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diff-delta: set size out-parameter to 0 for NULL delta 

When we cannot generate a delta, we return NULL but leave delta_size
untouched. This is generally OK, as callers rely on NULL to decide if
the output is usable or not. But it can confuse compilers; in
particular, gcc 9.2.1 with "-flto -O3" complains in fast-import's
store_object() that delta_len may be used uninitialized.

Let's change the diff-delta code to set the size explicitly to 0 for a
NULL return. That silences the compiler and makes it easier to reason
about the result.

Reported-by: Stephan Beyer <s-beyer@gmx.net>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Jeff King 2019-09-05 18:53:37 -04:00 committed by Junio C Hamano
parent 7140414d8b
commit e4b369069e
1 changed files with 2 additions and 0 deletions
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diff-delta.c
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@ -326,6 +326,8 @@ create_delta(const struct delta_index *index,
const unsigned char *ref_data, *ref_top, *data, *top;
unsigned char *out;
*delta_size = 0;
if (!trg_buf || !trg_size)
return NULL;