diff: clarify textconv interface

The memory allocation scheme for the textconv interface is a
bit tricky, and not well documented. It was originally
designed as an internal part of diff.c (matching
fill_mmfile), but gradually was made public.

Refactoring it is difficult, but we can at least improve the
situation by documenting the intended flow and enforcing it
with an in-code assertion.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 4 insertions, 1 deletions

diff --git a/diff.c b/diff.c
index f62b7f7..a09b0b6 100644
--- a/diff.c
+++ b/diff.c
@@ -4996,7 +4996,7 @@ size_t fill_textconv(struct userdiff_driver *driver,
 {
 	size_t size;
 
-	if (!driver || !driver->textconv) {
+	if (!driver) {
 		if (!DIFF_FILE_VALID(df)) {
 			*outbuf = "";
 			return 0;
@@ -5007,6 +5007,9 @@ size_t fill_textconv(struct userdiff_driver *driver,
 		return df->size;
 	}
 
+	if (!driver->textconv)
+		die("BUG: fill_textconv called with non-textconv driver");
+
 	if (driver->textconv_cache && df->sha1_valid) {
 		*outbuf = notes_cache_get(driver->textconv_cache, df->sha1,
 					  &size);