start up threadpool later

might help php and ruby web frameworks

see eg. https://github.com/libvips/php-vips-ext/issues/42

ChangeLog

@@ -1,9 +1,12 @@
-14/8/20 started 8.11.2
+14/7/21 started 8.11.3
+- build threadpool later [kleisauke]
+
+15/6/20 started 8.11.2
 - better libdir guessing [remi]
 - fix tiff pyramid creation with jp2k compression (was broken by 8.11.1)
 - don't load modules if we're built without modules
 
-14/8/20 started 8.11.1
+18/6/21 started 8.11.1
 - add more example code to C docs
 - update libtool support in configure.ac 
 - more startup info if VIPS_INFO is set

configure.ac

@@ -2,7 +2,7 @@
 
 # also update the version number in the m4 macros below
 
-AC_INIT([vips],[8.11.2],[vipsip@jiscmail.ac.uk])
+AC_INIT([vips],[8.11.3],[vipsip@jiscmail.ac.uk])
 # required for gobject-introspection
 AC_PREREQ([2.69])
 
@@ -18,7 +18,7 @@ AC_CONFIG_MACRO_DIR([m4])
 # user-visible library versioning
 m4_define([vips_major_version], [8])
 m4_define([vips_minor_version], [11])
-m4_define([vips_micro_version], [2])
+m4_define([vips_micro_version], [3])
 m4_define([vips_version],
           [vips_major_version.vips_minor_version.vips_micro_version])
 
@@ -43,7 +43,7 @@ VIPS_LIBS=""
 #   binary interface changes not backwards compatible?: reset age to 0
 
 LIBRARY_CURRENT=55
-LIBRARY_REVISION=1
+LIBRARY_REVISION=2
 LIBRARY_AGE=13
 
 # patched into include/vips/version.h

libvips/iofuncs/threadpool.c

@@ -731,6 +731,19 @@ vips_task_free( VipsTask *task )
 	VIPS_FREE( task );
 }
 
+static void *
+vips__thread_once_init( void *data )
+{
+	/* We can have many more than vips__concurrency threads -- each active
+	 * pipeline will make vips__concurrency more, see
+	 * vips_threadpool_run().
+	 */
+	vips__pool = g_thread_pool_new( vips_thread_main_loop, NULL,
+		-1, FALSE, NULL );
+
+	return( NULL );
+}
+
 /**
  * vips__thread_execute:
  * @name: a name for the thread
@@ -747,10 +760,14 @@ vips_task_free( VipsTask *task )
 int
 vips__thread_execute( const char *name, GFunc func, gpointer data )
 {
+	static GOnce once = G_ONCE_INIT;
+
 	VipsThreadExec *exec;
 	GError *error = NULL;
 	gboolean result;
 
+	VIPS_ONCE( &once, vips__thread_once_init, NULL );
+
 	exec = g_new( VipsThreadExec, 1 );
 	exec->name = name;
 	exec->func = func;
@@ -948,12 +965,12 @@ vips__threadpool_init( void )
 	if( vips__concurrency == 0 )
 		vips__concurrency = vips__concurrency_get_default();
 
-	/* We can have many more than vips__concurrency threads -- each active
-	 * pipeline will make vips__concurrency more, see
-	 * vips_threadpool_run().
+	/* The threadpool is built in the first vips__thread_execute()
+	 * call, since we want thread creation to happen as late as possible.
+	 *
+	 * Many web platforms start up in a base environment, then fork() for
+	 * each request. We must not make the threadpool before the fork.
 	 */
-	vips__pool = g_thread_pool_new( vips_thread_main_loop, NULL,
-		-1, FALSE, NULL );
 
 	VIPS_DEBUG_MSG( "vips__threadpool_init: (%p)\n", vips__pool );
 }