Re: [PATCH 3/9] strbuf: provide function to append whole lines

From: Junio C Hamano <gitster@pobox.com>
Date: Mar 27, 2020, 21:04 UTC
Message-ID: <xmqqpncxfr1p.fsf@gitster.c.googlers.com>
In-Reply-To: <4b0a963ea5c47a951b95aa0a03966315b3e8299d.1585129842.git.ps@pks.im>

Patrick Steinhardt <ps@pks.im> writes:

Show 9 quoted lines

> While the strbuf interface already provides functions to read a line
> into it that completely replaces its current contents, we do not have an
> interface that allows for appending lines without discarding current
> contents.
>
> Add a new function `strbuf_appendwholeline` that reads a line including
> its terminating character into a strbuf non-destructively. This is a
> preparatory step for git-update-ref(1) reading standard input line-wise
> instead of as a block.

Hmph. If we were to gain more uses of this function over time, the necessity for an extra strbuf_addbuf() becomes annoying. I wonder if we should be doing this patch the other way around, i.e. move the whole implementation, except for the early

	if (feof(fp))
		return EOF;
	strbuf_reset(sb);

part, and call it strbuf_addwholeline(), and strbuf_getwholeline() becomes a thin wrapper around it that resets the incoming buffer before calling strbuf_addwholeline(). The logic to determine EOF would need to be updated if we go that route (i.e. instead of checking sb->len is zero in the !HAVE_GETDELIM side of the implementation, we need to see if the number is the same as before) but it should be minor.

There is a stale comment in the HAVE_GETDELIM side of the curent implementation, by the way. I think our xrealloc no longer tries to "recover" from ENOMEM.

Having said all that, I am fine with this version, at least for now.

Show 5 quoted lines

> +int strbuf_appendwholeline(struct strbuf *sb, FILE *fp, int term)
> +{
> +	struct strbuf line = STRBUF_INIT;
> +	if (strbuf_getwholeline(&line, fp, term))
> +		return EOF;

So, if caller wants to read the stream until it runs out, it can do

	strbuf_init(&sb);
	while (strbuf_appendwholeline(&sb, fp, '\n') != EOF)
		; /* keep going */

If the caller knows how many lines to read, EOF-return can be used only for error checking, e.g.

	strbuf_init(&sb);
	for (count = 0; count < 5; count++)
		if (strbuf_appendwholeline(&sb, fp, '\n') == EOF)
			die("cannot grab 5 lines");

It becomes cumbersome if the input lines are terminated, though. The caller wouldn't be using strbuf_appendwholeline() interface, e.g.

	strbuf_init(&accum);
	while ((strbuf_getwholeline(&sb, fp, '\n') != EOF) &&
               strcmp(sb.buf, "done\n"))
		strbuf_addbuf(&accum, &sb);

Anyway, I was primarily wondering if returning EOF, which perfectly makes sense for getwholeline(), still makes sense for addwholeline(), and it seems that it is still useful.

Show 25 quoted lines

> +	strbuf_addbuf(sb, &line);
> +	strbuf_release(&line);
> +	return 0;
> +}
> +
>  static int strbuf_getdelim(struct strbuf *sb, FILE *fp, int term)
>  {
>  	if (strbuf_getwholeline(sb, fp, term))
> diff --git a/strbuf.h b/strbuf.h
> index ce8e49c0b2..411063ca76 100644
> --- a/strbuf.h
> +++ b/strbuf.h
> @@ -502,6 +502,12 @@ int strbuf_getline(struct strbuf *sb, FILE *file);
>   */
>  int strbuf_getwholeline(struct strbuf *sb, FILE *file, int term);
>  
> +/**
> + * Like `strbuf_getwholeline`, but appends the line instead of
> + * resetting the buffer first.
> + */
> +int strbuf_appendwholeline(struct strbuf *sb, FILE *file, int term);
> +
>  /**
>   * Like `strbuf_getwholeline`, but operates on a file descriptor.
>   * It reads one character at a time, so it is very slow.  Do not