[PATCH 03/11] reftable/block: check deflateInit() return value

["Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com>]

From: Johannes Schindelin <johannes.schindelin@gmx.de>

block_writer_init() allocates a z_stream and calls deflateInit()
to prepare it for compressing log records. The return value of
deflateInit() is silently discarded. If zlib initialization fails
(e.g., Z_MEM_ERROR when the system is under memory pressure), the
z_stream is left in an undefined state.

Subsequent deflate() calls in block_writer_finish() then operate
on this uninitialized stream. Depending on the zlib
implementation, this can produce silently corrupted compressed
data (which would be written to the reftable file and discovered
only when a later reader fails to inflate) or crash outright.

The function already uses REFTABLE_ZLIB_ERROR for deflate()
failures later in the code path (lines 171, 199), so returning
the same error code for deflateInit() failure is consistent.

Pointed out by Coverity.

Assisted-by: Claude Opus 4.6
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 reftable/block.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/reftable/block.c b/reftable/block.c
index 920b3f4486..ec81fd0493 100644
--- a/reftable/block.c
+++ b/reftable/block.c
@@ -87,7 +87,8 @@ int block_writer_init(struct block_writer *bw, uint8_t typ, uint8_t *block,
 		REFTABLE_CALLOC_ARRAY(bw->zstream, 1);
 		if (!bw->zstream)
 			return REFTABLE_OUT_OF_MEMORY_ERROR;
-		deflateInit(bw->zstream, 9);
+		if (deflateInit(bw->zstream, 9) != Z_OK)
+			return REFTABLE_ZLIB_ERROR;
 	}
 
 	return 0;
-- 
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