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update-ref: implement interactive transaction handling 

The git-update-ref(1) command can only handle queueing transactions
right now via its "--stdin" parameter, but there is no way for users to
handle the transaction itself in a more explicit way. E.g. in a
replicated scenario, one may imagine a coordinator that spawns
git-update-ref(1) for multiple repositories and only if all agree that
an update is possible will the coordinator send a commit. Such a
transactional session could look like

    > start
    < start: ok
    > update refs/heads/master $OLD $NEW
    > prepare
    < prepare: ok
    # All nodes have returned "ok"
    > commit
    < commit: ok

or

    > start
    < start: ok
    > create refs/heads/master $OLD $NEW
    > prepare
    < fatal: cannot lock ref 'refs/heads/master': reference already exists
    # On all other nodes:
    > abort
    < abort: ok

In order to allow for such transactional sessions, this commit
introduces four new commands for git-update-ref(1), which matches those
we have internally already with the exception of "start":

    - start: start a new transaction

    - prepare: prepare the transaction, that is try to lock all
               references and verify their current value matches the
               expected one

    - commit: explicitly commit a session, that is update references to
              match their new expected state

    - abort: abort a session and roll back all changes

By design, git-update-ref(1) will commit as soon as standard input is
being closed. While fine in a non-transactional world, it is definitely
unexpected in a transactional world. Because of this, as soon as any of
the new transactional commands is used, the default will change to
aborting without an explicit "commit". To avoid a race between queueing
updates and the first "prepare" that starts a transaction, the "start"
command has been added to start an explicit transaction.

Add some tests to exercise this new functionality.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
maint
Patrick Steinhardt 2020-04-02 09:10:02 +02:00 committed by Junio C Hamano
parent 94fd491a54
commit e48cf33b61
3 changed files with 255 additions and 8 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

26
Documentation/git-update-ref.txt
View File

@ -66,6 +66,10 @@ performs all modifications together. Specify commands of the form:
delete SP <ref> [SP <oldvalue>] LF
verify SP <ref> [SP <oldvalue>] LF
option SP <opt> LF
start LF
prepare LF
commit LF
abort LF

With `--create-reflog`, update-ref will create a reflog for each ref
even if one would not ordinarily be created.
@ -83,6 +87,10 @@ quoting:
delete SP <ref> NUL [<oldvalue>] NUL
verify SP <ref> NUL [<oldvalue>] NUL
option SP <opt> NUL
start NUL
prepare NUL
commit NUL
abort NUL

In this format, use 40 "0" to specify a zero value, and use the empty
string to specify a missing value.
@ -114,6 +122,24 @@ option::
The only valid option is `no-deref` to avoid dereferencing
a symbolic ref.

start::
Start a transaction. In contrast to a non-transactional session, a
transaction will automatically abort if the session ends without an
explicit commit.

prepare::
Prepare to commit the transaction. This will create lock files for all
queued reference updates. If one reference could not be locked, the
transaction will be aborted.

commit::
Commit all reference updates queued for the transaction, ending the
transaction.

abort::
Abort the transaction, releasing all locks if the transaction is in
prepared state.

If all <ref>s can be locked with matching <oldvalue>s
simultaneously, all modifications are performed. Otherwise, no
modifications are performed. Note that while each individual

106
builtin/update-ref.c
View File

@ -312,21 +312,80 @@ static void parse_cmd_option(struct ref_transaction *transaction,
die("option unknown: %s", next);
}

static void parse_cmd_start(struct ref_transaction *transaction,
const char *next, const char *end)
{
if (*next != line_termination)
die("start: extra input: %s", next);
puts("start: ok");
}

static void parse_cmd_prepare(struct ref_transaction *transaction,
const char *next, const char *end)
{
struct strbuf error = STRBUF_INIT;
if (*next != line_termination)
die("prepare: extra input: %s", next);
if (ref_transaction_prepare(transaction, &error))
die("prepare: %s", error.buf);
puts("prepare: ok");
}

static void parse_cmd_abort(struct ref_transaction *transaction,
const char *next, const char *end)
{
struct strbuf error = STRBUF_INIT;
if (*next != line_termination)
die("abort: extra input: %s", next);
if (ref_transaction_abort(transaction, &error))
die("abort: %s", error.buf);
puts("abort: ok");
}

static void parse_cmd_commit(struct ref_transaction *transaction,
const char *next, const char *end)
{
struct strbuf error = STRBUF_INIT;
if (*next != line_termination)
die("commit: extra input: %s", next);
if (ref_transaction_commit(transaction, &error))
die("commit: %s", error.buf);
puts("commit: ok");
ref_transaction_free(transaction);
}

enum update_refs_state {
/* Non-transactional state open for updates. */
UPDATE_REFS_OPEN,
/* A transaction has been started. */
UPDATE_REFS_STARTED,
/* References are locked and ready for commit */
UPDATE_REFS_PREPARED,
/* Transaction has been committed or closed. */
UPDATE_REFS_CLOSED,
};

static const struct parse_cmd {
const char *prefix;
void (*fn)(struct ref_transaction *, const char *, const char *);
unsigned args;
enum update_refs_state state;
} command[] = {
{ "update", parse_cmd_update, 3 },
{ "create", parse_cmd_create, 2 },
{ "delete", parse_cmd_delete, 2 },
{ "verify", parse_cmd_verify, 2 },
{ "option", parse_cmd_option, 1 },
{ "update", parse_cmd_update, 3, UPDATE_REFS_OPEN },
{ "create", parse_cmd_create, 2, UPDATE_REFS_OPEN },
{ "delete", parse_cmd_delete, 2, UPDATE_REFS_OPEN },
{ "verify", parse_cmd_verify, 2, UPDATE_REFS_OPEN },
{ "option", parse_cmd_option, 1, UPDATE_REFS_OPEN },
{ "start", parse_cmd_start, 0, UPDATE_REFS_STARTED },
{ "prepare", parse_cmd_prepare, 0, UPDATE_REFS_PREPARED },
{ "abort", parse_cmd_abort, 0, UPDATE_REFS_CLOSED },
{ "commit", parse_cmd_commit, 0, UPDATE_REFS_CLOSED },
};

static void update_refs_stdin(void)
{
struct strbuf input = STRBUF_INIT, err = STRBUF_INIT;
enum update_refs_state state = UPDATE_REFS_OPEN;
struct ref_transaction *transaction;
int i, j;

@ -374,14 +433,45 @@ static void update_refs_stdin(void)
if (strbuf_appendwholeline(&input, stdin, line_termination))
break;

switch (state) {
case UPDATE_REFS_OPEN:
case UPDATE_REFS_STARTED:
/* Do not downgrade a transaction to a non-transaction. */
if (cmd->state >= state)
state = cmd->state;
break;
case UPDATE_REFS_PREPARED:
if (cmd->state != UPDATE_REFS_CLOSED)
die("prepared transactions can only be closed");
state = cmd->state;
break;
case UPDATE_REFS_CLOSED:
die("transaction is closed");
break;
}

cmd->fn(transaction, input.buf + strlen(cmd->prefix) + !!cmd->args,
input.buf + input.len);
}

if (ref_transaction_commit(transaction, &err))
die("%s", err.buf);
switch (state) {
case UPDATE_REFS_OPEN:
/* Commit by default if no transaction was requested. */
if (ref_transaction_commit(transaction, &err))
die("%s", err.buf);
ref_transaction_free(transaction);
break;
case UPDATE_REFS_STARTED:
case UPDATE_REFS_PREPARED:
/* If using a transaction, we want to abort it. */
if (ref_transaction_abort(transaction, &err))
die("%s", err.buf);
break;
case UPDATE_REFS_CLOSED:
/* Otherwise no need to do anything, the transaction was closed already. */
break;
}

ref_transaction_free(transaction);
strbuf_release(&err);
strbuf_release(&input);
}

131
t/t1400-update-ref.sh
View File

@ -1404,4 +1404,135 @@ test_expect_success 'handle per-worktree refs in refs/bisect' '
! test_cmp main-head worktree-head
'

test_expect_success 'transaction handles empty commit' '
cat >stdin <<-EOF &&
start
prepare
commit
EOF
git update-ref --stdin <stdin >actual &&
printf "%s: ok\n" start prepare commit >expect &&
test_cmp expect actual
'

test_expect_success 'transaction handles empty commit with missing prepare' '
cat >stdin <<-EOF &&
start
commit
EOF
git update-ref --stdin <stdin >actual &&
printf "%s: ok\n" start commit >expect &&
test_cmp expect actual
'

test_expect_success 'transaction handles sole commit' '
cat >stdin <<-EOF &&
commit
EOF
git update-ref --stdin <stdin >actual &&
printf "%s: ok\n" commit >expect &&
test_cmp expect actual
'

test_expect_success 'transaction handles empty abort' '
cat >stdin <<-EOF &&
start
prepare
abort
EOF
git update-ref --stdin <stdin >actual &&
printf "%s: ok\n" start prepare abort >expect &&
test_cmp expect actual
'

test_expect_success 'transaction exits on multiple aborts' '
cat >stdin <<-EOF &&
abort
abort
EOF
test_must_fail git update-ref --stdin <stdin >actual 2>err &&
printf "%s: ok\n" abort >expect &&
test_cmp expect actual &&
grep "fatal: transaction is closed" err
'

test_expect_success 'transaction exits on start after prepare' '
cat >stdin <<-EOF &&
prepare
start
EOF
test_must_fail git update-ref --stdin <stdin 2>err >actual &&
printf "%s: ok\n" prepare >expect &&
test_cmp expect actual &&
grep "fatal: prepared transactions can only be closed" err
'

test_expect_success 'transaction handles empty abort with missing prepare' '
cat >stdin <<-EOF &&
start
abort
EOF
git update-ref --stdin <stdin >actual &&
printf "%s: ok\n" start abort >expect &&
test_cmp expect actual
'

test_expect_success 'transaction handles sole abort' '
cat >stdin <<-EOF &&
abort
EOF
git update-ref --stdin <stdin >actual &&
printf "%s: ok\n" abort >expect &&
test_cmp expect actual
'

test_expect_success 'transaction can handle commit' '
cat >stdin <<-EOF &&
start
create $a HEAD
commit
EOF
git update-ref --stdin <stdin >actual &&
printf "%s: ok\n" start commit >expect &&
test_cmp expect actual &&
git rev-parse HEAD >expect &&
git rev-parse $a >actual &&
test_cmp expect actual
'

test_expect_success 'transaction can handle abort' '
cat >stdin <<-EOF &&
start
create $b HEAD
abort
EOF
git update-ref --stdin <stdin >actual &&
printf "%s: ok\n" start abort >expect &&
test_cmp expect actual &&
test_path_is_missing .git/$b
'

test_expect_success 'transaction aborts by default' '
cat >stdin <<-EOF &&
start
create $b HEAD
EOF
git update-ref --stdin <stdin >actual &&
printf "%s: ok\n" start >expect &&
test_cmp expect actual &&
test_path_is_missing .git/$b
'

test_expect_success 'transaction with prepare aborts by default' '
cat >stdin <<-EOF &&
start
create $b HEAD
prepare
EOF
git update-ref --stdin <stdin >actual &&
printf "%s: ok\n" start prepare >expect &&
test_cmp expect actual &&
test_path_is_missing .git/$b
'

test_done