reftable/block: fix error handling when searching restart points

When doing the binary search over restart points in a block we need to
decode the record keys. This decoding step can result in an error when
the block is corrupted, which we indicate to the caller of the binary
search by setting `args.error = 1`. But the only caller that exists
mishandles this because it in fact performs the error check before
calling `binsearch()`.

Fix this bug by checking for errors at the right point in time.
Furthermore, refactor `binsearch()` so that it aborts the search in case
the callback function returns a negative value so that we don't
needlessly continue to search the block.

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

reftable/basics.c

@@ -39,8 +39,11 @@ size_t binsearch(size_t sz, int (*f)(size_t k, void *args), void *args)
 	 */
 	while (hi - lo > 1) {
 		size_t mid = lo + (hi - lo) / 2;
+		int ret = f(mid, args);
+		if (ret < 0)
+			return sz;
 
-		if (f(mid, args))
+		if (ret > 0)
 			hi = mid;
 		else
 			lo = mid;

reftable/basics.h

@@ -22,8 +22,9 @@ uint32_t get_be24(uint8_t *in);
 void put_be16(uint8_t *out, uint16_t i);
 
 /*
- * find smallest index i in [0, sz) at which f(i) is true, assuming
- * that f is ascending. Return sz if f(i) is false for all indices.
+ * find smallest index i in [0, sz) at which `f(i) > 0`, assuming that f is
+ * ascending. Return sz if `f(i) == 0` for all indices. The search is aborted
+ * and `sz` is returned in case `f(i) < 0`.
  *
  * Contrary to bsearch(3), this returns something useful if the argument is not
  * found.

reftable/block.c

@@ -387,11 +387,6 @@ int block_reader_seek(struct block_reader *br, struct block_iter *it,
 	int err = 0;
 	size_t i;
 
-	if (args.error) {
-		err = REFTABLE_FORMAT_ERROR;
-		goto done;
-	}
-
 	/*
 	 * Perform a binary search over the block's restart points, which
 	 * avoids doing a linear scan over the whole block. Like this, we
@@ -405,6 +400,10 @@ int block_reader_seek(struct block_reader *br, struct block_iter *it,
 	 * too many record.
 	 */
 	i = binsearch(br->restart_count, &restart_needle_less, &args);
+	if (args.error) {
+		err = REFTABLE_FORMAT_ERROR;
+		goto done;
+	}
 
 	/*
 	 * Now there are multiple cases: