[PATCH] object-file: reprepare alternates when necessary

["Derrick Stolee via GitGitGadget" <gitgitgadget@gmail.com>]

From: Derrick Stolee <derrickstolee@github.com>

When an object is not found in a repository's object store, we sometimes
call reprepare_packed_git() to see if the object was temporarily moved
into a new pack-file (and its old pack-file or loose object was
deleted). This process does a scan of each pack directory within each
odb, but does not reevaluate if the odb list needs updating.

Create a new reprepare_alt_odb() method that is a similar wrapper around
prepare_alt_odb(). Call it from reprepare_packed_git() under the object
read lock to avoid readers from interacting with a potentially
incomplete odb being added to the odb list.

prepare_alt_odb() already avoids adding duplicate odbs to the list
during its progress, so it is safe to call it again from
reprepare_alt_odb() without worrying about duplicate odbs.

This change is specifically for concurrent changes to the repository, so
it is difficult to create a test that guarantees this behavior is
correct. I manually verified by introducing a reprepare_packed_git() call
into get_revision() and stepped into that call in a debugger with a
parent 'git log' process. Multiple runs of reprepare_alt_odb() kept
the_repository->objects->odb as a single-item chain until I added a
.git/objects/info/alternates file in a different process. The next run
added the new odb to the chain and subsequent runs did not add to the
chain.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>
---
    object-file: reprepare alternates when necessary
    
    This subtlety was notice by Michael Haggerty due to how alternates are
    used server-side at $DAYJOB. Moving pack-files from a repository to the
    alternate occasionally causes failures because processes that start
    before the alternate exists don't know how to find that alternate at
    run-time.
    
    Thanks,
    
     * Stolee

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1490%2Fderrickstolee%2Fstolee%2Freprepare-alternates-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1490/derrickstolee/stolee/reprepare-alternates-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/1490

 object-file.c  | 6 ++++++
 object-store.h | 1 +
 packfile.c     | 1 +
 3 files changed, 8 insertions(+)

diff --git a/object-file.c b/object-file.c
index 939865c1ae0..22acc7fd8e9 100644
--- a/object-file.c
+++ b/object-file.c
@@ -944,6 +944,12 @@ void prepare_alt_odb(struct repository *r)
 	r->objects->loaded_alternates = 1;
 }
 
+void reprepare_alt_odb(struct repository *r)
+{
+	r->objects->loaded_alternates = 0;
+	prepare_alt_odb(r);
+}
+
 /* Returns 1 if we have successfully freshened the file, 0 otherwise. */
 static int freshen_file(const char *fn)
 {
diff --git a/object-store.h b/object-store.h
index 1a713d89d7c..750c29daa54 100644
--- a/object-store.h
+++ b/object-store.h
@@ -56,6 +56,7 @@ KHASH_INIT(odb_path_map, const char * /* key: odb_path */,
 	struct object_directory *, 1, fspathhash, fspatheq)
 
 void prepare_alt_odb(struct repository *r);
+void reprepare_alt_odb(struct repository *r);
 char *compute_alternate_path(const char *path, struct strbuf *err);
 struct object_directory *find_odb(struct repository *r, const char *obj_dir);
 typedef int alt_odb_fn(struct object_directory *, void *);
diff --git a/packfile.c b/packfile.c
index 79e21ab18e7..2b28918a05e 100644
--- a/packfile.c
+++ b/packfile.c
@@ -1008,6 +1008,7 @@ void reprepare_packed_git(struct repository *r)
 	struct object_directory *odb;
 
 	obj_read_lock();
+	reprepare_alt_odb(r);
 	for (odb = r->objects->odb; odb; odb = odb->next)
 		odb_clear_loose_cache(odb);
 

base-commit: d15644fe0226af7ffc874572d968598564a230dd
-- 
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