Added semaphore to resize_photo to prevent resource issues on server.

src/photo.c

@@ -8,6 +8,7 @@
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <unistd.h>
+#include <fcntl.h>
 #include <jpeg.h>
 #include <settings.h>
 
@@ -460,6 +461,22 @@ char* resize_photo(	struct photo_meta* dest,
 		}
 	}
 
+	/*
+	 * Check for semaphore... if it exists, wait for it to disappear.
+	 * For this, we use the open() syscall with O_CREAT|O_EXCL, which
+	 * will fail if the file already exists.
+	 */
+	char* sem_file = construct_path( "s/s/s", get_cgidir(),
+						CACHEDIR, "semaphore" );
+	int fd = open( sem_file, O_CREAT|O_EXCL, S_IRUSR|S_IWUSR );
+	while( fd == -1 ) {
+		sleep(1);
+		fd = open( sem_file, O_CREAT|O_EXCL, S_IRUSR|S_IWUSR );
+		dprintf( "resize_photo: waiting for semaphore %s (fd: %d)\n",
+			sem_file, fd );
+	}
+	close(fd);
+	
 	MagickWand* wand = NewMagickWand();
 	dprintf( "resize_photo: wand created at %p\n", wand);
 	if (MagickReadImage( wand, dest->filepath ) == MagickFalse) {
@@ -467,6 +484,11 @@ char* resize_photo(	struct photo_meta* dest,
 		char* description=MagickGetException(wand,&severity);
 		dprintf("%s %s %ld %s\n",GetMagickModule(),description);
 		gcfree( cache_file );
+	
+		/* Get rid of the semaphore */
+		unlink( sem_file );
+		gcfree( sem_file );
+
 		return NULL;
 	}
 	
@@ -496,11 +518,21 @@ char* resize_photo(	struct photo_meta* dest,
 		ExceptionType severity;
 		char* description=MagickGetException(wand,&severity);
 		dprintf("%s %s %ld %s\n",GetMagickModule(),description);
+	
+		/* Get rid of the semaphore */
+		unlink( sem_file );
+		gcfree( sem_file );
+
 		gcfree( cache_file );
 		cache_file = NULL;
 	}
 
 	DestroyMagickWand( wand );
+	
+	/* Get rid of the semaphore */
+	unlink( sem_file );
+	gcfree( sem_file );
+
 	gcfree( cache_path );
 	dprintf( "resize_photo: resized photo at %s (%p)\n", cache_file, cache_file );
 	return cache_file;