add vips_error_buffer_copy()

Add vips_error_buffer_copy() to fix a race in error buffer fetch.

See https://github.com/libvips/libvips/issues/1423

Thanks @dineshkannaa

ChangeLog

@@ -15,6 +15,7 @@
 - handle alpha in heifload / heifsave [meyermarcel]
 - add @interpretation and @format to rawload
 - nifti load/save uses double for all floating point metadata
+- add vips_error_buffer_copy()
 
 31/8/19 started 8.8.3
 - revert sharpen restoring the input colourspace

libvips/include/vips/error.h

@@ -36,6 +36,7 @@ extern "C" {
 #endif /*__cplusplus*/
 
 const char *vips_error_buffer( void );
+char *vips_error_buffer_copy( void );
 void vips_error_clear( void );
 
 void vips_error_freeze( void );

libvips/iofuncs/error.c

@@ -14,6 +14,8 @@
  * 	- gtkdoc comments
  * 24/6/10
  * 	- fmt to error_exit() may be NULL
+ * 12/9/19 [dineshkannaa]
+ * 	- add vips_error_buffer_copy()
  */
 
 /*
@@ -184,6 +186,26 @@ vips_error_buffer( void )
 	return( msg );
 }
 
+/**
+ * vips_error_buffer_copy: 
+ *
+ * Return a copy of the vips error buffer, and clear it. 
+ *
+ * Returns: a copy of the libvips error buffer
+ */
+char *
+vips_error_buffer_copy( void )
+{
+	char *msg;
+
+	g_mutex_lock( vips__global_lock );
+	msg = g_strdup( vips_buf_all( &vips_error_buf ) );
+	vips_error_clear();
+	g_mutex_unlock( vips__global_lock );
+
+	return( msg );
+}
+
 /* Some systems do not have va_copy() ... this might work (it does on MSVC),
  * apparently.
  *