fix a race in xyz to lab

the table build had a race condition

.gitignore

@@ -125,4 +125,5 @@ doc/reference/sgml.stamp
 doc/reference/xml/
 doc/html
 doc/pdf
+gtkdocerrors
 

ChangeLog

@@ -1,6 +1,7 @@
 26/7/11 started 7.26.1
 - doc fixups
 - oops, ==0 missing from a strcmp() in vips7compat
+- fixed a race in im_XYZ2Lab() table build
 
 26/7/11 started 7.26.0
 - version bunp for 7.26

libvips/colour/im_LabQ2disp.c

@@ -1,10 +1,6 @@
-/* @(#) Turn Lab 32bit packed format into displayable rgb. Fast, but very
- * @(#) inaccurate: for display only!
- * @(#) 
- * @(#) Usage:  
- * @(#)   int im_LabQ2disp( IMAGE *in, IMAGE *out, struct im_col_display *d )
- * @(#) 
- * @(#) Returns: -1 on error, else 0
+/* Turn Lab 32bit packed format into displayable rgb. Fast, but very
+ * inaccurate: for display only! Note especially that this dithers and will
+ * give different results on different runs.
  *
  * 5/11/97 Steve Perry
  *	- adapted from old ip code

libvips/colour/im_XYZ2Lab.c

@@ -15,6 +15,8 @@
  *	- ahem, build the LUT outside the eval thread
  * 2/11/09
  * 	- gtkdoc
+ * 3/8/11
+ * 	- fix a race in the table build
  */
 
 /*
@@ -74,15 +76,14 @@ imb_XYZ2Lab_tables( void )
 {
 	static int built_tables = 0;
 
-	int was_built;
 	int i;
 
 	g_mutex_lock( im__global_lock );
-	was_built = built_tables;
-	built_tables = 1;
-	g_mutex_unlock( im__global_lock );
-	if( was_built )
+
+	if( built_tables ) {
+		g_mutex_unlock( im__global_lock );
 		return;
+	}
 
 	for( i = 0; i < QUANT_ELEMENTS; i++ ) {
 		float Y = (double) i / QUANT_ELEMENTS;
@@ -92,6 +93,10 @@ imb_XYZ2Lab_tables( void )
 		else 
 			cbrt_table[i] = cbrt( Y );
 	}
+
+	built_tables = 1;
+
+	g_mutex_unlock( im__global_lock );
 }
 
 /* Process a buffer of data.