refs: set up ref consistency check infrastructure

The "struct ref_store" is the base class which contains the "be" pointer
which provides backend-specific functions whose interfaces are defined
in the "ref_storage_be". We could reuse this polymorphism to define only
one interface. For every backend, we need to provide its own function
pointer.

The interfaces defined in the `ref_storage_be` are carefully structured
in semantic. It's organized as the five parts:

1. The name and the initialization interfaces.
2. The ref transaction interfaces.
3. The ref internal interfaces (pack, rename and copy).
4. The ref filesystem interfaces.
5. The reflog related interfaces.

To keep consistent with the git-fsck(1), add a new interface named
"fsck_refs_fn" to the end of "ref_storage_be". This semantic cannot be
grouped into any above five categories. Explicitly add blank line to
make it different from others.

Last, implement placeholder functions for each ref backends.

Mentored-by: Patrick Steinhardt <ps@pks.im>
Mentored-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: shejialuo <shejialuo@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs.c

@@ -318,6 +318,11 @@ int check_refname_format(const char *refname, int flags)
 	return check_or_sanitize_refname(refname, flags, NULL);
 }
 
+int refs_fsck(struct ref_store *refs, struct fsck_options *o)
+{
+	return refs->be->fsck(refs, o);
+}
+
 void sanitize_refname_component(const char *refname, struct strbuf *out)
 {
 	if (check_or_sanitize_refname(refname, REFNAME_ALLOW_ONELEVEL, out))

refs.h

@@ -4,6 +4,7 @@
 #include "commit.h"
 #include "repository.h"
 
+struct fsck_options;
 struct object_id;
 struct ref_store;
 struct strbuf;
@@ -541,6 +542,13 @@ int refs_for_each_reflog(struct ref_store *refs, each_reflog_fn fn, void *cb_dat
  */
 int check_refname_format(const char *refname, int flags);
 
+/*
+ * Check the reference database for consistency. Return 0 if refs and
+ * reflogs are consistent, and non-zero otherwise. The errors will be
+ * written to stderr.
+ */
+int refs_fsck(struct ref_store *refs, struct fsck_options *o);
+
 /*
  * Apply the rules from check_refname_format, but mutate the result until it
  * is acceptable, and place the result in "out".

refs/debug.c

@@ -419,6 +419,15 @@ static int debug_reflog_expire(struct ref_store *ref_store, const char *refname,
 	return res;
 }
 
+static int debug_fsck(struct ref_store *ref_store,
+		      struct fsck_options *o)
+{
+	struct debug_ref_store *drefs = (struct debug_ref_store *)ref_store;
+	int res = drefs->refs->be->fsck(drefs->refs, o);
+	trace_printf_key(&trace_refs, "fsck: %d\n", res);
+	return res;
+}
+
 struct ref_storage_be refs_be_debug = {
 	.name = "debug",
 	.init = NULL,
@@ -451,4 +460,6 @@ struct ref_storage_be refs_be_debug = {
 	.create_reflog = debug_create_reflog,
 	.delete_reflog = debug_delete_reflog,
 	.reflog_expire = debug_reflog_expire,
+
+	.fsck = debug_fsck,
 };

refs/files-backend.c

@@ -3408,6 +3408,15 @@ static int files_ref_store_remove_on_disk(struct ref_store *ref_store,
 	return ret;
 }
 
+static int files_fsck(struct ref_store *ref_store,
+		      struct fsck_options *o)
+{
+	struct files_ref_store *refs =
+		files_downcast(ref_store, REF_STORE_READ, "fsck");
+
+	return refs->packed_ref_store->be->fsck(refs->packed_ref_store, o);
+}
+
 struct ref_storage_be refs_be_files = {
 	.name = "files",
 	.init = files_ref_store_init,
@@ -3434,5 +3443,7 @@ struct ref_storage_be refs_be_files = {
 	.reflog_exists = files_reflog_exists,
 	.create_reflog = files_create_reflog,
 	.delete_reflog = files_delete_reflog,
-	.reflog_expire = files_reflog_expire
+	.reflog_expire = files_reflog_expire,
+
+	.fsck = files_fsck,
 };

refs/packed-backend.c

@@ -1735,6 +1735,12 @@ static struct ref_iterator *packed_reflog_iterator_begin(struct ref_store *ref_s
 	return empty_ref_iterator_begin();
 }
 
+static int packed_fsck(struct ref_store *ref_store,
+		       struct fsck_options *o)
+{
+	return 0;
+}
+
 struct ref_storage_be refs_be_packed = {
 	.name = "packed",
 	.init = packed_ref_store_init,
@@ -1762,4 +1768,6 @@ struct ref_storage_be refs_be_packed = {
 	.create_reflog = NULL,
 	.delete_reflog = NULL,
 	.reflog_expire = NULL,
+
+	.fsck = packed_fsck,
 };

refs/refs-internal.h

@@ -4,6 +4,7 @@
 #include "refs.h"
 #include "iterator.h"
 
+struct fsck_options;
 struct ref_transaction;
 
 /*
@@ -650,6 +651,9 @@ typedef int read_raw_ref_fn(struct ref_store *ref_store, const char *refname,
 typedef int read_symbolic_ref_fn(struct ref_store *ref_store, const char *refname,
 				 struct strbuf *referent);
 
+typedef int fsck_fn(struct ref_store *ref_store,
+		    struct fsck_options *o);
+
 struct ref_storage_be {
 	const char *name;
 	ref_store_init_fn *init;
@@ -677,6 +681,8 @@ struct ref_storage_be {
 	create_reflog_fn *create_reflog;
 	delete_reflog_fn *delete_reflog;
 	reflog_expire_fn *reflog_expire;
+
+	fsck_fn *fsck;
 };
 
 extern struct ref_storage_be refs_be_files;

refs/reftable-backend.c

@@ -2303,6 +2303,12 @@ done:
 	return ret;
 }
 
+static int reftable_be_fsck(struct ref_store *ref_store,
+			    struct fsck_options *o)
+{
+	return 0;
+}
+
 struct ref_storage_be refs_be_reftable = {
 	.name = "reftable",
 	.init = reftable_be_init,
@@ -2330,4 +2336,6 @@ struct ref_storage_be refs_be_reftable = {
 	.create_reflog = reftable_be_create_reflog,
 	.delete_reflog = reftable_be_delete_reflog,
 	.reflog_expire = reftable_be_reflog_expire,
+
+	.fsck = reftable_be_fsck,
 };