Documentation/technical/reftable: improve repo layout
Previously, the list of tables was in .git/refs. This makes
repo detection fail in older clients, which is undesirable.
This is proposal was discussed and approved on the git@vger list at
https://lore.kernel.org/git/CAFQ2z_PvKiz==GyS6J1H1uG0FRPL86JvDj+LjX1We4-yCSVQ+g@mail.gmail.com/
For backward compatibility, JGit could detect a file under .git/refs and
use it as a reftable list.
Change-Id: Ic0b974fa250cfa905463b811957e2a4fdd7bbc6b
Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
diff --git a/Documentation/technical/reftable.md b/Documentation/technical/reftable.md
index 1236a79..ebef68f 100644
--- a/Documentation/technical/reftable.md
+++ b/Documentation/technical/reftable.md
@@ -773,9 +773,6 @@
### Layout
-The `$GIT_DIR/refs` path is a file when reftable is configured, not a
-directory. This prevents loose references from being stored.
-
A collection of reftable files are stored in the `$GIT_DIR/reftable/`
directory:
@@ -789,28 +786,38 @@
Log-only files use the `.log` extension, while ref-only and mixed ref
and log files use `.ref`. extension.
-The stack ordering file is `$GIT_DIR/refs` and lists the current
-files, one per line, in order, from oldest (base) to newest (most
-recent):
- $ cat .git/refs
+The stack ordering file is `$GIT_DIR/reftable/tables.list` and lists the current
+files, one per line, in order, from oldest (base) to newest (most recent):
+
+ $ cat .git/reftable/tables.list
00000001-00000001.log
00000002-00000002.ref
00000003-00000003.ref
-Readers must read `$GIT_DIR/refs` to determine which files are
-relevant right now, and search through the stack in reverse order
-(last reftable is examined first).
+Readers must read `$GIT_DIR/reftable/tables.list` to determine which files are
+relevant right now, and search through the stack in reverse order (last reftable
+is examined first).
-Reftable files not listed in `refs` may be new (and about to be added
+Reftable files not listed in `tables.list` may be new (and about to be added
to the stack by the active writer), or ancient and ready to be pruned.
+### Backward compatibility
+
+Older clients should continue to recognize the directory as a git repository so
+they don't look for an enclosing repository in parent directories. To this end,
+a reftable-enabled repository must contain the following dummy files
+
+* `.git/HEAD`, a regular file containing `ref: refs/heads/.invalid`.
+* `.git/refs/`, a directory
+* `.git/refs/heads`, a regular file
+
### Readers
Readers can obtain a consistent snapshot of the reference space by
following:
-1. Open and read the `refs` file.
+1. Open and read the `tables.list` file.
2. Open each of the reftable files that it mentions.
3. If any of the files is missing, goto 1.
4. Read from the now-open files as long as necessary.
@@ -820,13 +827,13 @@
Although reftables are immutable, mutations are supported by writing a
new reftable and atomically appending it to the stack:
-1. Acquire `refs.lock`.
-2. Read `refs` to determine current reftables.
+1. Acquire `tables.list.lock`.
+2. Read `tables.list` to determine current reftables.
3. Select `update_index` to be most recent file's `max_update_index + 1`.
4. Prepare temp reftable `tmp_XXXXXX`, including log entries.
5. Rename `tmp_XXXXXX` to `${update_index}-${update_index}.ref`.
-6. Copy `refs` to `refs.lock`, appending file from (5).
-7. Rename `refs.lock` to `refs`.
+6. Copy `tables.list` to `tables.list.lock`, appending file from (5).
+7. Rename `tables.list.lock` to `tables.list`.
During step 4 the new file's `min_update_index` and `max_update_index`
are both set to the `update_index` selected by step 3. All log
@@ -834,9 +841,9 @@
This enables later correlation of which references were updated by the
same transaction.
-Because a single `refs.lock` file is used to manage locking, the
+Because a single `tables.list.lock` file is used to manage locking, the
repository is single-threaded for writers. Writers may have to
-busy-spin (with backoff) around creating `refs.lock`, for up to an
+busy-spin (with backoff) around creating `tables.list.lock`, for up to an
acceptable wait period, aborting if the repository is too busy to
mutate. Application servers wrapped around repositories (e.g. Gerrit
Code Review) can layer their own lock/wait queue to improve fairness
@@ -864,21 +871,21 @@
reftable files (from oldest to newest): A, B, C, and D. The compactor
is going to compact B and C, leaving A and D alone.
-1. Obtain lock `refs.lock` and read the `refs` file.
+1. Obtain lock `tables.list.lock` and read the `tables.list` file.
2. Obtain locks `B.lock` and `C.lock`.
Ownership of these locks prevents other processes from trying
to compact these files.
-3. Release `refs.lock`.
+3. Release `tables.list.lock`.
4. Compact `B` and `C` into a temp file `${min_update_index}-${max_update_index}_XXXXXX`.
-5. Reacquire lock `refs.lock`.
+5. Reacquire lock `tables.list.lock`.
6. Verify that `B` and `C` are still in the stack, in that order. This
should always be the case, assuming that other processes are adhering
to the locking protocol.
7. Rename `${min_update_index}-${max_update_index}_XXXXXX` to
`${min_update_index}-${max_update_index}.ref`.
-8. Write the new stack to `refs.lock`, replacing `B` and `C` with the
+8. Write the new stack to `tables.list.lock`, replacing `B` and `C` with the
file from (4).
-9. Rename `refs.lock` to `refs`.
+9. Rename `tables.list.lock` to `tables.list`.
10. Delete `B` and `C`, perhaps after a short sleep to avoid forcing
readers to backtrack.
