merge-tree: handle directory/empty conflict correctly

git-merge-tree causes a null pointer dereference when a directory
entry exists in only one or two of the three trees being compared with
no corresponding entry in the other tree(s).

When this happens, we want to handle the entry as a directory and not
attempt to mark it as a file merge.  Do this by setting the entries bit
in the directory mask when the entry is missing or when it is a
directory, only performing the file comparison when we know that a file
entry exists.

Reported-by: Andreas Jacobsen <andreas@andreasjacobsen.com>
Signed-off-by: John Keeping <john@keeping.me.uk>
Tested-by: Andreas Jacobsen <andreas@andreasjacobsen.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/merge-tree.c

@@ -251,7 +251,11 @@ static void unresolved(const struct traverse_info *info, struct name_entry n[3])
 
 	for (i = 0; i < 3; i++) {
 		mask |= (1 << i);
-		if (n[i].mode && S_ISDIR(n[i].mode))
+		/*
+		 * Treat missing entries as directories so that we return
+		 * after unresolved_directory has handled this.
+		 */
+		if (!n[i].mode || S_ISDIR(n[i].mode))
 			dirmask |= (1 << i);
 	}
 

t/t4300-merge-tree.sh

@@ -259,6 +259,57 @@ EXPECTED
 	test_cmp expected actual
 '
 
+test_expect_success 'tree add A, B (same)' '
+	cat >expect <<-\EOF &&
+	EOF
+	git reset --hard initial &&
+	mkdir sub &&
+	test_commit "add sub/file" "sub/file" "file" add-tree-A &&
+	git merge-tree initial add-tree-A add-tree-A >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'tree add A, B (different)' '
+	cat >expect <<-\EOF &&
+	added in both
+	  our    100644 43d5a8ed6ef6c00ff775008633f95787d088285d sub/file
+	  their  100644 ba629238ca89489f2b350e196ca445e09d8bb834 sub/file
+	@@ -1 +1,5 @@
+	+<<<<<<< .our
+	 AAA
+	+=======
+	+BBB
+	+>>>>>>> .their
+	EOF
+	git reset --hard initial &&
+	mkdir sub &&
+	test_commit "add sub/file" "sub/file" "AAA" add-tree-a-b-A &&
+	git reset --hard initial &&
+	mkdir sub &&
+	test_commit "add sub/file" "sub/file" "BBB" add-tree-a-b-B &&
+	git merge-tree initial add-tree-a-b-A add-tree-a-b-B >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'tree unchanged A, removed B' '
+	cat >expect <<-\EOF &&
+	removed in remote
+	  base   100644 43d5a8ed6ef6c00ff775008633f95787d088285d sub/file
+	  our    100644 43d5a8ed6ef6c00ff775008633f95787d088285d sub/file
+	@@ -1 +0,0 @@
+	-AAA
+	EOF
+	git reset --hard initial &&
+	mkdir sub &&
+	test_commit "add sub/file" "sub/file" "AAA" tree-remove-b-initial &&
+	git rm sub/file &&
+	test_tick &&
+	git commit -m "remove sub/file" &&
+	git tag tree-remove-b-B &&
+	git merge-tree tree-remove-b-initial tree-remove-b-initial tree-remove-b-B >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success 'turn file to tree' '
 	git reset --hard initial &&
 	rm initial-file &&