add technical documentation about ref iteration

Signed-off-by: Heiko Voigt <hvoigt@hvoigt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/api-ref-iteration.txt

@@ -0,0 +1,81 @@
+ref iteration API
+=================
+
+
+Iteration of refs is done by using an iterate function which will call a
+callback function for every ref. The callback function has this
+signature:
+
+	int handle_one_ref(const char *refname, const unsigned char *sha1,
+			   int flags, void *cb_data);
+
+There are different kinds of iterate functions which all take a
+callback of this type. The callback is then called for each found ref
+until the callback returns nonzero. The returned value is then also
+returned by the iterate function.
+
+Iteration functions
+-------------------
+
+* `head_ref()` just iterates the head ref.
+
+* `for_each_ref()` iterates all refs.
+
+* `for_each_ref_in()` iterates all refs which have a defined prefix and
+  strips that prefix from the passed variable refname.
+
+* `for_each_tag_ref()`, `for_each_branch_ref()`, `for_each_remote_ref()`,
+  `for_each_replace_ref()` iterate refs from the respective area.
+
+* `for_each_glob_ref()` iterates all refs that match the specified glob
+  pattern.
+
+* `for_each_glob_ref_in()` the previous and `for_each_ref_in()` combined.
+
+* `head_ref_submodule()`, `for_each_ref_submodule()`,
+  `for_each_ref_in_submodule()`, `for_each_tag_ref_submodule()`,
+  `for_each_branch_ref_submodule()`, `for_each_remote_ref_submodule()`
+  do the same as the functions descibed above but for a specified
+  submodule.
+
+* `for_each_rawref()` can be used to learn about broken ref and symref.
+
+* `for_each_reflog()` iterates each reflog file.
+
+Submodules
+----------
+
+If you want to iterate the refs of a submodule you first need to add the
+submodules object database. You can do this by a code-snippet like
+this:
+
+	const char *path = "path/to/submodule"
+	if (!add_submodule_odb(path))
+		die("Error submodule '%s' not populated.", path);
+
+`add_submodule_odb()` will return an non-zero value on success. If you
+do not do this you will get an error for each ref that it does not point
+to a valid object.
+
+Note: As a side-effect of this you can not safely assume that all
+objects you lookup are available in superproject. All submodule objects
+will be available the same way as the superprojects objects.
+
+Example:
+--------
+
+----
+static int handle_remote_ref(const char *refname,
+		const unsigned char *sha1, int flags, void *cb_data)
+{
+	struct strbuf *output = cb_data;
+	strbuf_addf(output, "%s\n", refname);
+	return 0;
+}
+
+...
+
+	struct strbuf output = STRBUF_INIT;
+	for_each_remote_ref(handle_remote_ref, &output);
+	printf("%s", output.buf);
+----