Re: [PATCH] RFC: A new type of symbolic refs

> From: "Stefan Beller" <sbeller@google.com>
> Sent: Tuesday, July 11, 2017 2:06 AM
>>A symbolic ref can currently only point at (another symbolic) ref.
>> However are use cases for symbolic refs to point at other things
>> representing a commit-ish, such as gitlink entries in other
>> repositories.  In this use case we can use such a symbolic link
>> to strengthen the relationship between a submodule and a superproject.
>> Examples:
>
> If I understand this correctly, the new type is the 'starts_with(buf, ))'.

>
> It just wasn't obvious from the text that the new type is "repo:" as you 
> never spell it out in the commit message. Should it be included in the 
> message?
>
> Have I understood correctly?
>
> --
> Philip
> [I'll be off-line till Friday, so will pick up then]
>
>>
>> 1) It makes it easier to explain when we recurse into submodules and
>>   to which sha1 the submodule is updated.
>>
>>   Currently a submodule (or any repo) is either on a branch (i.e.
>>   HEAD is a symbolic ref) or is in 'detached HEAD' state (HEAD is
>>   a direct ref).
>>   For submodules it is wrong to be on its own branch if it wants to be
>>   controlled by the superproject as being on its own branch signals that
>>   the submodule is independent and can move the HEAD freely.
>>   Being in 'detached HEAD' state is the alternative to that and was
>>   chosen when git-submodule was implemented, but it is equally wrong;
>>   the lesser of two evils.
>>
>>   Semantically the correct way to state a submodule is part of the
>>   superproject is by pointing its HEAD to the superproject.
>>
>>   We do have "reset/checkout --recurse-submodules" now, but it is
>>   hard to explain what actually happens there (Which submodules are
>>   updated? All of them! -- But I want a subset only!)
>>
>>   With this new mode of symbolic refs, any submodule that tracks the
>>   superproject, would 'automatically follow' the superproject as the
>>   submodules HEAD moves when the superproject changes.
>>
>> 2) "git -C submodule commit" would behave the same as it would on branch
>>   nowadays: It would make the commit in the submodule and then change
>>   the target of the symbolic ref which would be the index of the
>>   superproject! That implies that you do not need to 'git add' the
>>   submodule to the superproject, but have it done automatically.
>>
>> Signed-off-by: Stefan Beller <sbeller@google.com>
>> ---
>> cache.h              |  2 ++
>> refs/files-backend.c | 10 ++++++++++
>> submodule.c          | 40 ++++++++++++++++++++++++++++++++++++++++
>> 3 files changed, 52 insertions(+)
>>
>> diff --git a/cache.h b/cache.h
>> index 71fe092644..4f79d23202 100644
>> --- a/cache.h
>> +++ b/cache.h
>> @@ -2029,4 +2029,6 @@ void sleep_millisec(int millisec);
>>  */
>> void safe_create_dir(const char *dir, int share);
>>
>> +extern int read_external_symref(struct strbuf *from, struct strbuf 
>> *out);
>> +
>> #endif /* CACHE_H */
>> diff --git a/refs/files-backend.c b/refs/files-backend.c
>> index 0404f2c233..f56f7b87ce 100644
>> --- a/refs/files-backend.c
>> +++ b/refs/files-backend.c
>> @@ -713,6 +713,16 @@ static int files_read_raw_ref(struct ref_store 
>> *ref_store,
>>  goto out;
>>  }
>>
>> + if (starts_with(buf, "repo:")) {
>> + if (read_external_symref(&sb_contents, referent)) {
>> + *type |= REF_ISBROKEN;
>> + ret = -1;
>> + goto out;
>> + }
>> + *type |= REF_ISSYMREF;
>> + ret = 0;
>> + }
>> +
>>  /*
>>  * Please note that FETCH_HEAD has additional
>>  * data after the sha.
>> diff --git a/submodule.c b/submodule.c
>> index da2b484879..7297f90485 100644
>> --- a/submodule.c
>> +++ b/submodule.c
>> @@ -2037,3 +2037,43 @@ int submodule_to_gitdir(struct strbuf *buf, const 
>> char *submodule)
>> cleanup:
>>  return ret;
>> }
>> +
>> +int read_external_symref(struct strbuf *from, struct strbuf *out)
>> +{
>> + struct child_process cp = CHILD_PROCESS_INIT;
>> + const char *repo, *gitlink;
>> + int hint, code;
>> + struct strbuf **split = strbuf_split(from, 0);
>> + struct strbuf cmd_out = STRBUF_INIT;
>> +
>> + if (!split[0] || !split[1])
>> + return -1;
>> +
>> + repo = split[0]->buf + 5; /* skip 'repo:' */
>> + gitlink = split[1]->buf;
>> +
>> + argv_array_pushl(&cp.args,
>> + "ignored-first-arg",
>> + "-C", repo,
>> + "ls-tree", "-z", "HEAD", "--", gitlink, NULL);
>> +
>> + /*
>> + * 17 accounts for '160000 commit ',
>> + * the \t before path and trailing \0.
>> + */
>> + hint = 17 + GIT_SHA1_HEXSZ + split[1]->len;
>> + code = capture_command(&cp, &cmd_out, hint);
>> +
>> + strbuf_release(split[0]);
>> + strbuf_release(split[1]);
>> +
>> + if (!code) {
>> + strbuf_reset(out);
>> + strbuf_add(out, cmd_out.buf + strlen("160000 commit "),
>> +    GIT_SHA1_HEXSZ);
>> + } else
>> + return -1;
>> +
>> + return 0;
>> +}
>> +
>> -- 
>> 2.13.2.695.g117ddefdb4
>>
>