Re: [PATCH v2] fetch: optionally allow disabling FETCH_HEAD update

> On 28/07/2020 17:37, Junio C Hamano wrote:
>> Derrick Stolee <stolee@gmail.com> writes:
>>
>>> On 7/27/2020 12:13 PM, Junio C Hamano wrote:
>>>> Derrick Stolee <stolee@gmail.com> writes:
>>>>
>>>>> I'll rebase onto jc/no-update-fetch-head for the next version, since
>>>>> that branch is based on v2.28.0-rc0, which is recent enough.
>>>>
>>>> I do not think it is wise to base a work on top of unfinished "you
>>>> could do it this way, perhaps?" demonstration patch the original
>>>> author does not have much inclination to finish, though.
>>>>
>>>> When I am really bored, I may go back to the topic to finish it, but
>>>> I wouldn't mind if you took ownership of it at all.
>>>
>>> Ah. I didn't understand the status of that branch. I'll pull it in
>>> to this topic.
>>
>> So here is with one of the two things that I found missing in the
>> first iteration of the patch: documentation.
>>
>> The other thing that I found iffy (and still missing from this
>> version) was what should be done when "git pull" is explicitly given
>> the "--no-write-fetch-head" option.
>>
>> I think (but didn't check the recent code) that 'git pull' would
>> pass only known-to-make-sense command line options to underlying
>> 'git fetch', so it probably will barf with "unknown option", which
>> is the best case.  We might want to make it sure with a new test in
>> 5521.  On the other hand, if we get anything other than "no such
>> option", we may want to think if we want to "fix" it or just leave
>> it inside "if it hurts, don't do it" territory.
>>
>> Thanks.
>>
>> The patch without doc was Reviewed-by: Taylor Blau <me@ttaylorr.com>
>> but this round has not been.
>>
>> -- >8 --
>>
>> If you run fetch but record the result in remote-tracking branches,
>> and either if you do nothing with the fetched refs (e.g. you are
>> merely mirroring) or if you always work from the remote-tracking
>> refs (e.g. you fetch and then merge origin/branchname separately),
>> you can get away with having no FETCH_HEAD at all.
>>
>> Teach "git fetch" a command line option "--[no-]write-fetch-head"
>> and "fetch.writeFetchHEAD" configuration variable.  Without either,
>> the default is to write FETCH_HEAD, and the usual rule that the
>> command line option defeats configured default applies.
>>
>> Note that under "--dry-run" mode, FETCH_HEAD is never written;
>> otherwise you'd see list of objects in the file that you do not
>> actually have.  Passing `--fetch-write-head` 
> 
> Typo, it should be `--write-fetch-head`
> 
>> does not force `git
>> fetch` to write the file.
>>
>> Also note that this option is explicitly passed when "git pull"
>> internally invokes "git fetch", so that those who configured their
>> "git fetch" not to write FETCH_HEAD would not be able to break the
>> cooperation between these two commands.  "git pull" must see what
>> "git fetch" got recorded in FETCH_HEAD to work correctly.
>>
>> Signed-off-by: Junio C Hamano <gitster@pobox.com>
>> ---
>>   Documentation/config/fetch.txt  |  7 ++++++
>>   Documentation/fetch-options.txt | 10 +++++++++
>>   builtin/fetch.c                 | 19 +++++++++++++---
>>   builtin/pull.c                  |  3 ++-
>>   t/t5510-fetch.sh                | 39 +++++++++++++++++++++++++++++++--
>>   5 files changed, 72 insertions(+), 6 deletions(-)
>>
>> diff --git a/Documentation/config/fetch.txt 
>> b/Documentation/config/fetch.txt
>> index b20394038d..0aaa05e8c0 100644
>> --- a/Documentation/config/fetch.txt
>> +++ b/Documentation/config/fetch.txt
>> @@ -91,3 +91,10 @@ fetch.writeCommitGraph::
>>       merge and the write may take longer. Having an updated commit-graph
>>       file helps performance of many Git commands, including `git 
>> merge-base`,
>>       `git push -f`, and `git log --graph`. Defaults to false.
>> +
>> +fetch.writeFetchHEAD::
>> +    Setting it to false tells `git fetch` not to write the list
>> +    of remote refs fetched in the `FETCH_HEAD` file directly
>> +    under `$GIT_DIR`.  Can be countermanded from the command
>> +    line with the `--[no-]write-fetch-head` option.  Defaults to
>> +    true.
>> diff --git a/Documentation/fetch-options.txt 
>> b/Documentation/fetch-options.txt
>> index 6e2a160a47..6775e8499f 100644
>> --- a/Documentation/fetch-options.txt
>> +++ b/Documentation/fetch-options.txt
>> @@ -64,6 +64,16 @@ documented in linkgit:git-config[1].
>>   --dry-run::
>>       Show what would be done, without making any changes.
>> +ifndef::git-pull[]
>> +--[no-]write-fetch-head::
>> +    Write the list of remote refs fetched in the `FETCH_HEAD`
>> +    file directly under `$GIT_DIR`.  This is the default unless
>> +    the configuration variable `fetch.writeFetchHEAD` is set to
>> +    false.  Passing `--no-write-fetch-head` from the command
>> +    line tells Git not to write the file.  Under `--dry-run`
>> +    option, the file is never written.
>> +endif::git-pull[]
>> +
>>   -f::
>>   --force::
>>       When 'git fetch' is used with `<src>:<dst>` refspec it may
>> diff --git a/builtin/fetch.c b/builtin/fetch.c
>> index 82ac4be8a5..3ccf69753f 100644
>> --- a/builtin/fetch.c
>> +++ b/builtin/fetch.c
>> @@ -56,6 +56,7 @@ static int prune_tags = -1; /* unspecified */
>>   #define PRUNE_TAGS_BY_DEFAULT 0 /* do we prune tags by default? */
>>   static int all, append, dry_run, force, keep, multiple, update_head_ok;
>> +static int write_fetch_head = 1;
>>   static int verbosity, deepen_relative, set_upstream;
>>   static int progress = -1;
>>   static int enable_auto_gc = 1;
>> @@ -118,6 +119,10 @@ static int git_fetch_config(const char *k, const 
>> char *v, void *cb)
>>           return 0;
>>       }
>> +    if (!strcmp(k, "fetch.writefetchhead")) {
>> +        write_fetch_head = git_config_bool(k, v);
>> +        return 0;
>> +    }
>>       return git_default_config(k, v, cb);
>>   }
>> @@ -162,6 +167,8 @@ static struct option builtin_fetch_options[] = {
>>               PARSE_OPT_OPTARG, option_fetch_parse_recurse_submodules),
>>       OPT_BOOL(0, "dry-run", &dry_run,
>>            N_("dry run")),
>> +    OPT_BOOL(0, "write-fetch-head", &write_fetch_head,
>> +         N_("write fetched references to the FETCH_HEAD file")),
>>       OPT_BOOL('k', "keep", &keep, N_("keep downloaded pack")),
>>       OPT_BOOL('u', "update-head-ok", &update_head_ok,
>>               N_("allow updating of HEAD ref")),
>> @@ -893,7 +900,9 @@ static int store_updated_refs(const char *raw_url, 
>> const char *remote_name,
>>       const char *what, *kind;
>>       struct ref *rm;
>>       char *url;
>> -    const char *filename = dry_run ? "/dev/null" : 
>> git_path_fetch_head(the_repository);
>> +    const char *filename = (!write_fetch_head
>> +                ? "/dev/null"
>> +                : git_path_fetch_head(the_repository));
> 
> I was suspicious of this as we haven't cleared write_fetch_head in the 
> --dry-run case yet but the test below seems to show that we still don't 
> write FETCH_HEAD if --dry-run is given. That makes we wonder what the 
> point of setting the filename based on the value of write_fetch_head is.

> The rest looks good to me, though it might be worth having a test to 
> check that pull does indeed reject --no-write-fetch-head
> 
> Best Wishes
> 
> Phillip
> 
>>       int want_status;
>>       int summary_width = transport_summary_width(ref_map);
>> @@ -1327,7 +1336,7 @@ static int do_fetch(struct transport *transport,
>>       }
>>       /* if not appending, truncate FETCH_HEAD */
>> -    if (!append && !dry_run) {
>> +    if (!append && write_fetch_head) {
>>           retcode = truncate_fetch_head();
>>           if (retcode)
>>               goto cleanup;
>> @@ -1594,7 +1603,7 @@ static int fetch_multiple(struct string_list 
>> *list, int max_children)
>>       int i, result = 0;
>>       struct argv_array argv = ARGV_ARRAY_INIT;
>> -    if (!append && !dry_run) {
>> +    if (!append && write_fetch_head) {
>>           int errcode = truncate_fetch_head();
>>           if (errcode)
>>               return errcode;
>> @@ -1795,6 +1804,10 @@ int cmd_fetch(int argc, const char **argv, 
>> const char *prefix)
>>       if (depth || deepen_since || deepen_not.nr)
>>           deepen = 1;
>> +    /* FETCH_HEAD never gets updated in --dry-run mode */
>> +    if (dry_run)
>> +        write_fetch_head = 0;
>> +
>>       if (all) {
>>           if (argc == 1)
>>               die(_("fetch --all does not take a repository argument"));
>> diff --git a/builtin/pull.c b/builtin/pull.c
>> index 8159c5d7c9..e988d92b53 100644
>> --- a/builtin/pull.c
>> +++ b/builtin/pull.c
>> @@ -527,7 +527,8 @@ static int run_fetch(const char *repo, const char 
>> **refspecs)
>>       struct argv_array args = ARGV_ARRAY_INIT;
>>       int ret;
>> -    argv_array_pushl(&args, "fetch", "--update-head-ok", NULL);
>> +    argv_array_pushl(&args, "fetch", "--update-head-ok",
>> +             "--write-fetch-head", NULL);
>>       /* Shared options */
>>       argv_push_verbosity(&args);
>> diff --git a/t/t5510-fetch.sh b/t/t5510-fetch.sh
>> index a66dbe0bde..3052c2d8d5 100755
>> --- a/t/t5510-fetch.sh
>> +++ b/t/t5510-fetch.sh
>> @@ -539,13 +539,48 @@ test_expect_success 'fetch into the current 
>> branch with --update-head-ok' '
>>   '
>> -test_expect_success 'fetch --dry-run' '
>> -
>> +test_expect_success 'fetch --dry-run does not touch FETCH_HEAD' '
>>       rm -f .git/FETCH_HEAD &&
>>       git fetch --dry-run . &&
>>       ! test -f .git/FETCH_HEAD
>>   '
>> +test_expect_success '--no-write-fetch-head does not touch FETCH_HEAD' '
>> +    rm -f .git/FETCH_HEAD &&
>> +    git fetch --no-write-fetch-head . &&
>> +    ! test -f .git/FETCH_HEAD
>> +'
>> +
>> +test_expect_success '--write-fetch-head gets defeated by --dry-run' '
>> +    rm -f .git/FETCH_HEAD &&
>> +    git fetch --dry-run --write-fetch-head . &&
>> +    ! test -f .git/FETCH_HEAD
>> +'
>> +
>> +test_expect_success 'fetch.writeFetchHEAD and FETCH_HEAD' '
>> +    rm -f .git/FETCH_HEAD &&
>> +    git -c fetch.writeFetchHEAD=no fetch . &&
>> +    ! test -f .git/FETCH_HEAD
>> +'
>> +
>> +test_expect_success 'fetch.writeFetchHEAD gets defeated by --dry-run' '
>> +    rm -f .git/FETCH_HEAD &&
>> +    git -c fetch.writeFetchHEAD=yes fetch --dry-run . &&
>> +    ! test -f .git/FETCH_HEAD
>> +'
>> +
>> +test_expect_success 'fetch.writeFetchHEAD and --no-write-fetch-head' '
>> +    rm -f .git/FETCH_HEAD &&
>> +    git -c fetch.writeFetchHEAD=yes fetch --no-write-fetch-head . &&
>> +    ! test -f .git/FETCH_HEAD
>> +'
>> +
>> +test_expect_success 'fetch.writeFetchHEAD and --write-fetch-head' '
>> +    rm -f .git/FETCH_HEAD &&
>> +    git -c fetch.writeFetchHEAD=no fetch --write-fetch-head . &&
>> +    test -f .git/FETCH_HEAD
>> +'
>> +
>>   test_expect_success "should be able to fetch with duplicate refspecs" '
>>       mkdir dups &&
>>       (
>>