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

From: Derrick Stolee <stolee@gmail.com>
Date: Jul 30, 2020, 15:17 UTC
Message-ID: <4b6f5458-f4f7-119f-ec8c-933f27dd3072@gmail.com>
In-Reply-To: <xmqqr1svegt4.fsf_-_@gitster.c.googlers.com>

On 7/28/2020 12:37 PM, Junio C Hamano wrote:

Show 32 quoted lines

> 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.

Here is the version I applied and updated. Please let me know what you think.

-->8--

From 3f60a0f0fd388447aa9c2b805b5646039df98d0b Mon Sep 17 00:00:00 2001
From: Junio C Hamano <gitster@pobox.com>
Date: Tue, 28 Jul 2020 09:37:59 -0700
Subject: [PATCH] fetch: optionally allow disabling FETCH_HEAD update

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 `--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>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
---
 Documentation/config/fetch.txt  |  7 ++++++
 Documentation/fetch-options.txt | 10 +++++++++
 builtin/fetch.c                 | 19 +++++++++++++---
 builtin/pull.c                  |  3 ++-
 t/t5510-fetch.sh                | 39 +++++++++++++++++++++++++++++++--
 t/t5521-pull-options.sh         | 16 ++++++++++++++
 6 files changed, 88 insertions(+), 6 deletions(-)

diff --git a/Documentation/config/fetch.txt b/Documentation/config/fetch.txt
index b20394038d1..0aaa05e8c0e 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 495bc8ab5a1..6972ad2522c 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 d8253f66e4c..1d7194aac26 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")),
@@ -895,7 +902,9 @@ 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));
 	int want_status;
 	int summary_width = transport_summary_width(ref_map);
 
@@ -1329,7 +1338,7 @@ 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;
@@ -1596,7 +1605,7 @@ 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;
@@ -1797,6 +1806,10 @@ 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 8159c5d7c96..e988d92b535 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 9850ecde5df..31c91d0ed2e 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 &&
 	(
diff --git a/t/t5521-pull-options.sh b/t/t5521-pull-options.sh
index 159afa7ac81..1acae3b9a4f 100755
--- a/t/t5521-pull-options.sh
+++ b/t/t5521-pull-options.sh
@@ -77,6 +77,7 @@ test_expect_success 'git pull -q -v --no-rebase' '
 	test_must_be_empty out &&
 	test -s err)
 '
+
 test_expect_success 'git pull --cleanup errors early on invalid argument' '
 	mkdir clonedcleanup &&
 	(cd clonedcleanup && git init &&
@@ -85,6 +86,21 @@ test_expect_success 'git pull --cleanup errors early on invalid argument' '
 	test -s err)
 '
 
+test_expect_success 'git pull --no-write-fetch-head fails' '
+	mkdir clonedwfh &&
+	(cd clonedwfh && git init &&
+	test_must_fail git pull --no-write-fetch-head "../parent" >out 2>err &&
+	test_must_be_empty out &&
+	test_i18ngrep "no-write-fetch-head" err)
+'
+
+test_expect_success 'git pull succeeds with fetch.writeFetchHEAD=false' '
+	mkdir clonedwfhconfig &&
+	(cd clonedwfhconfig && git init &&
+	git config fetch.writeFetchHEAD false &&
+	git pull "../parent" >out 2>err &&
+	grep FETCH_HEAD err)
+'
 
 test_expect_success 'git pull --force' '
 	mkdir clonedoldstyle &&

-- 
2.27.0.366.g34746c1d11e.dirty