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Fix decompress_peek_gz to cope with files starting on empty gzip blocks. #1643

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Jul 13, 2023
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Fix decompress_peek_gz to cope with files starting on empty gzip blocks. #1643

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20 changes: 17 additions & 3 deletions hts.c
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Original file line number Diff line number Diff line change
Expand Up @@ -324,10 +324,24 @@ decompress_peek_gz(hFILE *fp, unsigned char *dest, size_t destsize)
zs.avail_out = destsize;
if (inflateInit2(&zs, 31) != Z_OK) return -1;

while (zs.total_out < destsize)
if (inflate(&zs, Z_SYNC_FLUSH) != Z_OK) break;
int ret;
unsigned char *last_in = buffer;
while (zs.total_out < destsize) {
ret = inflate(&zs, Z_SYNC_FLUSH);
if (ret == Z_STREAM_END && zs.avail_in && zs.total_out < destsize) {
if (last_in == zs.next_in)
break; // paranoia to avoid potential looping
else
last_in = zs.next_in;
inflateReset(&zs);
continue;
}
if (ret != Z_OK)
break;
}

destsize = zs.total_out;
// zs.total_out can sometimes be wrong as inflateReset resets it
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If total_out may have been reset by inflateReset, is it still appropriate to use it in the loop above?

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Doh! Good point. No (although in practice avail_in seems to catch it).

I'll make a follow up fix with the similar next_out - dest logic instead.

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It appears I was being overly paranoid (although I left in the loop detection bit, even though I can't actually trigger zlib to consume no input without producing an error).

avail_out would have been the correct variable, as unlike total_out this isn't reset. However it turns out neither are necessary. Calling inflate with one of those as zero returns an error (Z_BUF_ERROR) so it promptly bails out the loop anyway, and even if it just returned Z_STREAM_END again it'd be caught by the other checks.

So it's now much simpler. See #1646 for 2nd try at fixing this :)

destsize = zs.next_out - dest;
inflateEnd(&zs);

return destsize;
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