Subject: Re: [PATCH 04/10] evolve: add support for parsing metacommits

From: Phillip Wood <[email-masked]>
Date: Mon, 26 Sep 2022 14:27:31 +0100

Hi Chris

On 23/09/2022 19:55, Stefan Xenos via GitGitGadget wrote:
> From: Stefan Xenos <[email-masked]>
> 
> This patch adds the get_metacommit_content method, which can classify
> commits as either metacommits or normal commits, determine whether they
> are abandoned, and extract the content commit's object id from the
> metacommit.
> 
> Signed-off-by: Stefan Xenos <[email-masked]>
> Signed-off-by: Chris Poucet <[email-masked]>
> ---
>   Makefile            |   1 +
>   metacommit-parser.c | 110 ++++++++++++++++++++++++++++++++++++++++++++
>   metacommit-parser.h |  19 ++++++++
>   3 files changed, 130 insertions(+)
>   create mode 100644 metacommit-parser.c
>   create mode 100644 metacommit-parser.h
> 
> diff --git a/Makefile b/Makefile
> index cac3452edb9..b2bcc00c289 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -999,6 +999,7 @@ LIB_OBJS += merge-ort.o
>   LIB_OBJS += merge-ort-wrappers.o
>   LIB_OBJS += merge-recursive.o
>   LIB_OBJS += merge.o
> +LIB_OBJS += metacommit-parser.o

There seems to be a problem with the indent here

>   LIB_OBJS += midx.o
>   LIB_OBJS += name-hash.o
>   LIB_OBJS += negotiator/default.o

 > diff --git a/metacommit-parser.h b/metacommit-parser.h
 > new file mode 100644
 > index 00000000000..1c74bd6d699
 > --- /dev/null
 > +++ b/metacommit-parser.h
 > @@ -0,0 +1,19 @@
 > +#ifndef METACOMMIT_PARSER_H
 > +#define METACOMMIT_PARSER_H
 > +
 > +#include "commit.h"
 > +#include "hash.h"
 > +
 > +/* Indicates a normal commit (non-metacommit) */
 > +#define METACOMMIT_TYPE_NONE 0
 > +/* Indicates a metacommit with normal content (non-abandoned) */
 > +#define METACOMMIT_TYPE_NORMAL 1
 > +/* Indicates a metacommit with abandoned content */
 > +#define METACOMMIT_TYPE_ABANDONED 2

Is it possible to define these as an enum? It would make the signature 
of get_meta_commit_content() nicer.

 > +struct commit;

What's this for? We're including commit.h above.

 > +extern int get_metacommit_content(
 > +	struct commit *commit, struct object_id *content);

> diff --git a/metacommit-parser.c b/metacommit-parser.c
> new file mode 100644
> index 00000000000..70c1428bfc6
> --- /dev/null
> +++ b/metacommit-parser.c
> @@ -0,0 +1,110 @@
> +#include "cache.h"
> +#include "metacommit-parser.h"
> +#include "commit.h"
> +
> +/*
> + * Search the commit buffer for a line starting with the given key. Unlike
> + * find_commit_header, this also searches the commit message body.
> + */

There is no explanation in the code or commit message as to why this 
function is needed. The documentation added in the first commit says 
that "parent-type" header is a commit header. I think the answer is that 
this series does not implement that header but uses the commit message 
instead. That's perfectly fine for a proof of concept but it is 
precisely the sort of detail that should be described it the commit 
message and probably flagged up in the cover letter.

> +static const char *find_key(const char *msg, const char *key, size_t *out_len)
> +{
> +	int key_len = strlen(key);
> +	const char *line = msg;
> +
> +	while (line) {
> +		const char *eol = strchrnul(line, '\n');
> +
> +		if (eol - line > key_len && !memcmp(line, key, key_len) &&
> +		    line[key_len] == ' ') {
> +			*out_len = eol - line - key_len - 1;
> +			return line + key_len + 1;
> +		}
> +		line = *eol ? eol + 1 : NULL;
> +	}
> +	return NULL;
> +}
> +
> +static struct commit *get_commit_by_index(struct commit_list *to_search, int index)
> +{
> +	while (to_search && index) {
> +		to_search = to_search->next;
> +		index--;
> +	}
> +
> +	if (!to_search)
> +		return NULL;
> +
> +	return to_search->item;
> +}

This function is a useful utility for struct commit_list and should live 
in commit.c. It could be used to simplify object-name.c:get_parent() for 
example.

> +/*
> + * Writes the index of the content parent to "result". Returns the metacommit
> + * type. See the METACOMMIT_TYPE_* constants.
> + */
> +static int index_of_content_commit(const char *buffer, int *result)

I found the signature confusing as it is returning an int but that is 
not the index. Switching to an enum for the metacommit types would 
clarify that.

> +{
> +	int index = 0;
> +	int ret = METACOMMIT_TYPE_NONE;
> +	size_t parent_types_size;
> +	const char *parent_types = find_key(buffer, "parent-type",
> +		&parent_types_size);
> +	const char *end;
> +	const char *enum_start = parent_types;
> +	int enum_length = 0;
> +
> +	if (!parent_types)
> +		return METACOMMIT_TYPE_NONE;
> +
> +	end = &parent_types[parent_types_size];
> +
> +	while (1) {
> +		char next = *parent_types;
> +		if (next == ' ' || parent_types >= end) {
> +			if (enum_length == 1) {

if enum_length != 1 then there is an error in the parent-type header and 
we should probably bail out.

> +				char first_char_in_enum = *enum_start;

It's not just the first character, it's the only character, do we really 
need such a long variable name? (how about just calling it "type")

I'll try and take at look at the next couple of patches later in the week.

Best Wishes

Phillip