[PATCH 3/8] refs: new ref types to make per-worktree refs visible to all worktrees

["Nguyễn Thái Ngọc Duy" <pclouds@gmail.com>]

One of the problems with multiple worktree is accessing per-worktree
refs of one worktree from another worktree. This was sort of solved by
multiple ref store, where the code can open the ref store of another
worktree and has access to the ref space of that worktree.

The problem with this is reporting. "HEAD" in another ref space is
also called "HEAD" like in the current ref space. In order to
differentiate them, all the code must somehow carry the ref store
around and print something like "HEAD from this ref store".

But that is not feasible (or possible with a _lot_ of work). With the
current design, we pass a reference around as a string (so called
"refname"). Extending this design to pass a string _and_ a ref store
is a nightmare, especially when handling extended SHA-1 syntax.

So we do it another way. Instead of entering a separate ref space, we
make refs from other worktrees available in the current ref space. So
"HEAD" is always HEAD of the current worktree, but then we can have
"worktrees/blah/HEAD" to denote HEAD from a worktree named
"blah". This syntax coincidentally matches the underlying directory
structure which makes implementation a bit easier.

The main worktree has to be treated specially because well.. it's
special from the beginning. So HEAD from the main worktree is
acccessible via the name "main/HEAD" (we can't use
"worktrees/main/HEAD" because "main" under "worktrees" is not
reserved).

This patch also makes it possible to specify refs from one worktree in
another one, e.g.

    git log worktrees/foo/HEAD

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
---
 refs.c                   | 20 ++++++++++++++++++++
 refs.h                   |  8 +++++---
 refs/files-backend.c     | 28 ++++++++++++++++++++++++++++
 t/t1415-worktree-refs.sh | 30 ++++++++++++++++++++++++++++++
 4 files changed, 83 insertions(+), 3 deletions(-)

diff --git a/refs.c b/refs.c
index a851ef085b..90b73c7334 100644
--- a/refs.c
+++ b/refs.c
@@ -641,12 +641,32 @@ static int is_pseudoref_syntax(const char *refname)
 	return 1;
 }
 
+static int is_main_pseudoref_syntax(const char *refname)
+{
+	return skip_prefix(refname, "main/", &refname) &&
+		is_pseudoref_syntax(refname);
+}
+
+static int is_other_pseudoref_syntax(const char *refname)
+{
+	if (!skip_prefix(refname, "worktrees/", &refname))
+		return 0;
+	refname = strchr(refname, '/');
+	if (!refname)
+		return 0;
+	return is_pseudoref_syntax(refname + 1);
+}
+
 enum ref_type ref_type(const char *refname)
 {
 	if (is_per_worktree_ref(refname))
 		return REF_TYPE_PER_WORKTREE;
 	if (is_pseudoref_syntax(refname))
 		return REF_TYPE_PSEUDOREF;
+	if (is_main_pseudoref_syntax(refname))
+		return REF_TYPE_MAIN_PSEUDOREF;
+	if (is_other_pseudoref_syntax(refname))
+		return REF_TYPE_OTHER_PSEUDOREF;
 	return REF_TYPE_NORMAL;
 }
 
diff --git a/refs.h b/refs.h
index bd52c1bbae..9b53dbeae8 100644
--- a/refs.h
+++ b/refs.h
@@ -704,9 +704,11 @@ int parse_hide_refs_config(const char *var, const char *value, const char *);
 int ref_is_hidden(const char *, const char *);
 
 enum ref_type {
-	REF_TYPE_PER_WORKTREE,
-	REF_TYPE_PSEUDOREF,
-	REF_TYPE_NORMAL,
+	REF_TYPE_PER_WORKTREE,	  /* refs inside refs/ but not shared       */
+	REF_TYPE_PSEUDOREF,	  /* refs outside refs/ in current worktree */
+	REF_TYPE_MAIN_PSEUDOREF,  /* pseudo refs from the main worktree     */
+	REF_TYPE_OTHER_PSEUDOREF, /* pseudo refs from other worktrees       */
+	REF_TYPE_NORMAL,	  /* normal/shared refs inside refs/        */
 };
 
 enum ref_type ref_type(const char *refname);
diff --git a/refs/files-backend.c b/refs/files-backend.c
index 416eafa453..bf9ed633b1 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -149,6 +149,23 @@ static struct files_ref_store *files_downcast(struct ref_store *ref_store,
 	return refs;
 }
 
+static void files_reflog_path_other_worktrees(struct files_ref_store *refs,
+					      struct strbuf *sb,
+					      const char *refname)
+{
+	const char *real_ref;
+
+	if (!skip_prefix(refname, "worktrees/", &real_ref))
+		BUG("refname %s is not a other-worktree ref", refname);
+	real_ref = strchr(real_ref, '/');
+	if (!real_ref)
+		BUG("refname %s is not a other-worktree ref", refname);
+	real_ref++;
+
+	strbuf_addf(sb, "%s/%.*slogs/%s", refs->gitcommondir,
+		    (int)(real_ref - refname), refname, real_ref);
+}
+
 static void files_reflog_path(struct files_ref_store *refs,
 			      struct strbuf *sb,
 			      const char *refname)
@@ -158,6 +175,12 @@ static void files_reflog_path(struct files_ref_store *refs,
 	case REF_TYPE_PSEUDOREF:
 		strbuf_addf(sb, "%s/logs/%s", refs->gitdir, refname);
 		break;
+	case REF_TYPE_OTHER_PSEUDOREF:
+		return files_reflog_path_other_worktrees(refs, sb, refname);
+	case REF_TYPE_MAIN_PSEUDOREF:
+		if (!skip_prefix(refname, "main/", &refname))
+			BUG("ref %s is not a main pseudoref", refname);
+		/* passthru */
 	case REF_TYPE_NORMAL:
 		strbuf_addf(sb, "%s/logs/%s", refs->gitcommondir, refname);
 		break;
@@ -176,6 +199,11 @@ static void files_ref_path(struct files_ref_store *refs,
 	case REF_TYPE_PSEUDOREF:
 		strbuf_addf(sb, "%s/%s", refs->gitdir, refname);
 		break;
+	case REF_TYPE_MAIN_PSEUDOREF:
+		if (!skip_prefix(refname, "main/", &refname))
+			BUG("ref %s is not a main pseudoref", refname);
+		/* passthru */
+	case REF_TYPE_OTHER_PSEUDOREF:
 	case REF_TYPE_NORMAL:
 		strbuf_addf(sb, "%s/%s", refs->gitcommondir, refname);
 		break;
diff --git a/t/t1415-worktree-refs.sh b/t/t1415-worktree-refs.sh
index 0c2d5f89a9..46ca7bfc19 100755
--- a/t/t1415-worktree-refs.sh
+++ b/t/t1415-worktree-refs.sh
@@ -33,4 +33,34 @@ test_expect_success 'refs/local are per-worktree' '
 	( cd wt2 && test_cmp_rev local/foo wt2 )
 '
 
+test_expect_success 'resolve main/HEAD' '
+	test_cmp_rev main/HEAD initial &&
+	( cd wt1 && test_cmp_rev main/HEAD initial ) &&
+	( cd wt2 && test_cmp_rev main/HEAD initial )
+'
+
+test_expect_success 'resolve worktrees/xx/HEAD' '
+	test_cmp_rev worktrees/wt1/HEAD wt1 &&
+	( cd wt1 && test_cmp_rev worktrees/wt1/HEAD wt1 ) &&
+	( cd wt2 && test_cmp_rev worktrees/wt1/HEAD wt1 )
+'
+
+test_expect_success 'reflog of main/HEAD' '
+	git reflog HEAD | sed "s/HEAD/main\/HEAD/" >expected &&
+	git reflog main/HEAD >actual &&
+	test_cmp expected actual &&
+	git -C wt1 reflog main/HEAD >actual.wt1 &&
+	test_cmp expected actual.wt1
+'
+
+test_expect_success 'reflog of worktrees/xx/HEAD' '
+	git -C wt2 reflog HEAD | sed "s/HEAD/worktrees\/wt2\/HEAD/" >expected &&
+	git reflog worktrees/wt2/HEAD >actual &&
+	test_cmp expected actual &&
+	git -C wt1 reflog worktrees/wt2/HEAD >actual.wt1 &&
+	test_cmp expected actual.wt1 &&
+	git -C wt2 reflog worktrees/wt2/HEAD >actual.wt2 &&
+	test_cmp expected actual.wt2
+'
+
 test_done
-- 
2.19.0.647.gb9a6049235