reftable: prevent 'update_index' changes after header write

The function `reftable_writer_set_limits()` allows updating the
'min_update_index' and 'max_update_index' of a reftable writer. These
values are written to both the writer's header and footer.

Since the header is written during the first block write, any subsequent
changes to the update index would create a mismatch between the header
and footer values. The footer would contain the newer values while the
header retained the original ones. To fix this bug, we now prevent
callers from updating these values after the header has been written.

Signed-off-by: Karthik Nayak <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: KarthikNayak

reftable/writer.c

@@ -182,6 +182,13 @@ int reftable_writer_new(struct reftable_writer **out,
 void reftable_writer_set_limits(struct reftable_writer *w, uint64_t min,
 				uint64_t max)
 {
+	/*
+	 * The limits shouldn't be modified post writing the first block, else
+	 * it would cause a mismatch between the header and the footer.
+	 */
+	if (w->next)
+		BUG("update index modified after writing first block");
+
 	w->min_update_index = min;
 	w->max_update_index = max;
 }