vcs-svn: cap number of bytes read from sliding view

Introduce a "max_off" field in struct sliding_view, roughly
representing a maximum number of bytes that can be read from "file".
If it is set to a nonnegative integer, a call to move_window()
attempting to put the right endpoint beyond that offset will return
an error instead.

The idea is to use this when applying Subversion-format deltas to
prevent reads past the end of the preimage (which has known length).
Without such a check, corrupt deltas would cause svn-fe to block
indefinitely when data in the input pipe is exhausted.

Inspired-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>

test-svn-fe.c

@@ -15,7 +15,7 @@ static int apply_delta(int argc, char *argv[])
 {
 	struct line_buffer preimage = LINE_BUFFER_INIT;
 	struct line_buffer delta = LINE_BUFFER_INIT;
-	struct sliding_view preimage_view = SLIDING_VIEW_INIT(&preimage);
+	struct sliding_view preimage_view = SLIDING_VIEW_INIT(&preimage, -1);
 
 	if (argc != 5)
 		usage(test_svnfe_usage);

vcs-svn/sliding_window.c

@@ -54,6 +54,8 @@ int move_window(struct sliding_view *view, off_t off, size_t width)
 		return -1;
 	if (off < view->off || off + width < view->off + view->width)
 		return error("invalid delta: window slides left");
+	if (view->max_off >= 0 && view->max_off < off + width)
+		return error("delta preimage ends early");
 
 	file_offset = view->off + view->buf.len;
 	if (off < file_offset) {

vcs-svn/sliding_window.h

@@ -7,10 +7,11 @@ struct sliding_view {
 	struct line_buffer *file;
 	off_t off;
 	size_t width;
+	off_t max_off;	/* -1 means unlimited */
 	struct strbuf buf;
 };
 
-#define SLIDING_VIEW_INIT(input)	{ (input), 0, 0, STRBUF_INIT }
+#define SLIDING_VIEW_INIT(input, len)	{ (input), 0, 0, (len), STRBUF_INIT }
 
 extern int move_window(struct sliding_view *view, off_t off, size_t width);