Refactor postmaster child process launching

Introduce new postmaster_child_launch() function that deals with the
differences in EXEC_BACKEND mode.

Refactor the mechanism of passing information from the parent to child
process. Instead of using different command-line arguments when
launching the child process in EXEC_BACKEND mode, pass a
variable-length blob of startup data along with all the global
variables. The contents of that blob depend on the kind of child
process being launched. In !EXEC_BACKEND mode, we use the same blob,
but it's simply inherited from the parent to child process.

Reviewed-by: Tristan Partin, Andres Freund
Discussion: https://www.postgresql.org/message-id/7a59b073-5b5b-151e-7ed3-8b01ff7ce9ef@iki.fi

1 files changed, 17 insertions, 3 deletions

diff --git a/src/backend/postmaster/bgworker.c b/src/backend/postmaster/bgworker.c
index b73e91f0c86..cf64a4beb20 100644
--- a/src/backend/postmaster/bgworker.c
+++ b/src/backend/postmaster/bgworker.c
@@ -720,15 +720,29 @@ bgworker_die(SIGNAL_ARGS)
  * Main entry point for background worker processes.
  */
 void
-BackgroundWorkerMain(void)
+BackgroundWorkerMain(char *startup_data, size_t startup_data_len)
 {
 	sigjmp_buf	local_sigjmp_buf;
-	BackgroundWorker *worker = MyBgworkerEntry;
+	BackgroundWorker *worker;
 	bgworker_main_type entrypt;
 
-	if (worker == NULL)
+	if (startup_data == NULL)
 		elog(FATAL, "unable to find bgworker entry");
+	Assert(startup_data_len == sizeof(BackgroundWorker));
+	worker = MemoryContextAlloc(TopMemoryContext, sizeof(BackgroundWorker));
+	memcpy(worker, startup_data, sizeof(BackgroundWorker));
 
+	/*
+	 * Now that we're done reading the startup data, release postmaster's
+	 * working memory context.
+	 */
+	if (PostmasterContext)
+	{
+		MemoryContextDelete(PostmasterContext);
+		PostmasterContext = NULL;
+	}
+
+	MyBgworkerEntry = worker;
 	MyBackendType = B_BG_WORKER;
 	init_ps_display(worker->bgw_name);