The low-level reference transactions used to update references are
currently completely opaque to the user. While certainly desirable in
most usecases, there are some which might want to hook into the
transaction to observe all queued reference updates as well as observing
the abortion or commit of a prepared transaction.
One such usecase would be to have a set of replicas of a given Git
repository, where we perform Git operations on all of the repositories
at once and expect the outcome to be the same in all of them. While
there exist hooks already for a certain subset of Git commands that
could be used to implement a voting mechanism for this, many others
currently don't have any mechanism for this.
The above scenario is the motivation for the new "reference-transaction"
hook that reaches directly into Git's reference transaction mechanism.
The hook receives as parameter the current state the transaction was
moved to ("prepared", "committed" or "aborted") and gets via its
standard input all queued reference updates. While the exit code gets
ignored in the "committed" and "aborted" states, a non-zero exit code in
the "prepared" state will cause the transaction to be aborted
prematurely.
Given the usecase described above, a voting mechanism can now be
implemented via this hook: as soon as it gets called, it will take all
of stdin and use it to cast a vote to a central service. When all
replicas of the repository agree, the hook will exit with zero,
otherwise it will abort the transaction by returning non-zero. The most
important upside is that this will catch _all_ commands writing
references at once, allowing to implement strong consistency for
reference updates via a single mechanism.
In order to test the impact on the case where we don't have any
"reference-transaction" hook installed in the repository, this commit
introduces a new performance test for git-update-refs(1). Run against an
empty repository, it produces the following results:
Test HEAD~ HEAD
------------------------------------------------------------------------------
1400.2: update existing reference 2.05(1.58+0.54) 2.08(1.58+0.57) +1.5%
1400.3: create and destroy reference 1.79(1.38+0.49) 1.82(1.39+0.51) +1.7%
So the overhead is around ~1.5%. Given that git-update-refs(1) is a
near-direct wrapper around reference transactions, there likely is no
other command that is impacted much worse than this.
Signed-off-by: Patrick Steinhardt <ps@xxxxxx>
---
The main adjustment in this second version is that I merged the
previously three hooks into a single one that gets the transaction state
as its first parameter, as proposed by Gábor. This is mainly to enable
atomic replacement of all three scripts, even though it could still be
that the hook gets replaced during a session. But I think it makes sense
regardless to merge these hooks into a single one.
I've also made changes based on Junio's feedback and added a benchmark
that exercises git-update-refs(1) as a proxy for this change. I guess
the ~1.5% penalty isn't too bad. It might be improved by caching hook
existence, but I don't think it necessary right now.
Thanks to both of you for your feedback!
Patrick
Documentation/githooks.txt | 29 ++++++++
refs.c | 72 +++++++++++++++++++-
t/perf/p1400-update-ref.sh | 31 +++++++++
t/t1416-ref-transaction-hooks.sh | 109 +++++++++++++++++++++++++++++++
4 files changed, 239 insertions(+), 2 deletions(-)
create mode 100755 t/perf/p1400-update-ref.sh
create mode 100755 t/t1416-ref-transaction-hooks.sh
diff --git a/Documentation/githooks.txt b/Documentation/githooks.txt
index 81f2a87e88..642471109f 100644
--- a/Documentation/githooks.txt
+++ b/Documentation/githooks.txt
@@ -404,6 +404,35 @@ Both standard output and standard error output are forwarded to
`git send-pack` on the other end, so you can simply `echo` messages
for the user.
+ref-transaction
+~~~~~~~~~~~~~~~
+
+This hook is invoked by any Git command that performs reference
+updates. It executes whenever a reference transaction is prepared,
+committed or aborted and may thus get called multiple times.
+
+The hook takes exactly one argument, which is the current state the
+given reference transaction is in:
+
+ - "prepared": All reference updates have been queued to the
+ transaction and references were locked on disk.
+
+ - "committed": The reference transaction was committed and all
+ references now have their respective new value.
+
+ - "aborted": The reference transaction was aborted, no changes
+ were performed and the locks have been released.
+
+For each reference update that was added to the transaction, the hook
+receives on standard input a line of the format:
+
+ <old-value> SP <new-value> SP <ref-name> LF
+
+The exit status of the hook is ignored for any state except for the
+"prepared" state. In the "prepared" state, a non-zero exit status will
+cause the transaction to be aborted. The hook will not be called with
+"aborted" state in that case.
+
push-to-checkout
~~~~~~~~~~~~~~~~
diff --git a/refs.c b/refs.c
index 224ff66c7b..af752e1759 100644
--- a/refs.c
+++ b/refs.c
@@ -9,6 +9,7 @@
#include "iterator.h"
#include "refs.h"
#include "refs/refs-internal.h"
+#include "run-command.h"
#include "object-store.h"
#include "object.h"
#include "tag.h"
@@ -16,6 +17,7 @@
#include "worktree.h"
#include "argv-array.h"
#include "repository.h"
+#include "sigchain.h"
/*
* List of all available backends
@@ -1986,10 +1988,61 @@ int ref_update_reject_duplicates(struct string_list *refnames,
return 0;
}
+static int run_transaction_hook(struct ref_transaction *transaction,
+ const char *state)
+{
+ struct child_process proc = CHILD_PROCESS_INIT;
+ struct strbuf buf = STRBUF_INIT;
+ int saved_errno = 0, ret, i;
+ const char *hook;
+
+ hook = find_hook("reference-transaction");
+ if (!hook)
+ return 0;
+
+ argv_array_pushl(&proc.args, hook, state, NULL);
+ proc.in = -1;
+ proc.stdout_to_stderr = 1;
+ proc.trace2_hook_name = hook;
+
+ ret = start_command(&proc);
+ if (ret)
+ return ret;
+
+ sigchain_push(SIGPIPE, SIG_IGN);
+
+ for (i = 0; i < transaction->nr; i++) {
+ struct ref_update *update = transaction->updates[i];
+
+ strbuf_reset(&buf);
+ strbuf_addf(&buf, "%s %s %s\n",
+ oid_to_hex(&update->old_oid),
+ oid_to_hex(&update->new_oid),
+ update->refname);
+
+ if (write_in_full(proc.in, buf.buf, buf.len) < 0) {
+ if (errno != EPIPE)
+ saved_errno = errno;
+ break;
+ }
+ }
+
+ close(proc.in);
+ sigchain_pop(SIGPIPE);
+ strbuf_release(&buf);
+
+ ret = finish_command(&proc);
+ if (ret)
+ return ret;
+
+ return saved_errno;
+}
+
int ref_transaction_prepare(struct ref_transaction *transaction,
struct strbuf *err)
{
struct ref_store *refs = transaction->ref_store;
+ int ret;
switch (transaction->state) {
case REF_TRANSACTION_OPEN:
@@ -2012,7 +2065,17 @@ int ref_transaction_prepare(struct ref_transaction *transaction,
return -1;
}
- return refs->be->transaction_prepare(refs, transaction, err);
+ ret = refs->be->transaction_prepare(refs, transaction, err);
+ if (ret)
+ return ret;
+
+ ret = run_transaction_hook(transaction, "prepared");
+ if (ret) {
+ ref_transaction_abort(transaction, err);
+ die(_("ref updates aborted by hook"));
+ }
+
+ return 0;
}
int ref_transaction_abort(struct ref_transaction *transaction,
@@ -2036,6 +2099,8 @@ int ref_transaction_abort(struct ref_transaction *transaction,
break;
}
+ run_transaction_hook(transaction, "aborted");
+
ref_transaction_free(transaction);
return ret;
}
@@ -2064,7 +2129,10 @@ int ref_transaction_commit(struct ref_transaction *transaction,
break;
}
- return refs->be->transaction_finish(refs, transaction, err);
+ ret = refs->be->transaction_finish(refs, transaction, err);
+ if (!ret)
+ run_transaction_hook(transaction, "committed");
+ return ret;
}
int refs_verify_refname_available(struct ref_store *refs,
diff --git a/t/perf/p1400-update-ref.sh b/t/perf/p1400-update-ref.sh
new file mode 100755
index 0000000000..4f4519529e
--- /dev/null
+++ b/t/perf/p1400-update-ref.sh
@@ -0,0 +1,31 @@
+#!/bin/sh
+
+test_description="Tests performance of update-ref"
+
+. ./perf-lib.sh
+
+test_perf_fresh_repo
+
+test_expect_success "setup" '
+ test_commit PRE &&
+ test_commit POST &&
+ git branch update-branch
+'
+
+test_perf "update existing reference" '
+ for i in $(test_seq 1000)
+ do
+ git update-ref refs/heads/update-branch PRE POST &&
+ git update-ref refs/heads/update-branch POST PRE
+ done
+'
+
+test_perf "create and destroy reference" '
+ for i in $(test_seq 1000)
+ do
+ git update-ref refs/heads/new POST
+ git update-ref -d refs/heads/new
+ done
+'
+
+test_done
diff --git a/t/t1416-ref-transaction-hooks.sh b/t/t1416-ref-transaction-hooks.sh
new file mode 100755
index 0000000000..da58d867a5
--- /dev/null
+++ b/t/t1416-ref-transaction-hooks.sh
@@ -0,0 +1,109 @@
+#!/bin/sh
+
+test_description='reference transaction hooks'
+
+. ./test-lib.sh
+
+test_expect_success setup '
+ mkdir -p .git/hooks &&
+ test_commit PRE &&
+ test_commit POST &&
+ POST_OID=$(git rev-parse POST)
+'
+
+test_expect_success 'hook allows updating ref if successful' '
+ test_when_finished "rm .git/hooks/reference-transaction" &&
+ git reset --hard PRE &&
+ write_script .git/hooks/reference-transaction <<-\EOF &&
+ echo "$*" >>actual
+ EOF
+ cat >expect <<-EOF &&
+ prepared
+ committed
+ EOF
+ git update-ref HEAD POST &&
+ test_cmp expect actual
+'
+
+test_expect_success 'hook aborts updating ref in prepared state' '
+ test_when_finished "rm .git/hooks/reference-transaction" &&
+ git reset --hard PRE &&
+ write_script .git/hooks/reference-transaction <<-\EOF &&
+ if test "$1" = prepared
+ then
+ exit 1
+ fi
+ EOF
+ test_must_fail git update-ref HEAD POST 2>err &&
+ test_i18ngrep "ref updates aborted by hook" err
+'
+
+test_expect_success 'hook gets all queued updates in prepared state' '
+ test_when_finished "rm .git/hooks/reference-transaction actual" &&
+ git reset --hard PRE &&
+ write_script .git/hooks/reference-transaction <<-\EOF &&
+ if test "$1" = prepared
+ then
+ while read -r line
+ do
+ printf "%s\n" "$line"
+ done >actual
+ fi
+ EOF
+ cat >expect <<-EOF &&
+ $ZERO_OID $POST_OID HEAD
+ $ZERO_OID $POST_OID refs/heads/master
+ EOF
+ git update-ref HEAD POST <<-EOF &&
+ update HEAD $ZERO_OID $POST_OID
+ update refs/heads/master $ZERO_OID $POST_OID
+ EOF
+ test_cmp expect actual
+'
+
+test_expect_success 'hook gets all queued updates in committed state' '
+ test_when_finished "rm .git/hooks/reference-transaction actual" &&
+ git reset --hard PRE &&
+ write_script .git/hooks/reference-transaction <<-\EOF &&
+ if test "$1" = committed
+ then
+ while read -r line
+ do
+ printf "%s\n" "$line"
+ done >actual
+ fi
+ EOF
+ cat >expect <<-EOF &&
+ $ZERO_OID $POST_OID HEAD
+ $ZERO_OID $POST_OID refs/heads/master
+ EOF
+ git update-ref HEAD POST &&
+ test_cmp expect actual
+'
+
+test_expect_success 'hook gets all queued updates in aborted state' '
+ test_when_finished "rm .git/hooks/reference-transaction actual" &&
+ git reset --hard PRE &&
+ write_script .git/hooks/reference-transaction <<-\EOF &&
+ if test "$1" = aborted
+ then
+ while read -r line
+ do
+ printf "%s\n" "$line"
+ done >actual
+ fi
+ EOF
+ cat >expect <<-EOF &&
+ $ZERO_OID $POST_OID HEAD
+ $ZERO_OID $POST_OID refs/heads/master
+ EOF
+ git update-ref --stdin <<-EOF &&
+ start
+ update HEAD POST $ZERO_OID
+ update refs/heads/master POST $ZERO_OID
+ abort
+ EOF
+ test_cmp expect actual
+'
+
+test_done
--
2.27.0
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