refs: introduce git_reference_remote_tracking_from_branch()

include/git2/refs.h

@@ -363,6 +363,27 @@ GIT_EXTERN(int) git_reference_foreach_glob(
  */
 GIT_EXTERN(int) git_reference_has_log(git_reference *ref);
 
+
+/**
+ * Return the reference supporting the remote tracking branch,
+ * given a reference branch.
+ *
+ * The input reference has to be located in the `refs/heads`
+ * namespace.
+ *
+ * @param tracking_ref Pointer where to store the retrieved
+ * reference.
+ *
+ * @param branch_ref A git local branch reference. 
+ *
+ * @return 0 on success; GIT_ENOTFOUND when no remote tracking
+ * reference exists, otherwise an error code.
+ */
+GIT_EXTERN(int) git_reference_remote_tracking_from_branch(
+	git_reference **tracking_ref,
+	git_reference *branch_ref
+);
+
 /** @} */
 GIT_END_DECL
 #endif

src/refs.c

@@ -11,6 +11,7 @@
 #include "fileops.h"
 #include "pack.h"
 #include "reflog.h"
+#include "config.h"
 
 #include <git2/tag.h>
 #include <git2/object.h>
@@ -1811,3 +1812,76 @@ int git_reference_has_log(
 
 	return result;
 }
+
+//TODO: How about also taking care of local tracking branches?
+//cf. http://alblue.bandlem.com/2011/07/git-tip-of-week-tracking-branches.html
+int git_reference_remote_tracking_from_branch(
+	git_reference **tracking_ref,
+	git_reference *branch_ref)
+{
+	git_config *config = NULL;
+	const char *name, *remote, *merge;
+	git_buf buf = GIT_BUF_INIT;
+	int error = -1;
+
+	assert(tracking_ref && branch_ref);
+
+	name = git_reference_name(branch_ref);
+
+	if (git__prefixcmp(name, GIT_REFS_HEADS_DIR)) {
+		giterr_set(
+			GITERR_INVALID,
+			"Failed to retrieve tracking reference - '%s' is not a branch.",
+			name);
+		return -1;
+	}
+
+	if (git_repository_config(&config, branch_ref->owner) < 0)
+		return -1;
+
+	if (git_buf_printf(
+		&buf,
+		"branch.%s.remote",
+		name + strlen(GIT_REFS_HEADS_DIR)) < 0)
+			goto cleanup;
+
+	if ((error = git_config_get_string(&remote, config, git_buf_cstr(&buf))) < 0)
+		goto cleanup;
+
+	error = -1;
+
+	git_buf_clear(&buf);
+
+	//TODO: Is it ok to fail when no merge target is found?
+	if (git_buf_printf(
+		&buf,
+		"branch.%s.merge",
+		name + strlen(GIT_REFS_HEADS_DIR)) < 0)
+			goto cleanup;
+
+	if (git_config_get_string(&merge, config, git_buf_cstr(&buf)) < 0)
+		goto cleanup;
+
+	//TODO: Should we test this?
+	if (git__prefixcmp(merge, GIT_REFS_HEADS_DIR))
+		goto cleanup;
+
+	git_buf_clear(&buf);
+
+	if (git_buf_printf(
+		&buf,
+		"refs/remotes/%s/%s",
+		remote,
+		merge + strlen(GIT_REFS_HEADS_DIR)) < 0)
+			goto cleanup;
+
+	error = git_reference_lookup(
+		tracking_ref,
+		branch_ref->owner,
+		git_buf_cstr(&buf));
+
+cleanup:
+	git_config_free(config);
+	git_buf_free(&buf);
+	return error;
+}

tests-clar/refs/remotetracking.c

@@ -0,0 +1,49 @@
+#include "clar_libgit2.h"
+
+static git_repository *g_repo;
+
+void test_refs_remotetracking__initialize(void)
+{
+	cl_git_pass(git_repository_open(&g_repo, cl_fixture("testrepo.git")));
+}
+
+void test_refs_remotetracking__cleanup(void)
+{
+	git_repository_free(g_repo);
+}
+
+void test_refs_remotetracking__unfound_returns_GIT_ENOTFOUND(void)
+{
+	git_reference *branch, *tracking;
+
+	cl_git_pass(git_reference_lookup(&branch, g_repo, "refs/heads/subtrees"));
+
+	cl_assert_equal_i(GIT_ENOTFOUND, git_reference_remote_tracking_from_branch(&tracking, branch));
+
+	git_reference_free(branch);
+}
+
+void test_refs_remotetracking__retrieving_from_a_non_head_fails(void)
+{
+	git_reference *branch, *tracking;
+
+	cl_git_pass(git_reference_lookup(&branch, g_repo, "refs/tags/e90810b"));
+
+	cl_git_fail(git_reference_remote_tracking_from_branch(&tracking, branch));
+
+	git_reference_free(branch);
+}
+
+void test_refs_remotetracking__can_retrieve_a_remote_tracking_branch_reference(void)
+{
+	git_reference *branch, *tracking;
+
+	cl_git_pass(git_reference_lookup(&branch, g_repo, "refs/heads/master"));
+
+	cl_git_pass(git_reference_remote_tracking_from_branch(&tracking, branch));
+
+	cl_assert_equal_s("refs/remotes/test/master", git_reference_name(tracking));
+
+	git_reference_free(branch);
+	git_reference_free(tracking);
+}