Brad King <brad.king@xxxxxxxxxxx> writes:
> Add 'struct ref_update' to encode the information needed to update or
> delete a ref (name, new sha1, optional old sha1, no-deref flag). Add
> function 'update_refs' accepting an array of updates to perform. First
> acquire locks on all refs with verified old values. Then update or
> delete all refs accordingly. Fail if any one lock cannot be obtained or
> any one old value does not match.
>
> Though the refs themeselves cannot be modified together in a single
> atomic transaction, this function does enable some useful semantics.
> For example, a caller may create a new branch starting from the head of
> another branch and rewind the original branch at the same time. This
> transfers ownership of commits between branches without risk of losing
> commits added to the original branch by a concurrent process, or risk of
> a concurrent process creating the new branch first.
>
> Signed-off-by: Brad King <brad.king@xxxxxxxxxxx>
> ---
> refs.c | 66 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> refs.h | 11 +++++++++++
> 2 files changed, 77 insertions(+)
>
> diff --git a/refs.c b/refs.c
> index 5a6c14e..0a0c19e 100644
> --- a/refs.c
> +++ b/refs.c
> @@ -3238,6 +3238,72 @@ int update_ref(const char *action, const char *refname,
> return update_ref_write(action, refname, sha1, lock, onerr);
> }
>
> +int update_refs(const char *action, struct ref_update *updates,
> + int n, enum action_on_err onerr)
> +{
> + int ret = 0, delnum = 0, i;
> + int *types;
> + struct ref_lock **locks;
> + const char **delnames;
> +
> + if (!updates || !n)
> + return 0;
> +
> + /* Allocate work space: */
> + types = xmalloc(sizeof(int)*n);
> + locks = xmalloc(sizeof(struct ref_lock*)*n);
> + delnames = xmalloc(sizeof(const char*)*n);
> +
> + /* Acquire all locks while verifying old values: */
> + for (i=0; i < n; ++i) {
Style:
for (i = 0; i < n; i++) {
> + locks[i] = update_ref_lock(updates[i].ref_name,
> + updates[i].old_sha1,
> + updates[i].flags,
> + &types[i], onerr);
> + if (!locks[i])
> + break;
> + }
Is it asking for AB-BA deadlock? If so, is the caller responsible
for avoiding it?
> + /* Abort if we did not get all locks: */
> + if (i < n) {
> + while (--i >= 0)
> + unlock_ref(locks[i]);
> + free(types);
> + free(locks);
> + free(delnames);
> + return 1;
> + }
> +
> + /* Perform updates first so live commits remain referenced: */
> + for (i=0; i < n; ++i)
> + if (!is_null_sha1(updates[i].new_sha1)) {
> + ret |= update_ref_write(action,
> + updates[i].ref_name,
> + updates[i].new_sha1,
> + locks[i], onerr);
> + locks[i] = 0; /* freed by update_ref_write */
> + }
> +
> + /* Perform deletes now that updates are safely completed: */
> + for (i=0; i < n; ++i)
> + if (locks[i]) {
> + delnames[delnum++] = locks[i]->ref_name;
> + ret |= delete_ref_loose(locks[i], types[i]);
> + }
> + ret |= repack_without_refs(delnames, delnum);
> + for (i=0; i < delnum; ++i)
> + unlink_or_warn(git_path("logs/%s", delnames[i]));
> + clear_loose_ref_cache(&ref_cache);
> + for (i=0; i < n; ++i)
> + if (locks[i])
> + unlock_ref(locks[i]);
> +
> + free(types);
> + free(locks);
> + free(delnames);
> + return ret;
> +}
> +
> struct ref *find_ref_by_name(const struct ref *list, const char *name)
> {
> for ( ; list; list = list->next)
> diff --git a/refs.h b/refs.h
> index 2cd307a..5763f3a 100644
> --- a/refs.h
> +++ b/refs.h
> @@ -214,6 +214,17 @@ int update_ref(const char *action, const char *refname,
> const unsigned char *sha1, const unsigned char *oldval,
> int flags, enum action_on_err onerr);
>
> +struct ref_update {
> + const char *ref_name;
> + unsigned char new_sha1[20];
> + unsigned char *old_sha1;
This asymmetry is rather curious and will later become problematic
when we have more users of this API. Maybe your callers happen have
static storage to keep old_sha1[] outside this structure but do not
have it for new_sha1[] for some unknown reason, which may not apply
to later callers we may want to add.
> + int flags;
> +};
> +
> +/** lock all refs and then write all of them */
> +int update_refs(const char *action, struct ref_update *updates,
> + int n, enum action_on_err onerr);
> +
> extern int parse_hide_refs_config(const char *var, const char *value, const char *);
> extern int ref_is_hidden(const char *);
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