Re: [PATCH v16 01/14] Write pseudorefs through ref backends.

> From: Han-Wen Nienhuys <hanwen@google.com>
> 
> Pseudorefs store transient data in in the repository. Examples are HEAD,
> CHERRY_PICK_HEAD, etc.
> 
> These refs have always been read through the ref backends, but they were written
> in a one-off routine that wrote an object ID or symref directly into
> .git/<pseudo_ref_name>.
> 
> This causes problems when introducing a new ref storage backend. To remedy this,
> extend the ref backend implementation with a write_pseudoref_fn and
> update_pseudoref_fn.

> [...]
> diff --git a/refs.h b/refs.h
> index e010f8aec28..4dad8f24914 100644
> --- a/refs.h
> +++ b/refs.h
> @@ -732,6 +732,17 @@ int update_ref(const char *msg, const char *refname,
>  	       const struct object_id *new_oid, const struct object_id *old_oid,
>  	       unsigned int flags, enum action_on_err onerr);
>  
> +/* Pseudorefs (eg. HEAD, CHERRY_PICK_HEAD) have a separate routines for updating
> +   and deletion as they cannot take part in normal transactional updates.
> +   Pseudorefs should only be written for the main repository.
> +*/
> +int refs_write_pseudoref(struct ref_store *refs, const char *pseudoref,
> +			 const struct object_id *oid,
> +			 const struct object_id *old_oid, struct strbuf *err);
> +int refs_delete_pseudoref(struct ref_store *refs, const char *pseudoref,
> +			  const struct object_id *old_oid);
> +int delete_pseudoref(const char *pseudoref, const struct object_id *old_oid);
> +

>  int parse_hide_refs_config(const char *var, const char *value, const char *);
>  
>  /*
> diff --git a/refs/files-backend.c b/refs/files-backend.c
> index 6516c7bc8c8..df7553f4cc3 100644
> --- a/refs/files-backend.c
> +++ b/refs/files-backend.c
> @@ -731,6 +731,115 @@ static int lock_raw_ref(struct files_ref_store *refs,
>  	return ret;
>  }
>  
> +static int files_write_pseudoref(struct ref_store *ref_store,
> +				 const char *pseudoref,
> +				 const struct object_id *oid,
> +				 const struct object_id *old_oid,
> +				 struct strbuf *err)
> +{
> +	struct files_ref_store *refs =
> +		files_downcast(ref_store, REF_STORE_READ, "write_pseudoref");
> +	int fd;
> +	struct lock_file lock = LOCK_INIT;
> +	struct strbuf filename = STRBUF_INIT;
> +	struct strbuf buf = STRBUF_INIT;
> +	int ret = -1;
> +
> +	if (!oid)
> +		return 0;
> +
> +	strbuf_addf(&filename, "%s/%s", refs->gitdir, pseudoref);
> +	fd = hold_lock_file_for_update_timeout(&lock, filename.buf, 0,
> +					       get_files_ref_lock_timeout_ms());
> +	if (fd < 0) {
> +		strbuf_addf(err, _("could not open '%s' for writing: %s"),
> +			    buf.buf, strerror(errno));
> +		goto done;
> +	}
> +
> +	if (old_oid) {
> +		struct object_id actual_old_oid;
> +
> +		if (read_ref(pseudoref, &actual_old_oid)) {
> +			if (!is_null_oid(old_oid)) {
> +				strbuf_addf(err, _("could not read ref '%s'"),
> +					    pseudoref);
> +				rollback_lock_file(&lock);
> +				goto done;
> +			}
> +		} else if (is_null_oid(old_oid)) {
> +			strbuf_addf(err, _("ref '%s' already exists"),
> +				    pseudoref);
> +			rollback_lock_file(&lock);
> +			goto done;
> +		} else if (!oideq(&actual_old_oid, old_oid)) {
> +			strbuf_addf(err,
> +				    _("unexpected object ID when writing '%s'"),
> +				    pseudoref);
> +			rollback_lock_file(&lock);
> +			goto done;
> +		}
> +	}
> +
> +	strbuf_addf(&buf, "%s\n", oid_to_hex(oid));
> +	if (write_in_full(fd, buf.buf, buf.len) < 0) {
> +		strbuf_addf(err, _("could not write to '%s'"), filename.buf);
> +		rollback_lock_file(&lock);
> +		goto done;
> +	}
> +
> +	commit_lock_file(&lock);
> +	ret = 0;
> +done:
> +	strbuf_release(&buf);
> +	strbuf_release(&filename);
> +	return ret;
> +}
> +
> +static int files_delete_pseudoref(struct ref_store *ref_store,
> +				  const char *pseudoref,
> +				  const struct object_id *old_oid)
> +{
> +	struct files_ref_store *refs =
> +		files_downcast(ref_store, REF_STORE_READ, "delete_pseudoref");
> +	struct strbuf filename = STRBUF_INIT;
> +	int ret = -1;
> +
> +	strbuf_addf(&filename, "%s/%s", refs->gitdir, pseudoref);
> +
> +	if (old_oid && !is_null_oid(old_oid)) {
> +		struct lock_file lock = LOCK_INIT;
> +		int fd;
> +		struct object_id actual_old_oid;
> +
> +		fd = hold_lock_file_for_update_timeout(
> +			&lock, filename.buf, 0,
> +			get_files_ref_lock_timeout_ms());
> +		if (fd < 0) {
> +			error_errno(_("could not open '%s' for writing"),
> +				    filename.buf);
> +			goto done;
> +		}
> +		if (read_ref(pseudoref, &actual_old_oid))
> +			die(_("could not read ref '%s'"), pseudoref);
> +		if (!oideq(&actual_old_oid, old_oid)) {
> +			error(_("unexpected object ID when deleting '%s'"),
> +			      pseudoref);
> +			rollback_lock_file(&lock);
> +			goto done;
> +		}
> +
> +		unlink(filename.buf);

> +		rollback_lock_file(&lock);
> +	} else {
> +		unlink(filename.buf);
> +	}
> +	ret = 0;
> +done:
> +	strbuf_release(&filename);
> +	return ret;
> +}
> +
>  struct files_ref_iterator {
>  	struct ref_iterator base;
>  
> @@ -3189,6 +3298,9 @@ struct ref_storage_be refs_be_files = {
>  	files_rename_ref,
>  	files_copy_ref,
>  
> +	files_write_pseudoref,
> +	files_delete_pseudoref,
> +
>  	files_ref_iterator_begin,
>  	files_read_raw_ref,
>  
> @@ -3198,5 +3310,5 @@ struct ref_storage_be refs_be_files = {
>  	files_reflog_exists,
>  	files_create_reflog,
>  	files_delete_reflog,
> -	files_reflog_expire
> +	files_reflog_expire,
>  };
> diff --git a/refs/packed-backend.c b/refs/packed-backend.c
> index 4458a0f69cc..08e8253a893 100644
> --- a/refs/packed-backend.c
> +++ b/refs/packed-backend.c
> @@ -1590,6 +1590,22 @@ static int packed_copy_ref(struct ref_store *ref_store,
>  	BUG("packed reference store does not support copying references");
>  }
>  
> +static int packed_write_pseudoref(struct ref_store *ref_store,
> +				  const char *pseudoref,
> +				  const struct object_id *oid,
> +				  const struct object_id *old_oid,
> +				  struct strbuf *err)
> +{
> +	BUG("packed reference store does not support writing pseudo-references");
> +}
> +
> +static int packed_delete_pseudoref(struct ref_store *ref_store,
> +				   const char *pseudoref,
> +				   const struct object_id *old_oid)
> +{
> +	BUG("packed reference store does not support deleting pseudo-references");
> +}
> +
>  static struct ref_iterator *packed_reflog_iterator_begin(struct ref_store *ref_store)
>  {
>  	return empty_ref_iterator_begin();
> @@ -1656,6 +1672,9 @@ struct ref_storage_be refs_be_packed = {
>  	packed_rename_ref,
>  	packed_copy_ref,
>  
> +	packed_write_pseudoref,
> +	packed_delete_pseudoref,
> +
>  	packed_ref_iterator_begin,
>  	packed_read_raw_ref,
>  
> @@ -1665,5 +1684,5 @@ struct ref_storage_be refs_be_packed = {
>  	packed_reflog_exists,
>  	packed_create_reflog,
>  	packed_delete_reflog,
> -	packed_reflog_expire
> +	packed_reflog_expire,
>  };
> diff --git a/refs/refs-internal.h b/refs/refs-internal.h
> index 4271362d264..59b053d53a2 100644
> --- a/refs/refs-internal.h
> +++ b/refs/refs-internal.h
> @@ -556,6 +556,21 @@ typedef int copy_ref_fn(struct ref_store *ref_store,
>  			  const char *oldref, const char *newref,
>  			  const char *logmsg);
>  
> +typedef int write_pseudoref_fn(struct ref_store *ref_store,
> +			       const char *pseudoref,
> +			       const struct object_id *oid,
> +			       const struct object_id *old_oid,
> +			       struct strbuf *err);
> +
> +/*
> + * Deletes a pseudoref. Deletion always succeeds (even if the pseudoref doesn't
> + * exist.), except if old_oid is specified. If it is, it can fail due to lock
> + * failure, failure reading the old OID, or an OID mismatch
> + */
> +typedef int delete_pseudoref_fn(struct ref_store *ref_store,
> +				const char *pseudoref,
> +				const struct object_id *old_oid);
> +
>  /*
>   * Iterate over the references in `ref_store` whose names start with
>   * `prefix`. `prefix` is matched as a literal string, without regard
> @@ -655,6 +670,9 @@ struct ref_storage_be {
>  	rename_ref_fn *rename_ref;
>  	copy_ref_fn *copy_ref;
>  
> +	write_pseudoref_fn *write_pseudoref;
> +	delete_pseudoref_fn *delete_pseudoref;
> +
>  	ref_iterator_begin_fn *iterator_begin;
>  	read_raw_ref_fn *read_raw_ref;
>  
>