refs: alternate reftable ref backend implementation

This introduces the reftable backend as an alternative to the packed
backend. This simplifies the code, because we now longer have to worry
about:

- pseudorefs and the HEAD ref
- worktrees
- commands that blur together files and references (cherry-pick, rebase)

This deviates from the spec that in
Documentation/technical/reftable.txt. It might be possible to update
the code such that all writes by default go to reftable directly. Then
the result would be compatible with an implementation that writes only
reftable (the reftable lock would still prevent races) relative to an
implementation that disregards loose refs. Or,  JGit could be adapted
to follow this implementation.

For this incremental path, the reftable format is arguably more
complex than necessary, as

- packed-refs doesn't support symrefs
- reflogs aren't moved into reftable/

on the other hand, the code is already there, and it's well-structured
and well-tested.

This implementation is a prototype. To test, you need to do `export
GIT_TEST_REFTABLE=1`. Doing so creates a handful of errors in the
test-suite, most seemingly related to the new behavior of pack-refs
(which creates a reftable/ dir and not a packed-refs file.), but it
seems overseeable.

Signed-off-by: Han-Wen Nienhuys <[email protected]>

Makefile

@@ -1118,6 +1118,7 @@ LIB_OBJS += reflog.o
 LIB_OBJS += refs.o
 LIB_OBJS += refs/debug.o
 LIB_OBJS += refs/files-backend.o
+LIB_OBJS += refs/reftable-backend.o
 LIB_OBJS += refs/iterator.o
 LIB_OBJS += refs/packed-backend.o
 LIB_OBJS += refs/ref-cache.o

config.mak.uname

@@ -743,7 +743,7 @@ vcxproj:
 
 	# Make .vcxproj files and add them
 	perl contrib/buildsystems/generate -g Vcxproj
-	git add -f git.sln {*,*/lib,t/helper/*}/*.vcxproj
+	git add -f git.sln {*,*/lib,*/libreftable,t/helper/*}/*.vcxproj
 
 	# Generate the LinkOrCopyBuiltins.targets and LinkOrCopyRemoteHttp.targets file
 	(echo '<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">' && \

contrib/workdir/git-new-workdir

@@ -79,7 +79,7 @@ trap cleanup $siglist
 # create the links to the original repo.  explicitly exclude index, HEAD and
 # logs/HEAD from the list since they are purely related to the current working
 # directory, and should not be shared.
-for x in config refs logs/refs objects info hooks packed-refs remotes rr-cache svn
+for x in config refs logs/refs objects info hooks packed-refs remotes rr-cache svn reftable
 do
 	# create a containing directory if needed
 	case $x in

refs/files-backend.c

@@ -9,6 +9,7 @@
 #include "refs-internal.h"
 #include "ref-cache.h"
 #include "packed-backend.h"
+#include "reftable-backend.h"
 #include "../ident.h"
 #include "../iterator.h"
 #include "../dir-iterator.h"
@@ -103,8 +104,13 @@ static struct ref_store *files_ref_store_create(struct repository *repo,
 	refs->store_flags = flags;
 	get_common_dir_noenv(&sb, gitdir);
 	refs->gitcommondir = strbuf_detach(&sb, NULL);
+
+	/* TODO: should look at the repo to decide whether to use packed-refs or
+	 * reftable. */
 	refs->packed_ref_store =
-		packed_ref_store_create(repo, refs->gitcommondir, flags);
+		git_env_bool("GIT_TEST_REFTABLE", 0)  
+		? git_reftable_ref_store_create(repo, refs->gitcommondir, flags)
+		: packed_ref_store_create(repo, refs->gitcommondir, flags);
 
 	chdir_notify_reparent("files-backend $GIT_DIR", &refs->base.gitdir);
 	chdir_notify_reparent("files-backend $GIT_COMMONDIR",

refs/packed-backend.c

@@ -1153,7 +1153,7 @@ static int write_packed_entry(FILE *fh, const char *refname,
 	return 0;
 }
 
-int packed_refs_lock(struct ref_store *ref_store, int flags, struct strbuf *err)
+static int packed_refs_lock(struct ref_store *ref_store, int flags, struct strbuf *err)
 {
 	struct packed_ref_store *refs =
 		packed_downcast(ref_store, REF_STORE_WRITE | REF_STORE_MAIN,
@@ -1212,7 +1212,7 @@ int packed_refs_lock(struct ref_store *ref_store, int flags, struct strbuf *err)
 	return 0;
 }
 
-void packed_refs_unlock(struct ref_store *ref_store)
+static void packed_refs_unlock(struct ref_store *ref_store)
 {
 	struct packed_ref_store *refs = packed_downcast(
 			ref_store,
@@ -1224,16 +1224,6 @@ void packed_refs_unlock(struct ref_store *ref_store)
 	rollback_lock_file(&refs->lock);
 }
 
-int packed_refs_is_locked(struct ref_store *ref_store)
-{
-	struct packed_ref_store *refs = packed_downcast(
-			ref_store,
-			REF_STORE_READ | REF_STORE_WRITE,
-			"packed_refs_is_locked");
-
-	return is_lock_file_locked(&refs->lock);
-}
-
 /*
  * The packed-refs header line that we write out. Perhaps other traits
  * will be added later.

refs/refs-internal.h

@@ -700,6 +700,7 @@ struct ref_storage_be {
 };
 
 extern struct ref_storage_be refs_be_files;
+extern struct ref_storage_be refs_be_reftable;
 extern struct ref_storage_be refs_be_packed;
 
 /*