attr_file: fix unescaping of escapes required for fnmatch

When parsing attribute patterns, we will eventually unescape the
parsed pattern. This is required because we require custom
escapes for whitespace characters, as normally they are used to
terminate the current pattern. Thing is, we don't only unescape
those whitespace characters, but in fact all escaped sequences.
So for example if the pattern was "\*", we unescape that to "*".
As this is directly passed to fnmatch(3) later, fnmatch would
treat it as a simple glob matching all files where it should
instead only match a file with name "*".

Fix the issue by unescaping spaces, only. Add a bunch of tests to
exercise escape parsing.

src/attr_file.c

@@ -574,6 +574,34 @@ static size_t trailing_space_length(const char *p, size_t len)
 	return len - n;
 }
 
+static size_t unescape_spaces(char *str)
+{
+	char *scan, *pos = str;
+	bool escaped = false;
+
+	if (!str)
+		return 0;
+
+	for (scan = str; *scan; scan++) {
+		if (!escaped && *scan == '\\') {
+			escaped = true;
+			continue;
+		}
+
+		/* Only insert the escape character for escaped non-spaces */
+		if (escaped && !git__isspace(*scan))
+			*pos++ = '\\';
+
+		*pos++ = *scan;
+		escaped = false;
+	}
+
+	if (pos != scan)
+		*pos = '\0';
+
+	return (pos - str);
+}
+
 /*
  * This will return 0 if the spec was filled out,
  * GIT_ENOTFOUND if the fnmatch does not require matching, or
@@ -701,8 +729,8 @@ int git_attr_fnmatch__parse(
 		*base = git__next_line(pattern);
 		return -1;
 	} else {
-		/* strip '\' that might have be used for internal whitespace */
-		spec->length = git__unescape(spec->pattern);
+		/* strip '\' that might have been used for internal whitespace */
+		spec->length = unescape_spaces(spec->pattern);
 		/* TODO: convert remaining '\' into '/' for POSIX ??? */
 	}
 

tests/ignore/path.c

@@ -459,3 +459,65 @@ void test_ignore_path__negative_directory_rules_only_match_directories(void)
 	assert_is_ignored(false, "src/A.keep");
 	assert_is_ignored(false, ".gitignore");
 }
+
+void test_ignore_path__escaped_character(void)
+{
+	cl_git_rewritefile("attr/.gitignore", "\\c\n");
+	assert_is_ignored(true, "c");
+	assert_is_ignored(false, "\\c");
+}
+
+void test_ignore_path__escaped_newline(void)
+{
+	cl_git_rewritefile(
+		"attr/.gitignore",
+		"\\\nnewline\n"
+	);
+
+	assert_is_ignored(true, "\nnewline");
+}
+
+void test_ignore_path__escaped_glob(void)
+{
+	cl_git_rewritefile("attr/.gitignore", "\\*\n");
+	assert_is_ignored(true, "*");
+	assert_is_ignored(false, "foo");
+}
+
+void test_ignore_path__escaped_comments(void)
+{
+	cl_git_rewritefile(
+		"attr/.gitignore",
+		"#foo\n"
+		"\\#bar\n"
+		"\\##baz\n"
+		"\\#\\\\#qux\n"
+	);
+
+	assert_is_ignored(false, "#foo");
+	assert_is_ignored(true, "#bar");
+	assert_is_ignored(false, "\\#bar");
+	assert_is_ignored(true, "##baz");
+	assert_is_ignored(false, "\\##baz");
+	assert_is_ignored(true, "#\\#qux");
+	assert_is_ignored(false, "##qux");
+	assert_is_ignored(false, "\\##qux");
+}
+
+void test_ignore_path__escaped_slash(void)
+{
+	cl_git_rewritefile(
+		"attr/.gitignore",
+		"\\\\\n"
+		"\\\\preceding\n"
+		"inter\\\\mittent\n"
+		"trailing\\\\\n"
+	);
+
+#ifndef GIT_WIN32
+	assert_is_ignored(true, "\\");
+	assert_is_ignored(true, "\\preceding");
+#endif
+	assert_is_ignored(true, "inter\\mittent");
+	assert_is_ignored(true, "trailing\\");
+}