include "access/parallel.h"
 #include "postmaster/bgworker_internals.h"
 #include "postmaster/postmaster.h"
 #include "storage/barrier.h"
@@ -93,6 +94,25 @@ struct BackgroundWorkerHandle
 
 static BackgroundWorkerArray *BackgroundWorkerData;
 
+/*
+ * List of internal background workers. These are used for mapping the
+ * function name to actual function when building with EXEC_BACKEND and also
+ * to allow these to be loaded outside of shared_preload_libraries.
+ */
+typedef struct InternalBGWorkerMain
+{
+	char			   *bgw_function_name;
+	bgworker_main_type	bgw_main;
+} InternalBGWorkerMain;
+
+static const InternalBGWorkerMain InternalBGWorkers[] = {
+	{"ParallelWorkerMain", ParallelWorkerMain},
+	/* Dummy entry marking end of the array. */
+	{NULL, NULL}
+};
+
+static bgworker_main_type GetInternalBgWorkerMain(BackgroundWorker *worker);
+
 /*
  * Calculate shared memory needed.
  */
@@ -695,22 +715,27 @@ StartBackgroundWorker(void)
 #endif
 	}
 
+	/* For internal workers set the entry point to known function address. */
+	entrypt = GetInternalBgWorkerMain(worker);
+
 	/*
-	 * If bgw_main is set, we use that value as the initial entrypoint.
-	 * However, if the library containing the entrypoint wasn't loaded at
-	 * postmaster startup time, passing it as a direct function pointer is not
-	 * possible.  To work around that, we allow callers for whom a function
-	 * pointer is not available to pass a library name (which will be loaded,
-	 * if necessary) and a function name (which will be looked up in the named
+	 * Otherwise, if bgw_main is set, we use that value as the initial
+	 * entrypoint. This does not work well EXEC_BACKEND outside Windows but
+	 * we keep the logic for backwards compatibility. In other cases use
+	 * the entry point specified by library name (which will be loaded, if
+	 * necessary) and a function name (which will be looked up in the named
 	 * library).
 	 */
-	if (worker->bgw_main != NULL)
-		entrypt = worker->bgw_main;
-	else
-		entrypt = (bgworker_main_type)
-			load_external_function(worker->bgw_library_name,
-								   worker->bgw_function_name,
-								   true, NULL);
+	if (entrypt == NULL)
+	{
+		if (worker->bgw_main != NULL)
+			entrypt = worker->bgw_main;
+		else
+			entrypt = (bgworker_main_type)
+				load_external_function(worker->bgw_library_name,
+									   worker->bgw_function_name,
+									   true, NULL);
+	}
 
 	/*
 	 * Note that in normal processes, we would call InitPostgres here.  For a
@@ -1078,3 +1103,28 @@ TerminateBackgroundWorker(BackgroundWorkerHandle *handle)
 	if (signal_postmaster)
 		SendPostmasterSignal(PMSIGNAL_BACKGROUND_WORKER_CHANGE);
 }
+
+/*
+ * Search the known internal worker array and return its main function
+ * pointer if found.
+ *
+ * Returns NULL if not known internal worker.
+ */
+static bgworker_main_type
+GetInternalBgWorkerMain(BackgroundWorker *worker)
+{
+	int i;
+
+	/* Internal workers always have to use postgres as library name. */
+	if (strncmp(worker->bgw_library_name, "postgres", BGW_MAXLEN) != 0)
+		return NULL;
+
+	for (i = 0; InternalBGWorkers[i].bgw_function_name; i++)
+	{
+		if (strncmp(InternalBGWorkers[i].bgw_function_name,
+					worker->bgw_function_name, BGW_MAXLEN) == 0)
+			return InternalBGWorkers[i].bgw_main;
+	}
+
+	return NULL;
+}

src/include/access/parallel.h

@@ -65,4 +65,6 @@ extern void AtEOXact_Parallel(bool isCommit);
 extern void AtEOSubXact_Parallel(bool isCommit, SubTransactionId mySubId);
 extern void ParallelWorkerReportLastRecEnd(XLogRecPtr last_xlog_end);
 
+extern void ParallelWorkerMain(Datum main_arg);
+
 #endif   /* PARALLEL_H */

src/test/modules/worker_spi/worker_spi.c

@@ -346,7 +346,9 @@ _PG_init(void)
 		BGWORKER_BACKEND_DATABASE_CONNECTION;
 	worker.bgw_start_time = BgWorkerStart_RecoveryFinished;
 	worker.bgw_restart_time = BGW_NEVER_RESTART;
-	worker.bgw_main = worker_spi_main;
+	worker.bgw_main = NULL;
+	sprintf(worker.bgw_library_name, "worker_spi");
+	sprintf(worker.bgw_function_name, "worker_spi_main");
 	worker.bgw_notify_pid = 0;
 
 	/*
@@ -377,7 +379,7 @@ worker_spi_launch(PG_FUNCTION_ARGS)
 		BGWORKER_BACKEND_DATABASE_CONNECTION;
 	worker.bgw_start_time = BgWorkerStart_RecoveryFinished;
 	worker.bgw_restart_time = BGW_NEVER_RESTART;
-	worker.bgw_main = NULL;		/* new worker might not have library loaded */
+	worker.bgw_main = NULL;
 	sprintf(worker.bgw_library_name, "worker_spi");
 	sprintf(worker.bgw_function_name, "worker_spi_main");
 	snprintf(worker.bgw_name, BGW_MAXLEN, "worker %d", i);