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 --- 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: + + SP SP 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