Add session_preload_libraries configuration parameter

This is like shared_preload_libraries except that it takes effect at
backend start and can be changed without a full postmaster restart.  It
is like local_preload_libraries except that it is still only settable by
a superuser.  This can be a better way to load modules such as
auto_explain.

Since there are now three preload parameters, regroup the documentation
a bit.  Put all parameters into one section, explain common
functionality only once, update the descriptions to reflect current and
future realities.
Reviewed-by: Dimitri Fontaine <dimitri@2ndQuadrant.fr>

doc/src/sgml/auto-explain.sgml

@@ -24,7 +24,8 @@ LOAD 'auto_explain';
 </programlisting>
 
   (You must be superuser to do that.)  More typical usage is to preload
-  it into all sessions by including <literal>auto_explain</> in
+  it into some or all sessions by including <literal>auto_explain</> in
+  <xref linkend="guc-session-preload-libraries"> or
   <xref linkend="guc-shared-preload-libraries"> in
   <filename>postgresql.conf</>.  Then you can track unexpectedly slow queries
   no matter when they happen.  Of course there is a price in overhead for
@@ -185,7 +186,7 @@ LOAD 'auto_explain';
 
 <programlisting>
 # postgresql.conf
-shared_preload_libraries = 'auto_explain'
+session_preload_libraries = 'auto_explain'
 
 auto_explain.log_min_duration = '3s'
 </programlisting>

src/backend/tcop/postgres.c

@@ -3743,7 +3743,7 @@ PostgresMain(int argc, char *argv[],
 	 * process any libraries that should be preloaded at backend start (this
 	 * likewise can't be done until GUC settings are complete)
 	 */
-	process_local_preload_libraries();
+	process_session_preload_libraries();
 
 	/*
 	 * Send this backend's cancellation info to the frontend.

src/backend/utils/init/miscinit.c

@@ -1222,6 +1222,7 @@ ValidatePgVersion(const char *path)
  * GUC variables: lists of library names to be preloaded at postmaster
  * start and at backend start
  */
+char	   *session_preload_libraries_string = NULL;
 char	   *shared_preload_libraries_string = NULL;
 char	   *local_preload_libraries_string = NULL;
 
@@ -1318,8 +1319,11 @@ process_shared_preload_libraries(void)
  * process any libraries that should be preloaded at backend start
  */
 void
-process_local_preload_libraries(void)
+process_session_preload_libraries(void)
 {
+	load_libraries(session_preload_libraries_string,
+				   "session_preload_libraries",
+				   false);
 	load_libraries(local_preload_libraries_string,
 				   "local_preload_libraries",
 				   true);

src/backend/utils/misc/guc.c

@@ -591,6 +591,8 @@ const char *const config_group_names[] =
 	gettext_noop("Client Connection Defaults / Statement Behavior"),
 	/* CLIENT_CONN_LOCALE */
 	gettext_noop("Client Connection Defaults / Locale and Formatting"),
+	/* CLIENT_CONN_PRELOAD */
+	gettext_noop("Client Connection Defaults / Shared Library Preloading"),
 	/* CLIENT_CONN_OTHER */
 	gettext_noop("Client Connection Defaults / Other Defaults"),
 	/* LOCK_MANAGEMENT */
@@ -2770,7 +2772,18 @@ static struct config_string ConfigureNamesString[] =
 	},
 
 	{
-		{"shared_preload_libraries", PGC_POSTMASTER, RESOURCES_KERNEL,
+		{"session_preload_libraries", PGC_SUSET, CLIENT_CONN_PRELOAD,
+			gettext_noop("Lists shared libraries to preload into each backend."),
+			NULL,
+			GUC_LIST_INPUT | GUC_LIST_QUOTE | GUC_SUPERUSER_ONLY
+		},
+		&session_preload_libraries_string,
+		"",
+		NULL, NULL, NULL
+	},
+
+	{
+		{"shared_preload_libraries", PGC_POSTMASTER, CLIENT_CONN_PRELOAD,
 			gettext_noop("Lists shared libraries to preload into server."),
 			NULL,
 			GUC_LIST_INPUT | GUC_LIST_QUOTE | GUC_SUPERUSER_ONLY
@@ -2781,8 +2794,8 @@ static struct config_string ConfigureNamesString[] =
 	},
 
 	{
-		{"local_preload_libraries", PGC_BACKEND, CLIENT_CONN_OTHER,
-			gettext_noop("Lists shared libraries to preload into each backend."),
+		{"local_preload_libraries", PGC_BACKEND, CLIENT_CONN_PRELOAD,
+			gettext_noop("Lists unprivileged shared libraries to preload into each backend."),
 			NULL,
 			GUC_LIST_INPUT | GUC_LIST_QUOTE
 		},

src/include/miscadmin.h

@@ -403,6 +403,7 @@ extern void BaseInit(void);
 /* in utils/init/miscinit.c */
 extern bool IgnoreSystemIndexes;
 extern PGDLLIMPORT bool process_shared_preload_libraries_in_progress;
+extern char *session_preload_libraries_string;
 extern char *shared_preload_libraries_string;
 extern char *local_preload_libraries_string;
 
@@ -438,7 +439,7 @@ extern void TouchSocketLockFiles(void);
 extern void AddToDataDirLockFile(int target_line, const char *str);
 extern void ValidatePgVersion(const char *path);
 extern void process_shared_preload_libraries(void);
-extern void process_local_preload_libraries(void);
+extern void process_session_preload_libraries(void);
 extern void pg_bindtextdomain(const char *domain);
 extern bool has_rolreplication(Oid roleid);
 

src/include/utils/guc_tables.h

@@ -88,6 +88,7 @@ enum config_group
 	CLIENT_CONN,
 	CLIENT_CONN_STATEMENT,
 	CLIENT_CONN_LOCALE,
+	CLIENT_CONN_PRELOAD,
 	CLIENT_CONN_OTHER,
 	LOCK_MANAGEMENT,
 	COMPAT_OPTIONS,