patchfetch-pack: make unexpected peek result non-fatal

> When a Git server responds to a fetch request, it may send optional
> sections before the packfile section. To handle this, the Git client
> calls packet_reader_peek() (see process_section_header()) in order to
> see what's next without consuming the line.
>
> However, as implemented, Git errors out whenever what's peeked is not an
> ordinary line. This is not only unexpected (here, we only need to know
> whether the upcoming line is the section header we want) but causes
> errors to include the name of a section header that is irrelevant to the
> cause of the error. For example, at $DAYJOB, we have seen "fatal: error
> reading section header 'shallow-info'" error messages when none of the
> repositories involved are shallow.
>
> Therefore, fix this so that the peek returns 1 if the upcoming line is
> the wanted section header and nothing else. Because of this change,
> reader->line may now be NULL later in the function, so update the error
> message printing code accordingly.

> Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
> ---
>  fetch-pack.c | 19 +++++++++++--------
>  1 file changed, 11 insertions(+), 8 deletions(-)
>
> diff --git a/fetch-pack.c b/fetch-pack.c
> index 4e1e88eea0..6d0d271259 100644
> --- a/fetch-pack.c
> +++ b/fetch-pack.c
> @@ -1370,17 +1370,20 @@ static int send_fetch_request(struct fetch_negotiator *negotiator, int fd_out,
>  static int process_section_header(struct packet_reader *reader,
>  				  const char *section, int peek)
>  {
> -	int ret;
> -
> -	if (packet_reader_peek(reader) != PACKET_READ_NORMAL)
> -		die(_("error reading section header '%s'"), section);
> +	int ret = 0;
>  
> -	ret = !strcmp(reader->line, section);
> +	if (packet_reader_peek(reader) == PACKET_READ_NORMAL &&
> +	    !strcmp(reader->line, section))
> +		ret = 1;
>  
>  	if (!peek) {
> -		if (!ret)
> -			die(_("expected '%s', received '%s'"),
> -			    section, reader->line);
> +		if (!ret) {
> +			if (reader->line)
> +				die(_("expected '%s', received '%s'"),
> +				    section, reader->line);
> +			else
> +				die(_("expected '%s'"), section);
> +		}
>  		packet_reader_read(reader);
>  	}

> We used to die unless READ_NORMAL was returned, so the code after
> packet_reader_peek() can rely on reader->line to be populated.  Now
> that code must handle cases where an earlier _peek() gave something
> other than READ_NORMAL and in such an error case reader->line points
> at NULL.  So any code that assumed reader->line was populated needs
> to be updated.
> 
> Makes sense.