refdb_backend: improve callback documentation

The callbacks are currently sparsely documented, making it really hard
to implement a new backend without taking a look at the existing
refdb_fs backend. Add documentation to make this task hopefully easier
to achieve.

include/git2/sys/refdb_backend.h

@@ -64,6 +64,12 @@ struct git_refdb_backend {
 	 * Queries the refdb backend for the existence of a reference.
 	 *
 	 * A refdb implementation must provide this function.
+	 *
+	 * @arg exists The implementation shall set this to `0` if a ref does
+	 *             not exist, otherwise to `1`.
+	 * @arg ref_name The reference's name that should be checked for
+	 *               existence.
+	 * @return `0` on success, a negative error value code.
 	 */
 	int GIT_CALLBACK(exists)(
 		int *exists,
@@ -74,6 +80,13 @@ struct git_refdb_backend {
 	 * Queries the refdb backend for a given reference.
 	 *
 	 * A refdb implementation must provide this function.
+	 *
+	 * @arg out The implementation shall set this to the allocated
+	 *          reference, if it could be found, otherwise to `NULL`.
+	 * @arg ref_name The reference's name that should be checked for
+	 *               existence.
+	 * @return `0` on success, `GIT_ENOTFOUND` if the reference does
+	 *         exist, otherwise a negative error code.
 	 */
 	int GIT_CALLBACK(lookup)(
 		git_reference **out,
@@ -84,6 +97,16 @@ struct git_refdb_backend {
 	 * Allocate an iterator object for the backend.
 	 *
 	 * A refdb implementation must provide this function.
+	 *
+	 * @arg out The implementation shall set this to the allocated
+	 *          reference iterator. A custom structure may be used with an
+	 *          embedded `git_reference_iterator` structure. Both `next`
+	 *          and `next_name` functions of `git_reference_iterator` need
+	 *          to be populated.
+	 * @arg glob A pattern to filter references by. If given, the iterator
+	 *           shall only return references that match the glob when
+	 *           passed to `wildmatch`.
+	 * @return `0` on success, otherwise a negative error code.
 	 */
 	int GIT_CALLBACK(iterator)(
 		git_reference_iterator **iter,
@@ -94,6 +117,27 @@ struct git_refdb_backend {
 	 * Writes the given reference to the refdb.
 	 *
 	 * A refdb implementation must provide this function.
+	 *
+	 * @arg ref The reference to persist. May either be a symbolic or
+	 *          direct reference.
+	 * @arg force Whether to write the reference if a reference with the
+	 *            same name already exists.
+	 * @arg who The person updating the reference. Shall be used to create
+	 *          a reflog entry.
+	 * @arg message The message detailing what kind of reference update is
+	 *              performed. Shall be used to create a reflog entry.
+	 * @arg old If not `NULL` and `force` is not set, then the
+	 *          implementation needs to ensure that the reference is currently at
+	 *          the given OID before writing the new value. If both `old`
+	 *          and `old_target` are `NULL`, then the reference should not
+	 *          exist at the point of writing.
+	 * @arg old_target If not `NULL` and `force` is not set, then the
+	 *                 implementation needs to ensure that the symbolic
+	 *                 reference is currently at the given target before
+	 *                 writing the new value. If both `old` and
+	 *                 `old_target` are `NULL`, then the reference should
+	 *                 not exist at the point of writing.
+	 * @return `0` on success, otherwise a negative error code.
 	 */
 	int GIT_CALLBACK(write)(git_refdb_backend *backend,
 		     const git_reference *ref, int force,
@@ -104,6 +148,18 @@ struct git_refdb_backend {
 	 * Rename a reference in the refdb.
 	 *
 	 * A refdb implementation must provide this function.
+	 *
+	 * @arg out The implementation shall set this to the newly created
+	 *          reference or `NULL` on error.
+	 * @arg old_name The current name of the reference that is to be renamed.
+	 * @arg new_name The new name that the old reference shall be renamed to.
+	 * @arg force Whether to write the reference if a reference with the
+	 *            target name already exists.
+	 * @arg who The person updating the reference. Shall be used to create
+	 *          a reflog entry.
+	 * @arg message The message detailing what kind of reference update is
+	 *              performed. Shall be used to create a reflog entry.
+	 * @return `0` on success, otherwise a negative error code.
 	 */
 	int GIT_CALLBACK(rename)(
 		git_reference **out, git_refdb_backend *backend,
@@ -116,6 +172,16 @@ struct git_refdb_backend {
 	 * If it exists, its reflog should be deleted as well.
 	 *
 	 * A refdb implementation must provide this function.
+	 *
+	 * @arg ref_name The name of the reference name that shall be deleted.
+	 * @arg old_id If not `NULL` and `force` is not set, then the
+	 *             implementation needs to ensure that the reference is currently at
+	 *             the given OID before writing the new value.
+	 * @arg old_target If not `NULL` and `force` is not set, then the
+	 *                 implementation needs to ensure that the symbolic
+	 *                 reference is currently at the given target before
+	 *                 writing the new value.
+	 * @return `0` on success, otherwise a negative error code.
 	 */
 	int GIT_CALLBACK(del)(git_refdb_backend *backend, const char *ref_name, const git_oid *old_id, const char *old_target);
 
@@ -127,13 +193,21 @@ struct git_refdb_backend {
 	 *
 	 * A refdb implementation may provide this function; if it is not
 	 * provided, nothing will be done.
+	 *
+	 * @return `0` on success a negative error code otherwise
 	 */
 	int GIT_CALLBACK(compress)(git_refdb_backend *backend);
 
 	/**
 	 * Query whether a particular reference has a log (may be empty)
 	 *
+	 * Shall return 1 if it has a reflog, 0 it it doesn't and negative in
+	 * case an error occurred.
+	 *
 	 * A refdb implementation must provide this function.
+	 *
+	 * @return `0` on success, `1` if the reflog for the given reference
+	 *         exists, a negative error code otherwise
 	 */
 	int GIT_CALLBACK(has_log)(git_refdb_backend *backend, const char *refname);
 
@@ -142,6 +216,8 @@ struct git_refdb_backend {
 	 * will be appended to on writes.
 	 *
 	 * A refdb implementation must provide this function.
+	 *
+	 * @return `0` on success, a negative error code otherwise
 	 */
 	int GIT_CALLBACK(ensure_log)(git_refdb_backend *backend, const char *refname);
 
@@ -157,6 +233,8 @@ struct git_refdb_backend {
 	 * Read the reflog for the given reference name.
 	 *
 	 * A refdb implementation must provide this function.
+	 *
+	 * @return `0` on success, a negative error code otherwise
 	 */
 	int GIT_CALLBACK(reflog_read)(git_reflog **out, git_refdb_backend *backend, const char *name);
 
@@ -164,6 +242,11 @@ struct git_refdb_backend {
 	 * Write a reflog to disk.
 	 *
 	 * A refdb implementation must provide this function.
+	 *
+	 * @arg reflog The complete reference log for a given reference. Note
+	 *             that this may contain entries that have already been
+	 *             written to disk.
+	 * @return `0` on success, a negative error code otherwise
 	 */
 	int GIT_CALLBACK(reflog_write)(git_refdb_backend *backend, git_reflog *reflog);
 
@@ -171,6 +254,10 @@ struct git_refdb_backend {
 	 * Rename a reflog.
 	 *
 	 * A refdb implementation must provide this function.
+	 *
+	 * @arg old_name The name of old reference whose reflog shall be renamed from.
+	 * @arg new_name The name of new reference whose reflog shall be renamed to.
+	 * @return `0` on success, a negative error code otherwise
 	 */
 	int GIT_CALLBACK(reflog_rename)(git_refdb_backend *_backend, const char *old_name, const char *new_name);
 
@@ -178,16 +265,22 @@ struct git_refdb_backend {
 	 * Remove a reflog.
 	 *
 	 * A refdb implementation must provide this function.
+	 *
+	 * @arg name The name of the reference whose reflog shall be deleted.
+	 * @return `0` on success, a negative error code otherwise
 	 */
 	int GIT_CALLBACK(reflog_delete)(git_refdb_backend *backend, const char *name);
 
 	/**
 	 * Lock a reference.
 	 *
-	 * The opaque parameter will be passed to the unlock function.
-	 *
 	 * A refdb implementation may provide this function; if it is not
 	 * provided, the transaction API will fail to work.
+	 *
+	 * @arg payload_out Opaque parameter that will be passed verbosely to
+	 *                  `unlock`.
+	 * @arg refname Reference that shall be locked.
+	 * @return `0` on success, a negative error code otherwise
 	 */
 	int GIT_CALLBACK(lock)(void **payload_out, git_refdb_backend *backend, const char *refname);
 
@@ -200,6 +293,20 @@ struct git_refdb_backend {
 	 *
 	 * A refdb implementation must provide this function if a `lock`
 	 * implementation is provided.
+	 *
+	 * @arg payload The payload returned by `lock`.
+	 * @arg success `1` if a reference should be updated, `2` if
+	 *              a reference should be deleted, `0` if the lock must be
+	 *              discarded.
+	 * @arg update_reflog `1` in case the reflog should be updated, `0`
+	 *                    otherwise.
+	 * @arg ref The reference which should be unlocked.
+	 * @arg who The person updating the reference. Shall be used to create
+	 *          a reflog entry in case `update_reflog` is set.
+	 * @arg message The message detailing what kind of reference update is
+	 *              performed. Shall be used to create a reflog entry in
+	 *              case `update_reflog` is set.
+	 * @return `0` on success, a negative error code otherwise
 	 */
 	int GIT_CALLBACK(unlock)(git_refdb_backend *backend, void *payload, int success, int update_reflog,
 		      const git_reference *ref, const git_signature *sig, const char *message);