Re: [RFC/PATCH 4/3] create_symref: drop support for writing symbolic links

[Junio C Hamano <gitster@pobox.com>]

Jeff King <peff@peff.net> writes:

> A conservative choice would probably be to issue a deprecation warning
> when we see it defined, wait a few versions, and then apply the patch
> below.

I agree with the analysis below.  And I agree that in the ideal
world, it would have been better not to add "prefer symlink refs"
configuration in the first place.  But we do not live in the ideal
world, and we already have it, so deprecation would need the usual
multi-step process.

Thanks.

> -- >8 --
> Long ago, in 2fabd21 (Disable USE_SYMLINK_HEAD by default,
> 2005-11-15), we switched git's default behavior to writing
> text symrefs instead of symbolic links. Any scripts
> accustomed to looking directly at .git/HEAD were updated to
> use `rev-parse` instead. The Linux kernel's setlocalversion
> script was one, and it was fixed in 117a93d (kbuild: Use git
> in scripts/setlocalversion, 2006-01-04).
>
> However, the problem still happened when bisecting the
> kernel; pre-117a93d kernels would not build properly with a
> newer git, because they wanted to look directly at HEAD. To
> solve this, we added 9f0bb90 (core.prefersymlinkrefs: use
> symlinks for .git/HEAD, 2006-05-02), which lets the user
> turn on the old behavior, theoretically letting you bisect
> older kernel history.
>
> But there are a few complications with this solution:
>
>   - packed-refs means you are limited in what you can do
>     with .git/HEAD. If it is a symlink, you may `readlink`
>     it to see where it points, but you cannot necessarily
>     `cat .git/HEAD` to get the sha1, as the pointed-to ref
>     may be packed.
>
>     In particular, this means that the pre-117a93d kbuild
>     script would sometimes work and sometimes not.
>
>   - These days, we bisect on a detached HEAD. So during
>     bisection, .git/HEAD _is_ a regular file with the
>     sha1, and it works to `cat` it, whether or not you set
>     core.preferSymlinkRefs.
>
> Such scripts will all be broken again if we move to
> alternate ref backends. They should have learned to use
> `rev-parse` long ago, and it is only bisecting ancient
> history that is a problem.
>
> Now that almost ten years have passed, it seems less likely
> that developers will bisect so far back in history. And
> moreover, this is but one of many possible problems
> developers run into when trying to build versions. The
> standard solution is to apply a "fixup" patch or other
> workaround while test-building the project, and that would
> work here, too.
>
> This patch therefore drops core.preferSymlinkRefs
> completely. There are a few reasons to want to do so:
>
>   1. It drops some code that is probably exercised very
>      rarely.
>
>   2. The symlink code is not up to the same standards as the
>      rest of the ref code. In particular, it is racy with
>      respect to simultaneous readers and writers.
>
>   3. If we want to eventually drop the symlink-reading code,
>      this is a good first step to deprecating it.
>
> Signed-off-by: Jeff King <peff@peff.net>
> ---
>  Documentation/config.txt               |  6 ------
>  cache.h                                |  1 -
>  config.c                               |  5 -----
>  contrib/completion/git-completion.bash |  1 -
>  environment.c                          |  1 -
>  refs/files-backend.c                   | 20 --------------------
>  t/t7201-co.sh                          | 12 ------------
>  t/t9903-bash-prompt.sh                 |  9 ---------
>  8 files changed, 55 deletions(-)
>
> diff --git a/Documentation/config.txt b/Documentation/config.txt
> index f617886..06a2f2a 100644
> --- a/Documentation/config.txt
> +++ b/Documentation/config.txt
> @@ -433,12 +433,6 @@ CIFS/Microsoft Windows.
>  +
>  False by default.
>  
> -core.preferSymlinkRefs::
> -	Instead of the default "symref" format for HEAD
> -	and other symbolic reference files, use symbolic links.
> -	This is sometimes needed to work with old scripts that
> -	expect HEAD to be a symbolic link.
> -
>  core.bare::
>  	If true this repository is assumed to be 'bare' and has no
>  	working directory associated with it.  If this is the case a
> diff --git a/cache.h b/cache.h
> index c63fcc1..5ff7df2 100644
> --- a/cache.h
> +++ b/cache.h
> @@ -625,7 +625,6 @@ extern int has_symlinks;
>  extern int minimum_abbrev, default_abbrev;
>  extern int ignore_case;
>  extern int assume_unchanged;
> -extern int prefer_symlink_refs;
>  extern int log_all_ref_updates;
>  extern int warn_ambiguous_refs;
>  extern int warn_on_object_refname_ambiguity;
> diff --git a/config.c b/config.c
> index 86a5eb2..f479eaa 100644
> --- a/config.c
> +++ b/config.c
> @@ -726,11 +726,6 @@ static int git_default_core_config(const char *var, const char *value)
>  		return 0;
>  	}
>  
> -	if (!strcmp(var, "core.prefersymlinkrefs")) {
> -		prefer_symlink_refs = git_config_bool(var, value);
> -		return 0;
> -	}
> -
>  	if (!strcmp(var, "core.logallrefupdates")) {
>  		log_all_ref_updates = git_config_bool(var, value);
>  		return 0;
> diff --git a/contrib/completion/git-completion.bash b/contrib/completion/git-completion.bash
> index 6956807..31d517d 100644
> --- a/contrib/completion/git-completion.bash
> +++ b/contrib/completion/git-completion.bash
> @@ -2046,7 +2046,6 @@ _git_config ()
>  		core.packedGitLimit
>  		core.packedGitWindowSize
>  		core.pager
> -		core.preferSymlinkRefs
>  		core.preloadindex
>  		core.quotepath
>  		core.repositoryFormatVersion
> diff --git a/environment.c b/environment.c
> index 2da7fe2..80a1c0c 100644
> --- a/environment.c
> +++ b/environment.c
> @@ -19,7 +19,6 @@ int has_symlinks = 1;
>  int minimum_abbrev = 4, default_abbrev = 7;
>  int ignore_case;
>  int assume_unchanged;
> -int prefer_symlink_refs;
>  int is_bare_repository_cfg = -1; /* unspecified */
>  int log_all_ref_updates = -1; /* unspecified */
>  int warn_ambiguous_refs = 1;
> diff --git a/refs/files-backend.c b/refs/files-backend.c
> index 180c837..6e19953 100644
> --- a/refs/files-backend.c
> +++ b/refs/files-backend.c
> @@ -2811,21 +2811,6 @@ static int commit_ref_update(struct ref_lock *lock,
>  	return 0;
>  }
>  
> -static int create_ref_symlink(struct ref_lock *lock, const char *target)
> -{
> -	int ret = -1;
> -#ifndef NO_SYMLINK_HEAD
> -	char *ref_path = get_locked_file_path(lock->lk);
> -	unlink(ref_path);
> -	ret = symlink(target, ref_path);
> -	free(ref_path);
> -
> -	if (ret)
> -		fprintf(stderr, "no symlink - falling back to symbolic ref\n");
> -#endif
> -	return ret;
> -}
> -
>  static void update_symref_reflog(struct ref_lock *lock, const char *refname,
>  				 const char *target, const char *logmsg)
>  {
> @@ -2841,11 +2826,6 @@ static void update_symref_reflog(struct ref_lock *lock, const char *refname,
>  static int create_symref_locked(struct ref_lock *lock, const char *refname,
>  				const char *target, const char *logmsg)
>  {
> -	if (prefer_symlink_refs && !create_ref_symlink(lock, target)) {
> -		update_symref_reflog(lock, refname, target, logmsg);
> -		return 0;
> -	}
> -
>  	if (!fdopen_lock_file(lock->lk, "w"))
>  		return error("unable to fdopen %s: %s",
>  			     lock->lk->tempfile.filename.buf, strerror(errno));
> diff --git a/t/t7201-co.sh b/t/t7201-co.sh
> index 8859236..715dc00 100755
> --- a/t/t7201-co.sh
> +++ b/t/t7201-co.sh
> @@ -427,18 +427,6 @@ test_expect_success 'checkout w/--track from tag fails' '
>      test "z$(git rev-parse master^0)" = "z$(git rev-parse HEAD)"
>  '
>  
> -test_expect_success 'detach a symbolic link HEAD' '
> -    git checkout master &&
> -    git config --bool core.prefersymlinkrefs yes &&
> -    git checkout side &&
> -    git checkout master &&
> -    it=$(git symbolic-ref HEAD) &&
> -    test "z$it" = zrefs/heads/master &&
> -    here=$(git rev-parse --verify refs/heads/master) &&
> -    git checkout side^ &&
> -    test "z$(git rev-parse --verify refs/heads/master)" = "z$here"
> -'
> -
>  test_expect_success \
>      'checkout with --track fakes a sensible -b <name>' '
>      git config remote.origin.fetch "+refs/heads/*:refs/remotes/origin/*" &&
> diff --git a/t/t9903-bash-prompt.sh b/t/t9903-bash-prompt.sh
> index af82049..25066f35 100755
> --- a/t/t9903-bash-prompt.sh
> +++ b/t/t9903-bash-prompt.sh
> @@ -46,15 +46,6 @@ test_expect_success 'prompt - branch name' '
>  	test_cmp expected "$actual"
>  '
>  
> -test_expect_success SYMLINKS 'prompt - branch name - symlink symref' '
> -	printf " (master)" >expected &&
> -	test_when_finished "git checkout master" &&
> -	test_config core.preferSymlinkRefs true &&
> -	git checkout master &&
> -	__git_ps1 >"$actual" &&
> -	test_cmp expected "$actual"
> -'
> -
>  test_expect_success 'prompt - unborn branch' '
>  	printf " (unborn)" >expected &&
>  	git checkout --orphan unborn &&