Trailer readability cleanups

These patches were created while digging into the trailer code to better understand how it works, in preparation for making the trailer.{c,h} files as small as possible to make them available as a library for external users. This series was originally created as part of [1], but are sent here separately because the changes here are arguably more subjective in nature.

These patches do not add or change any features. Instead, their goal is to make the code easier to understand for new contributors (like myself), by making various cleanups and improvements. Ultimately, my hope is that with such cleanups, we are better positioned to make larger changes (especially the broader libification effort, as in "Introduce Git Standard Library" [2]).

Updates in v5
=============

* Patch 4 ("trailer: only use trailer_block_* variables if trailers were found") has been dropped.
* Patch 2 returns early if "--no-divider" is true, avoiding unnecessary loop iterations (thanks Jonathan).
* Added missing Reported-by trailer for Patch 3 (it was originally Glen's idea to use offsets).
* Patch 3: Fixed typo in "trailer.h" that referred to an obsolete function name ("find_true_end_of_input()", instead of "find_end_of_log_message()").

Updates in v4
=============

* The first 3 patches in v3 were merged into 'master'. Necessarily, those 3 patches have been dropped.
* Patch 4 in v3 ("trailer: rename *_DEFAULT enums to *_UNSPECIFIED") has been dropped, as well as Patch 9 in v3 ("trailer: make stack variable names match field names"). These were dropped to simplify this series for what I think is the more immediate, important change (see next bullet point).
* Patches 5-8 in v3 are the only ones remaining in this series. They still solely deal with --no-divider and trailer block start/end cleanups.

Updates in v3
=============

* Patches 4 and 6 (--no-divider and trailer block start/end cleanups) have been reorganized to Patches 5-8. This ended up touching commit.c in a minor way, but otherwise all of the changes here are cleanups and do not change any behavior.

Updates in v2
=============

* Patch 1: Drop the use of a #define. Instead just use an anonymous struct named internal.
* Patch 2: Don't free info out parameter inside parse_trailers(). Instead free it from the caller, process_trailers(). Update comment in parse_trailers().
* Patch 3: Reword commit message.
* Patch 4: Mention be3d654343 (commit: pass --no-divider to interpret-trailers, 2023-06-17) in commit message.
* Added Patch 6 to make trailer_info use offsets for trailer_start and trailer_end (thanks to Glen Choo for the suggestion).

[1]
https://lore.kernel.org/git/[email protected]/T/#mb044012670663d8eb7a548924bbcc933bef116de
[2]
https://lore.kernel.org/git/[email protected]/
[3]
https://lore.kernel.org/git/[email protected]/
[4]
https://lore.kernel.org/git/6b4cb31b17077181a311ca87e82464a1e2ad67dd.1686797630.git.gitgitgadget@gmail.com/
[5]
https://lore.kernel.org/git/[email protected]/T/#m0131f9829c35d8e0103ffa88f07d8e0e43dd732c

Linus Arver (3):
commit: ignore_non_trailer computes number of bytes to ignore
trailer: find the end of the log message
trailer: use offsets for trailer_start/trailer_end

 builtin/commit.c |  2 +-
 builtin/merge.c  |  2 +-
 commit.c         |  2 +-
 commit.h         |  4 +--
 sequencer.c      |  2 +-
 trailer.c        | 85 +++++++++++++++++++++++++++++-------------------
 trailer.h        | 10 +++---
 7 files changed, 62 insertions(+), 45 deletions(-)

base-commit: bcb6cae2966cc407ca1afc77413b3ef11103c175
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1563%2Flistx%2Ftrailer-libification-prep-v5
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1563/listx/trailer-libification-prep-v5
Pull-Request: https://github.com/gitgitgadget/git/pull/1563

Range-diff vs v4:

 1:  4ce5cf77005 = 1:  4ce5cf77005 commit: ignore_non_trailer computes number of bytes to ignore
 2:  c904caba7e1 ! 2:  ce25420db29 trailer: find the end of the log message
     @@ Commit message
          the starting point which find_trailer_start() needs to start searching
          backward to parse individual trailers (if any).
      
     +    Helped-by: Jonathan Tan <[email protected]>
          Helped-by: Junio C Hamano <[email protected]>
          Signed-off-by: Linus Arver <[email protected]>
      
     @@ trailer.c: static ssize_t last_line(const char *buf, size_t len)
      +static size_t find_end_of_log_message(const char *input, int no_divider)
       {
      +	size_t end;
     -+
       	const char *s;
       
      -	for (s = str; *s; s = next_line(s)) {
      +	/* Assume the naive end of the input is already what we want. */
      +	end = strlen(input);
      +
     ++	if (no_divider) {
     ++		return end;
     ++	}
     ++
      +	/* Optionally skip over any patch part ("---" line and below). */
      +	for (s = input; *s; s = next_line(s)) {
       		const char *v;
       
      -		if (skip_prefix(s, "---", &v) && isspace(*v))
      -			return s - str;
     -+		if (!no_divider && skip_prefix(s, "---", &v) && isspace(*v)) {
     ++		if (skip_prefix(s, "---", &v) && isspace(*v)) {
      +			end = s - input;
      +			break;
      +		}

3: 796e47c1e5f ! 3: e3a7b150241 trailer: use offsets for trailer_start/trailer_end
@@ Commit message
more explicit about these offsets (that they are for the entire trailer block including other trailers). Ditto for trailer_end.

+ Reported-by: Glen Choo <[email protected]>

## sequencer.c ##
@@ trailer.h: int trailer_set_if_missing(enum trailer_if_missing *item, const char
- * input string.
+ * Offsets to the trailer block start and end positions in the input
+ * string. If no trailer block is found, these are both set to the
-+ * "true" end of the input, per find_true_end_of_input().
-+ *
-+ * NOTE: This will be changed so that these point to 0 in the next
-+ * patch if no trailers are found.
++ * "true" end of the input (find_end_of_log_message()).
*/
- const char *trailer_start, *trailer_end;
+ size_t trailer_block_start, trailer_block_end;
4: 64e1bd4e4be < -: ----------- trailer: only use trailer_block_* variables if trailers were found

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