Add .INCOMPLETE file marker to flexnbd listen

We drop a marker onto the filesystem to say when we know the image we're
serving is not yet ready.

README.txt

@@ -81,6 +81,12 @@ the source in the interim.
 To support transparently replacing an existing server, flexnbd can
 switch addresses once it has received a successful migration. 
 
+When `flexnbd listen` is run, it will create a file named
+<FILE>.INCOMPLETE next to FILE.  This will be removed when a migration
+has successfully completed.  The .INCOMPLETE file will be created before
+the listening socket is opened.
+
+
 Options
 ^^^^^^^
 As for 'serve', with these additions:

src/flexnbd.c

@@ -283,6 +283,85 @@ int flexnbd_default_deny( struct flexnbd * flexnbd )
 }
 
 
+char * flexnbd_incomplete_filename( struct flexnbd * flexnbd )
+{
+	NULLCHECK( flexnbd );
+	struct server * serve = flexnbd_server( flexnbd );
+
+	return serve->filename_incomplete;
+}
+
+void make_writable( const char * filename )
+{
+	NULLCHECK( filename );
+	FATAL_IF_NEGATIVE( chmod( filename, S_IWUSR ),
+			"Couldn't chmod %s: %s",
+			filename,
+			strerror( errno ) );
+}
+
+/** Drops a marker file on the filesystem to show that the image we're
+ * serving hasn't yet finished its migration yet
+ */
+void flexnbd_mark_incomplete( struct flexnbd * flexnbd )
+{
+	char * filename = 
+		flexnbd_incomplete_filename( flexnbd );
+	int fd;
+
+	NULLCHECK( filename );
+
+	/* It's OK if the file already exists - it's perfectly possible
+	 * that a previous process died part-way through and left it
+	 * behind.  However, we might have left the file mode in a bad
+	 * state.
+	 */
+
+	struct stat ignored;
+	int exists = stat( filename, &ignored ) == 0;
+	if ( exists ) {
+		/* definitely there, need to chmod */
+		debug( "%s exists, making it writable", filename );
+		make_writable( filename );
+	}
+	else if ( ENOENT != errno ) {
+		/* Can't tell if it's there or not, weirdness. */
+		fatal( "Unable to stat %s", filename );
+	}
+	else { /* definitely not there. NOP. */ }
+
+	
+	fd = open( filename, O_CREAT|O_WRONLY, 0 );
+	FATAL_IF_NEGATIVE( fd, 
+			"Couldn't open %s: %s", 
+			filename,
+			strerror( errno ) );
+
+	/* Minor race here - in principle we could see the file
+	 * disappear before the chmod */
+	close( fd );
+}
+
+
+/** Removes the .INCOMPLETE marker file from the filesystem. Call this
+ * only when you know the migration has completed successfully.
+ */
+void flexnbd_mark_complete( struct flexnbd * flexnbd )
+{
+	char * filename = 
+		flexnbd_incomplete_filename( flexnbd );
+
+	NULLCHECK( filename );
+
+	make_writable( filename );
+
+	FATAL_IF_NEGATIVE( unlink( filename ),
+			"Couldn't unlink %s: %s",
+			filename,
+			strerror( errno ) );
+}
+
+
 int flexnbd_serve( struct flexnbd * flexnbd )
 {
 	NULLCHECK( flexnbd );

src/flexnbd.h

@@ -69,6 +69,9 @@ void flexnbd_set_server( struct flexnbd * flexnbd, struct server * serve );
 void flexnbd_switch( struct flexnbd * flexnbd, struct server *(listen_cb)(struct listen *) );
 int flexnbd_signal_fd( struct flexnbd * flexnbd );
 
+void flexnbd_mark_incomplete( struct flexnbd * flexnbd );
+void flexnbd_mark_complete( struct flexnbd * flexnbd );
+
 
 int flexnbd_serve( struct flexnbd * flexnbd );
 struct server * flexnbd_server( struct flexnbd * flexnbd );

src/listen.c

@@ -83,6 +83,7 @@ int do_listen( struct listen * listen )
 	flexnbd_lock_switch( listen->flexnbd );
 	{
 		flexnbd_set_server( listen->flexnbd, listen->init_serve );
+		flexnbd_mark_incomplete( listen->flexnbd );
 	}
 	flexnbd_unlock_switch( listen->flexnbd );
 
@@ -93,6 +94,8 @@ int do_listen( struct listen * listen )
 
 
 	if( have_control ) {
+		flexnbd_mark_complete( listen->flexnbd );
+
 		info( "Taking control.");
 		flexnbd_switch( listen->flexnbd, listen_switch );
 		/* WATCH FOR RACES HERE: the server hasn't been