process startup: Split single user code out of PostgresMain().

It was harder than necessary to understand PostgresMain() because the code for
a normal backend was interspersed with single-user mode specific code. Split
most of the single-user mode code into its own function
PostgresSingleUserMain(), that does all the necessary setup for single-user
mode, and then hands off after that to PostgresMain().

There still is some single-user mode code in InitPostgres(), and it'd likely
be worth moving at least some of it out. But that's for later.

Reviewed-By: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Author: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/20210802164124.ufo5buo4apl6yuvs@alap3.anarazel.de

src/backend/main/main.c

@@ -192,9 +192,8 @@ main(int argc, char *argv[])
 	else if (argc > 1 && strcmp(argv[1], "--describe-config") == 0)
 		GucInfoMain();
 	else if (argc > 1 && strcmp(argv[1], "--single") == 0)
-		PostgresMain(argc, argv,
-					 NULL,		/* no dbname */
-					 strdup(get_user_name_or_exit(progname)));
+		PostgresSingleUserMain(argc, argv,
+							   strdup(get_user_name_or_exit(progname)));
 	else
 		PostmasterMain(argc, argv);
 	/* the functions above should not return */

src/backend/postmaster/postmaster.c

@@ -4551,19 +4551,13 @@ BackendInitialize(Port *port)
 static void
 BackendRun(Port *port)
 {
-	char	   *av[2];
-	const int	ac = 1;
-
-	av[0] = "postgres";
-	av[1] = NULL;
-
 	/*
 	 * Make sure we aren't in PostmasterContext anymore.  (We can't delete it
 	 * just yet, though, because InitPostgres will need the HBA data.)
 	 */
 	MemoryContextSwitchTo(TopMemoryContext);
 
-	PostgresMain(ac, av, port->database_name, port->user_name);
+	PostgresMain(port->database_name, port->user_name);
 }
 
 

src/backend/tcop/postgres.c

@@ -3654,7 +3654,7 @@ get_stats_option_name(const char *arg)
 
 /* ----------------------------------------------------------------
  * process_postgres_switches
- *	   Parse command line arguments for PostgresMain
+ *	   Parse command line arguments for backends
  *
  * This is called twice, once for the "secure" options coming from the
  * postmaster or command line, and once for the "insecure" options coming
@@ -3915,40 +3915,30 @@ process_postgres_switches(int argc, char *argv[], GucContext ctx,
 }
 
 
-/* ----------------------------------------------------------------
- * PostgresMain
- *	   postgres main loop -- all backends, interactive or otherwise start here
+/*
+ * PostgresSingleUserMain
+ *     Entry point for single user mode. argc/argv are the command line
+ *     arguments to be used.
  *
- * argc/argv are the command line arguments to be used.  (When being forked
- * by the postmaster, these are not the original argv array of the process.)
- * dbname is the name of the database to connect to, or NULL if the database
- * name should be extracted from the command line arguments or defaulted.
- * username is the PostgreSQL user name to be used for the session.
- * ----------------------------------------------------------------
+ * Performs single user specific setup then calls PostgresMain() to actually
+ * process queries. Single user mode specific setup should go here, rather
+ * than PostgresMain() or InitPostgres() when reasonably possible.
  */
 void
-PostgresMain(int argc, char *argv[],
-			 const char *dbname,
-			 const char *username)
+PostgresSingleUserMain(int argc, char *argv[],
+					   const char *username)
 {
-	int			firstchar;
-	StringInfoData input_message;
-	sigjmp_buf	local_sigjmp_buf;
-	volatile bool send_ready_for_query = true;
-	bool		idle_in_transaction_timeout_enabled = false;
-	bool		idle_session_timeout_enabled = false;
+	const char *dbname = NULL;
 
-	/* Initialize startup process environment if necessary. */
-	if (!IsUnderPostmaster)
-		InitStandaloneProcess(argv[0]);
+	Assert(!IsUnderPostmaster);
 
-	SetProcessingMode(InitProcessing);
+	/* Initialize startup process environment. */
+	InitStandaloneProcess(argv[0]);
 
 	/*
 	 * Set default values for command-line options.
 	 */
-	if (!IsUnderPostmaster)
-		InitializeGUCOptions();
+	InitializeGUCOptions();
 
 	/*
 	 * Parse command-line options.
@@ -3966,12 +3956,75 @@ PostgresMain(int argc, char *argv[],
 							progname)));
 	}
 
-	/* Acquire configuration parameters, unless inherited from postmaster */
-	if (!IsUnderPostmaster)
-	{
-		if (!SelectConfigFiles(userDoption, progname))
-			proc_exit(1);
-	}
+	/* Acquire configuration parameters */
+	if (!SelectConfigFiles(userDoption, progname))
+		proc_exit(1);
+
+	/*
+	 * Validate we have been given a reasonable-looking DataDir and change
+	 * into it.
+	 */
+	checkDataDir();
+	ChangeToDataDir();
+
+	/*
+	 * Create lockfile for data directory.
+	 */
+	CreateDataDirLockFile(false);
+
+	/* read control file (error checking and contains config ) */
+	LocalProcessControlFile(false);
+
+	/* Initialize MaxBackends */
+	InitializeMaxBackends();
+
+	CreateSharedMemoryAndSemaphores();
+
+	/*
+	 * Remember stand-alone backend startup time,roughly at the same point
+	 * during startup that postmaster does so.
+	 */
+	PgStartTime = GetCurrentTimestamp();
+
+	/*
+	 * Create a per-backend PGPROC struct in shared memory. We must do this
+	 * before we can use LWLocks.
+	 */
+	InitProcess();
+
+	/*
+	 * Now that sufficient infrastructure has been initialized, PostgresMain()
+	 * can do the rest.
+	 */
+	PostgresMain(dbname, username);
+}
+
+
+/* ----------------------------------------------------------------
+ * PostgresMain
+ *	   postgres main loop -- all backends, interactive or otherwise loop here
+ *
+ * dbname is the name of the database to connect to, username is the
+ * PostgreSQL user name to be used for the session.
+ *
+ * NB: Single user mode specific setup should go to PostgresSingleUserMain()
+ * if reasonably possible.
+ * ----------------------------------------------------------------
+ */
+void
+PostgresMain(const char *dbname, const char *username)
+{
+	int			firstchar;
+	StringInfoData input_message;
+	sigjmp_buf	local_sigjmp_buf;
+	volatile bool send_ready_for_query = true;
+	bool		idle_in_transaction_timeout_enabled = false;
+	bool		idle_session_timeout_enabled = false;
+
+	AssertArg(dbname != NULL);
+	AssertArg(username != NULL);
+
+	SetProcessingMode(InitProcessing);
 
 	/*
 	 * Set up signal handlers.  (InitPostmasterChild or InitStandaloneProcess
@@ -4029,43 +4082,6 @@ PostgresMain(int argc, char *argv[],
 									 * platforms */
 	}
 
-	if (!IsUnderPostmaster)
-	{
-		/*
-		 * Validate we have been given a reasonable-looking DataDir (if under
-		 * postmaster, assume postmaster did this already).
-		 */
-		checkDataDir();
-
-		/* Change into DataDir (if under postmaster, was done already) */
-		ChangeToDataDir();
-
-		/*
-		 * Create lockfile for data directory.
-		 */
-		CreateDataDirLockFile(false);
-
-		/* read control file (error checking and contains config ) */
-		LocalProcessControlFile(false);
-
-		/* Initialize MaxBackends (if under postmaster, was done already) */
-		InitializeMaxBackends();
-
-		CreateSharedMemoryAndSemaphores();
-
-		/*
-		 * Remember stand-alone backend startup time, roughly at the same
-		 * point during startup that postmaster does so.
-		 */
-		PgStartTime = GetCurrentTimestamp();
-
-		/*
-		 * Create a per-backend PGPROC struct in shared memory. We must do
-		 * this before we can use LWLocks.
-		 */
-		InitProcess();
-	}
-
 	/* Early initialization */
 	BaseInit();
 

src/include/tcop/tcopprot.h

@@ -75,8 +75,9 @@ extern void ProcessClientWriteInterrupt(bool blocked);
 
 extern void process_postgres_switches(int argc, char *argv[],
 									  GucContext ctx, const char **dbname);
-extern void PostgresMain(int argc, char *argv[],
-						 const char *dbname,
+extern void PostgresSingleUserMain(int argc, char *argv[],
+								   const char *username) pg_attribute_noreturn();
+extern void PostgresMain(const char *dbname,
 						 const char *username) pg_attribute_noreturn();
 extern long get_stack_depth_rlimit(void);
 extern void ResetUsage(void);