Process session_preload_libraries within InitPostgres's transaction.

Previously we did this after InitPostgres, at a somewhat randomly chosen
place within PostgresMain.  However, since commit a0ffa885e doing this
outside a transaction can cause a crash, if we need to check permissions
while replacing a placeholder GUC.  (Besides which, a preloaded library
could itself want to do database access within _PG_init.)

To avoid needing an additional transaction start/end in every session,
move the process_session_preload_libraries call to within InitPostgres's
transaction.  That requires teaching the code not to call it when
InitPostgres is called from somewhere other than PostgresMain, since
we don't want session_preload_libraries to affect background workers.
The most future-proof solution here seems to be to add an additional
flag parameter to InitPostgres; fortunately, we're not yet very worried
about API stability for v15.

Doing this also exposed the fact that we're currently honoring
session_preload_libraries in walsenders, even those not connected to
any database.  This seems, at minimum, a POLA violation: walsenders
are not interactive sessions.  Let's stop doing that.

(All these comments also apply to local_preload_libraries, of course.)

Per report from Gurjeet Singh (thanks also to Nathan Bossart and Kyotaro
Horiguchi for review).  Backpatch to v15 where a0ffa885e came in.

Discussion: https://postgr.es/m/CABwTF4VEpwTHhRQ+q5MiC5ucngN-whN-PdcKeufX7eLSoAfbZA@mail.gmail.com

src/backend/bootstrap/bootstrap.c

@@ -354,7 +354,7 @@ BootstrapModeMain(int argc, char *argv[], bool check_only)
 	if (pg_link_canary_is_frontend())
 		elog(ERROR, "backend is incorrectly linked to frontend functions");
 
-	InitPostgres(NULL, InvalidOid, NULL, InvalidOid, NULL, false);
+	InitPostgres(NULL, InvalidOid, NULL, InvalidOid, false, false, NULL);
 
 	/* Initialize stuff for bootstrap-file processing */
 	for (i = 0; i < MAXATTR; i++)

src/backend/postmaster/autovacuum.c

@@ -475,7 +475,7 @@ AutoVacLauncherMain(int argc, char *argv[])
 	/* Early initialization */
 	BaseInit();
 
-	InitPostgres(NULL, InvalidOid, NULL, InvalidOid, NULL, false);
+	InitPostgres(NULL, InvalidOid, NULL, InvalidOid, false, false, NULL);
 
 	SetProcessingMode(NormalProcessing);
 
@@ -1694,12 +1694,13 @@ AutoVacWorkerMain(int argc, char *argv[])
 		pgstat_report_autovac(dbid);
 
 		/*
-		 * Connect to the selected database
+		 * Connect to the selected database, specifying no particular user
 		 *
 		 * Note: if we have selected a just-deleted database (due to using
 		 * stale stats info), we'll fail and exit here.
 		 */
-		InitPostgres(NULL, dbid, NULL, InvalidOid, dbname, false);
+		InitPostgres(NULL, dbid, NULL, InvalidOid, false, false,
+					 dbname);
 		SetProcessingMode(NormalProcessing);
 		set_ps_display(dbname);
 		ereport(DEBUG1,

src/backend/postmaster/postmaster.c

@@ -5654,7 +5654,11 @@ BackgroundWorkerInitializeConnection(const char *dbname, const char *username, u
 				(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
 				 errmsg("database connection requirement not indicated during registration")));
 
-	InitPostgres(dbname, InvalidOid, username, InvalidOid, NULL, (flags & BGWORKER_BYPASS_ALLOWCONN) != 0);
+	InitPostgres(dbname, InvalidOid,	/* database to connect to */
+				 username, InvalidOid,	/* role to connect as */
+				 false,			/* never honor session_preload_libraries */
+				 (flags & BGWORKER_BYPASS_ALLOWCONN) != 0,	/* ignore datallowconn? */
+				 NULL);			/* no out_dbname */
 
 	/* it had better not gotten out of "init" mode yet */
 	if (!IsInitProcessingMode())
@@ -5677,7 +5681,11 @@ BackgroundWorkerInitializeConnectionByOid(Oid dboid, Oid useroid, uint32 flags)
 				(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
 				 errmsg("database connection requirement not indicated during registration")));
 
-	InitPostgres(NULL, dboid, NULL, useroid, NULL, (flags & BGWORKER_BYPASS_ALLOWCONN) != 0);
+	InitPostgres(NULL, dboid,	/* database to connect to */
+				 NULL, useroid, /* role to connect as */
+				 false,			/* never honor session_preload_libraries */
+				 (flags & BGWORKER_BYPASS_ALLOWCONN) != 0,	/* ignore datallowconn? */
+				 NULL);			/* no out_dbname */
 
 	/* it had better not gotten out of "init" mode yet */
 	if (!IsInitProcessingMode())

src/backend/tcop/postgres.c

@@ -4076,7 +4076,11 @@ PostgresMain(const char *dbname, const char *username)
 	 * it inside InitPostgres() instead.  In particular, anything that
 	 * involves database access should be there, not here.
 	 */
-	InitPostgres(dbname, InvalidOid, username, InvalidOid, NULL, false);
+	InitPostgres(dbname, InvalidOid,	/* database to connect to */
+				 username, InvalidOid,	/* role to connect as */
+				 !am_walsender, /* honor session_preload_libraries? */
+				 false,			/* don't ignore datallowconn */
+				 NULL);			/* no out_dbname */
 
 	/*
 	 * If the PostmasterContext is still around, recycle the space; we don't
@@ -4112,12 +4116,6 @@ PostgresMain(const char *dbname, const char *username)
 	if (am_walsender)
 		InitWalSender();
 
-	/*
-	 * process any libraries that should be preloaded at backend start (this
-	 * likewise can't be done until GUC settings are complete)
-	 */
-	process_session_preload_libraries();
-
 	/*
 	 * Send this backend's cancellation info to the frontend.
 	 */

src/backend/utils/init/postinit.c

@@ -622,29 +622,48 @@ BaseInit(void)
  * InitPostgres
  *		Initialize POSTGRES.
  *
+ * Parameters:
+ *	in_dbname, dboid: specify database to connect to, as described below
+ *	username, useroid: specify role to connect as, as described below
+ *	load_session_libraries: TRUE to honor [session|local]_preload_libraries
+ *	override_allow_connections: TRUE to connect despite !datallowconn
+ *	out_dbname: optional output parameter, see below; pass NULL if not used
+ *
  * The database can be specified by name, using the in_dbname parameter, or by
- * OID, using the dboid parameter.  In the latter case, the actual database
+ * OID, using the dboid parameter.  Specify NULL or InvalidOid respectively
+ * for the unused parameter.  If dboid is provided, the actual database
  * name can be returned to the caller in out_dbname.  If out_dbname isn't
  * NULL, it must point to a buffer of size NAMEDATALEN.
  *
- * Similarly, the username can be passed by name, using the username parameter,
+ * Similarly, the role can be passed by name, using the username parameter,
  * or by OID using the useroid parameter.
  *
- * In bootstrap mode no parameters are used.  The autovacuum launcher process
+ * In bootstrap mode the database and username parameters are NULL/InvalidOid.
- * doesn't use any parameters either, because it only goes far enough to be
+ * The autovacuum launcher process doesn't specify these parameters either,
- * able to read pg_database; it doesn't connect to any particular database.
+ * because it only goes far enough to be able to read pg_database; it doesn't
- * In walsender mode only username is used.
+ * connect to any particular database.  An autovacuum worker specifies a
+ * database but not a username; conversely, a physical walsender specifies
+ * username but not database.
  *
- * As of PostgreSQL 8.2, we expect InitProcess() was already called, so we
+ * By convention, load_session_libraries should be passed as true in
- * already have a PGPROC struct ... but it's not completely filled in yet.
+ * "interactive" sessions (including standalone backends), but false in
+ * background processes such as autovacuum.  Note in particular that it
+ * shouldn't be true in parallel worker processes; those have another
+ * mechanism for replicating their leader's set of loaded libraries.
+ *
+ * We expect that InitProcess() was already called, so we already have a
+ * PGPROC struct ... but it's not completely filled in yet.
  *
  * Note:
  *		Be very careful with the order of calls in the InitPostgres function.
  * --------------------------------
  */
 void
-InitPostgres(const char *in_dbname, Oid dboid, const char *username,
+InitPostgres(const char *in_dbname, Oid dboid,
-			 Oid useroid, char *out_dbname, bool override_allow_connections)
+			 const char *username, Oid useroid,
+			 bool load_session_libraries,
+			 bool override_allow_connections,
+			 char *out_dbname)
 {
 	bool		bootstrap = IsBootstrapProcessingMode();
 	bool		am_superuser;
@@ -1108,6 +1127,16 @@ InitPostgres(const char *in_dbname, Oid dboid, const char *username,
 	/* Initialize this backend's session state. */
 	InitializeSession();
 
+	/*
+	 * If this is an interactive session, load any libraries that should be
+	 * preloaded at backend start.  Since those are determined by GUCs, this
+	 * can't happen until GUC settings are complete, but we want it to happen
+	 * during the initial transaction in case anything that requires database
+	 * access needs to be done.
+	 */
+	if (load_session_libraries)
+		process_session_preload_libraries();
+
 	/* report this backend in the PgBackendStatus array */
 	if (!bootstrap)
 		pgstat_bestart();

src/include/miscadmin.h

@@ -449,8 +449,11 @@ extern PGDLLIMPORT AuxProcType MyAuxProcType;
 /* in utils/init/postinit.c */
 extern void pg_split_opts(char **argv, int *argcp, const char *optstr);
 extern void InitializeMaxBackends(void);
-extern void InitPostgres(const char *in_dbname, Oid dboid, const char *username,
+extern void InitPostgres(const char *in_dbname, Oid dboid,
-						 Oid useroid, char *out_dbname, bool override_allow_connections);
+						 const char *username, Oid useroid,
+						 bool load_session_libraries,
+						 bool override_allow_connections,
+						 char *out_dbname);
 extern void BaseInit(void);
 
 /* in utils/init/miscinit.c */