Subject: Re: [PATCH v9 1/5] sha1_file: support reading from a loose object of unknown type

From: karthik nayak <[email-masked]>
Date: Wed, 29 Apr 2015 23:20:46 +0530

On 04/29/2015 08:19 PM, Junio C Hamano wrote:
> Karthik Nayak <[email-masked]> writes:
>
>> Update sha1_loose_object_info() to optionally allow it to read
>> from a loose object file of unknown/bogus type; as the function
>> usually returns the type of the object it read in the form of enum
>> for known types, add an optional "typename" field to receive the
>> name of the type in textual form and a flag to indicate the reading
>> of a loose object file of unknown/bogus type.
>>
>> Add parse_sha1_header_extended() which acts as a wrapper around
>> parse_sha1_header() allowing more information to be obtained.
>
> Thanks.  This mostly looks good modulo a nit.

Sorry didn't get what you meant by "modulo a nit.".
>
>> diff --git a/sha1_file.c b/sha1_file.c
>> index 980ce6b..9a15634 100644
>> --- a/sha1_file.c
>> +++ b/sha1_file.c
>> @@ -1564,6 +1564,33 @@ int unpack_sha1_header(git_zstream *stream, unsigned char *map, unsigned long ma
>>   	return git_inflate(stream, 0);
>>   }
>>
>> +static int unpack_sha1_header_to_strbuf(git_zstream *stream, unsigned char *map,
>> +					unsigned long mapsize, void *buffer,
>> +					unsigned long bufsiz, struct strbuf *header)
>
> This function in this round looks somewhat tricky.
>
>> +{
>> +	unsigned char *cp;
>> +	int status;
>> +
>> +	status = unpack_sha1_header(stream, map, mapsize, buffer, bufsiz);
>
> Here, we unpack into the caller-supplied buffer, without touching
> the caller-supplied header strbuf.
>
>> +	/*
>> +	 * Check if entire header is unpacked in the first iteration.
>> +	 */
>> +	if (memchr(buffer, '\0', stream->next_out - (unsigned char *)buffer))
>> +		return 0;
>
> And we return the object header in the buffer without ever touching
> the header strbuf.  We expect that the caller would know that the
> buffer it gave us would contain the full object header line.
>
>> +	strbuf_add(header, buffer, stream->next_out - (unsigned char *)buffer);
>> +	do {
>> +		status = git_inflate(stream, 0);
>> +		strbuf_add(header, buffer, stream->next_out - (unsigned char *)buffer);
>> +		if (memchr(buffer, '\0', stream->next_out - (unsigned char *)buffer))
>> +			return 0;
>
> However, here, we return the object header in the caller-supplied
> header strbuf, while using the caller-supplied buffer as a scratch
> area.  We expect that the caller would know that the header strbuf
> is what it has to use to find the object header.
>
> Which is good in the sense that there is no unnecessary copies, but
> the caller needs to be careful to do something like:
>
> 	if (!unpack_to_strbuf(... buffer, sizeof(buffer), &header)) {
>      		if (header.len)
>                  	object_header = header.buf;
> 		else
>                  	object_header = buffer;
> 	} else {
>          	error("cannot unpack the object header");
> 	}
>
> It is a good trade-off between complexity and efficiency.  The
> complexity is isolated as the function is file-scope-static and it
> is perfectly fine to force the callers to be extra careful.
>
> But this suggests that the patch to add tests should try at least
> two, preferably three, kinds of test input.  A bogus type that needs
> a header longer than the caller's fixed buffer, a bogus type whose
> header would fit within the fixed buffer, and optionally a correct
> type whose header should always fit within the fixed buffer.
Yes it is a tradeoff, and it is complex as in the user has to check the 
strbuf provided to see if its been used. But this like you said I guess 
its a good tradeoff.
About the three tests, My patch checks "a bogus type whose header would 
fit within the fixed buffer" and "correct type whose header should 
always fit within the fixed buffer" but will write a test to check "A 
bogus type that needs a header longer than the caller's fixed buffer"
>
>> @@ -1614,27 +1641,38 @@ static void *unpack_sha1_rest(git_zstream *stream, void
>> ...
>> +	/*
>> +	 * Set type to 0 if its an unknown object and
>> +	 * we're obtaining the type using '--allow-unkown-type'
>> +	 * option.
>> +	 */
>> +	if ((flags & LOOKUP_UNKNOWN_OBJECT) && (type == -1))
>> +		type = 0;
>> +	else if (type == -1)
>> +		die("invalid object type");
>
> Would "type == -2" or any other negative value, if existed, mean
> something different?  I do not think so, and hence I would prefer
> these two checks be done with "type < 0" instead.
Thanks will change this.
>
>> @@ -2522,13 +2572,15 @@ struct packed_git *find_sha1_pack(const unsigned char *sha1,
>>   }
>>
>>   static int sha1_loose_object_info(const unsigned char *sha1,
>> -				  struct object_info *oi)
>> +				  struct object_info *oi,
>> +				  int flags)
>>   {
>> -	int status;
>> -	unsigned long mapsize, size;
>> +	int status = 0;
>> +	unsigned long mapsize;
>>   	void *map;
>>   	git_zstream stream;
>>   	char hdr[32];
>> +	struct strbuf hdrbuf = STRBUF_INIT;
>> ...
>> +	if ((flags & LOOKUP_UNKNOWN_OBJECT)) {
>> +		if (unpack_sha1_header_to_strbuf(&stream, map, mapsize, hdr, sizeof(hdr), &hdrbuf) < 0)
>> +			status = error("unable to unpack %s header with --allow-unknown-type",
>> +				       sha1_to_hex(sha1));
>> +	} else if (unpack_sha1_header(&stream, map, mapsize, hdr, sizeof(hdr)) < 0)
>>   		status = error("unable to unpack %s header",
>>   			       sha1_to_hex(sha1));
>> -	else if ((status = parse_sha1_header(hdr, &size)) < 0)
>> +	if (hdrbuf.len) {
>
> And here the caller is being careful, but the way it does is a bit
> subtle.  Even if it has a codepath that entirely bypasses a call to
> unpack_to_strbuf(), it knows that the other codepath does not touch
> the hdrbuf strbuf, which means that this block can be reached only
> when it called unpack_to_strbuf(), the callee found a long object
> header, and the unpacking succeeded.  Otherwise, it will make a
> call to parse_sha1_header_extended() using the fixed hdr[] in the
> "else if" clause that corresponds to this "if".
>
>> +		if ((status = parse_sha1_header_extended(hdrbuf.buf, oi, flags)) < 0)
>> +			status = error("unable to parse %s header with --allow-unknown-type",
>> +				       sha1_to_hex(sha1));
>> +	} else if ((status = parse_sha1_header_extended(hdr, oi, flags)) < 0)
>>   		status = error("unable to parse %s header", sha1_to_hex(sha1));
>
> However, I wonder what happens when "status" is already set after
> the call to unpack functions above.  hdrbuf.len may be zero so this
> "else if" part would be taken, and it then still try to feed the
> hdr[] to parse_sha1_header_extended(), no?
>
> In other words, shouldn't the flow be more like this?
>
> 	if (lookup-unknown) {
>          	if (unpack-to-strbuf)
> 			status = error();
> 	} else if (unpack-to-hdr)
> 		status = error();
>
> +	if (status < 0)
> +        	; /* do not do anything */
> +	else if (hdrbuf.len)
> -	if (hdrbuf.len)
> 		parse(hdrbuf.buf);
> 	else
>          	parse(hdr);
>
Yes you're right here, you had mentioned it before too, I missed it 
somehow. Thanks