Subject: Re: [PATCH v3 2/5] fetch: use strbuf to format FETCH_HEAD updates

From: Junio C Hamano <[email-masked]>
Date: Mon, 11 Jan 2021 15:21:35 -0800

Patrick Steinhardt <[email-masked]> writes:

> @@ -909,6 +910,7 @@ static int open_fetch_head(struct fetch_head *fetch_head)
>  		fetch_head->fp = fopen(filename, "a");
>  		if (!fetch_head->fp)
>  			return error_errno(_("cannot open %s"), filename);
> +		strbuf_init(&fetch_head->buf, 0);
>  	} else {
>  		fetch_head->fp = NULL;
>  	}

Leaving fetch_head->buf uninitialized is probably OK as the caller
of open_fetch_head() would (or at least should) immediately barf
upon seeing an error return?

Ah, no.  Under dry-run mode, we will return success but leave buf
uninitializesd.  It is safe because we do not use the strbuf when fp
is NULL.  OK.

> @@ -941,14 +943,17 @@ static void append_fetch_head(struct fetch_head *fetch_head,
>  			return;
>  	}
>  
> -	fprintf(fetch_head->fp, "%s\t%s\t%s",
> -		oid_to_hex_r(old_oid_hex, old_oid), merge_status_marker, note);
> +	strbuf_addf(&fetch_head->buf, "%s\t%s\t%s",
> +		    oid_to_hex_r(old_oid_hex, old_oid), merge_status_marker, note);
>  	for (i = 0; i < url_len; ++i)
>  		if ('\n' == url[i])
> -			fputs("\\n", fetch_head->fp);
> +			strbuf_addstr(&fetch_head->buf, "\\n");
>  		else
> -			fputc(url[i], fetch_head->fp);
> -	fputc('\n', fetch_head->fp);
> +			strbuf_addch(&fetch_head->buf, url[i]);
> +	strbuf_addch(&fetch_head->buf, '\n');
> +
> +	strbuf_write(&fetch_head->buf, fetch_head->fp);
> +	strbuf_reset(&fetch_head->buf);

This gets us closer to fixing the "one record can be written out
with multiple write(2) calls, allowing parallel fetches to corrupt
FETCH_HEAD file by intermixed records" problem (even though this
change alone would not solve it---for that we need to do write
ourselves, not letting stdio do the flushing).

>  }
>  
>  static void commit_fetch_head(struct fetch_head *fetch_head)
> @@ -962,6 +967,7 @@ static void close_fetch_head(struct fetch_head *fetch_head)
>  		return;
>  
>  	fclose(fetch_head->fp);
> +	strbuf_release(&fetch_head->buf);
>  }
>  
>  static const char warn_show_forced_updates[] =