Introduce GIT_MERGE_ANALYSIS_UNBORN

include/git2/merge.h

@@ -259,6 +259,13 @@ typedef enum {
 	 * given merge input.
 	 */
 	GIT_MERGE_ANALYSIS_FASTFORWARD = (1 << 2),
+
+	/**
+	 * The HEAD of the current repository is "unborn" and does not point to
+	 * a valid commit.  No merge can be performed, but the caller may wish
+	 * to simply set HEAD to the target commit(s).
+	 */
+	GIT_MERGE_ANALYSIS_UNBORN = (1 << 3),
 } git_merge_analysis_t;
 
 /**

src/merge.c

@@ -2524,26 +2524,41 @@ int git_merge_analysis(
 	size_t their_heads_len)
 {
 	git_merge_head *ancestor_head = NULL, *our_head = NULL;
-	int error;
+	int error = 0;
 
 	assert(out && repo && their_heads);
 
-	*out = GIT_MERGE_ANALYSIS_NORMAL;
+	*out = GIT_MERGE_ANALYSIS_NONE;
+
+	if (git_repository_head_unborn(repo)) {
+		*out = GIT_MERGE_ANALYSIS_UNBORN;
+		goto done;
+	}
+
+	if (their_heads_len != 1) {
+		giterr_set(GITERR_MERGE, "Can only merge a single branch");
+		error = -1;
+		goto done;
+	}
 
 	if ((error = merge_heads(&ancestor_head, &our_head, repo, their_heads, their_heads_len)) < 0)
 		goto done;
 
-	if (their_heads_len == 1 && ancestor_head != NULL) {
-		/* We're up-to-date if we're trying to merge our own common ancestor. */
-		if (git_oid_equal(&ancestor_head->oid, &their_heads[0]->oid))
-			*out = GIT_MERGE_ANALYSIS_UP_TO_DATE;
+	/* We're up-to-date if we're trying to merge our own common ancestor. */
+	if (ancestor_head && git_oid_equal(&ancestor_head->oid, &their_heads[0]->oid))
+		*out = GIT_MERGE_ANALYSIS_UP_TO_DATE;
 
-		/* We're fastforwardable if we're our own common ancestor. */
-		else if (git_oid_equal(&ancestor_head->oid, &our_head->oid))
-			*out = GIT_MERGE_ANALYSIS_FASTFORWARD | GIT_MERGE_ANALYSIS_NORMAL;
-	}
+	/* We're fastforwardable if we're our own common ancestor. */
+	else if (ancestor_head && git_oid_equal(&ancestor_head->oid, &our_head->oid))
+		*out = GIT_MERGE_ANALYSIS_FASTFORWARD | GIT_MERGE_ANALYSIS_NORMAL;
+
+	/* Otherwise, just a normal merge is possible. */
+	else
+		*out = GIT_MERGE_ANALYSIS_NORMAL;
 
 done:
+	git_merge_head_free(ancestor_head);
+	git_merge_head_free(our_head);
 	return error;
 }
 

tests/merge/workdir/analysis.c

@@ -5,6 +5,7 @@
 #include "merge.h"
 #include "../merge_helpers.h"
 #include "refs.h"
+#include "posix.h"
 
 static git_repository *repo;
 static git_index *repo_index;
@@ -39,18 +40,18 @@ static git_merge_analysis_t analysis_from_branch(const char *branchname)
 {
 	git_buf refname = GIT_BUF_INIT;
 	git_reference *their_ref;
-	git_merge_head *their_heads[1];
+	git_merge_head *their_head;
 	git_merge_analysis_t analysis;
 
 	git_buf_printf(&refname, "%s%s", GIT_REFS_HEADS_DIR, branchname);
 
 	cl_git_pass(git_reference_lookup(&their_ref, repo, git_buf_cstr(&refname)));
-	cl_git_pass(git_merge_head_from_ref(&their_heads[0], repo, their_ref));
+	cl_git_pass(git_merge_head_from_ref(&their_head, repo, their_ref));
 
-	cl_git_pass(git_merge_analysis(&analysis, repo, their_heads, 1));
+	cl_git_pass(git_merge_analysis(&analysis, repo, (const git_merge_head **)&their_head, 1));
 
 	git_buf_free(&refname);
-	git_merge_head_free(their_heads[0]);
+	git_merge_head_free(their_head);
 	git_reference_free(their_ref);
 
 	return analysis;
@@ -88,3 +89,18 @@ void test_merge_workdir_analysis__uptodate_merging_prev_commit(void)
 	analysis = analysis_from_branch(PREVIOUS_BRANCH);
 	cl_assert_equal_i(GIT_MERGE_ANALYSIS_UP_TO_DATE, analysis);
 }
+
+void test_merge_workdir_analysis__unborn(void)
+{
+	git_merge_analysis_t analysis;
+	git_buf master = GIT_BUF_INIT;
+
+	git_buf_joinpath(&master, git_repository_path(repo), "refs/heads/master");
+	p_unlink(git_buf_cstr(&master));
+
+	analysis = analysis_from_branch(NOFASTFORWARD_BRANCH);
+	cl_assert_equal_i(GIT_MERGE_ANALYSIS_UNBORN, analysis);
+
+	git_buf_free(&master);
+}
+