mingw: implement a platform-specific strbuf_realpath()

There is a Win32 API function to resolve symbolic links, and we can use
that instead of resolving them manually. Even better, this function also
resolves NTFS junction points (which are somewhat similar to bind
mounts).

This fixes https://github.com/git-for-windows/git/issues/2481.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

compat/mingw.c

@@ -1207,6 +1207,82 @@ struct tm *localtime_r(const time_t *timep, struct tm *result)
 }
 #endif
 
+char *mingw_strbuf_realpath(struct strbuf *resolved, const char *path)
+{
+	wchar_t wpath[MAX_PATH];
+	HANDLE h;
+	DWORD ret;
+	int len;
+	const char *last_component = NULL;
+	char *append = NULL;
+
+	if (xutftowcs_path(wpath, path) < 0)
+		return NULL;
+
+	h = CreateFileW(wpath, 0,
+			FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE, NULL,
+			OPEN_EXISTING, FILE_FLAG_BACKUP_SEMANTICS, NULL);
+
+	/*
+	 * strbuf_realpath() allows the last path component to not exist. If
+	 * that is the case, now it's time to try without last component.
+	 */
+	if (h == INVALID_HANDLE_VALUE &&
+	    GetLastError() == ERROR_FILE_NOT_FOUND) {
+		/* cut last component off of `wpath` */
+		wchar_t *p = wpath + wcslen(wpath);
+
+		while (p != wpath)
+			if (*(--p) == L'/' || *p == L'\\')
+				break; /* found start of last component */
+
+		if (p != wpath && (last_component = find_last_dir_sep(path))) {
+			append = xstrdup(last_component + 1); /* skip directory separator */
+			/*
+			 * Do not strip the trailing slash at the drive root, otherwise
+			 * the path would be e.g. `C:` (which resolves to the
+			 * _current_ directory on that drive).
+			 */
+			if (p[-1] == L':')
+				p[1] = L'\0';
+			else
+				*p = L'\0';
+			h = CreateFileW(wpath, 0, FILE_SHARE_READ |
+					FILE_SHARE_WRITE | FILE_SHARE_DELETE,
+					NULL, OPEN_EXISTING,
+					FILE_FLAG_BACKUP_SEMANTICS, NULL);
+		}
+	}
+
+	if (h == INVALID_HANDLE_VALUE) {
+realpath_failed:
+		FREE_AND_NULL(append);
+		return NULL;
+	}
+
+	ret = GetFinalPathNameByHandleW(h, wpath, ARRAY_SIZE(wpath), 0);
+	CloseHandle(h);
+	if (!ret || ret >= ARRAY_SIZE(wpath))
+		goto realpath_failed;
+
+	len = wcslen(wpath) * 3;
+	strbuf_grow(resolved, len);
+	len = xwcstoutf(resolved->buf, normalize_ntpath(wpath), len);
+	if (len < 0)
+		goto realpath_failed;
+	resolved->len = len;
+
+	if (append) {
+		/* Use forward-slash, like `normalize_ntpath()` */
+		strbuf_complete(resolved, '/');
+		strbuf_addstr(resolved, append);
+		FREE_AND_NULL(append);
+	}
+
+	return resolved->buf;
+
+}
+
 char *mingw_getcwd(char *pointer, int len)
 {
 	wchar_t cwd[MAX_PATH], wpointer[MAX_PATH];

compat/mingw.h

@@ -457,6 +457,9 @@ static inline void convert_slashes(char *path)
 #define PATH_SEP ';'
 char *mingw_query_user_email(void);
 #define query_user_email mingw_query_user_email
+struct strbuf;
+char *mingw_strbuf_realpath(struct strbuf *resolved, const char *path);
+#define platform_strbuf_realpath mingw_strbuf_realpath
 #if !defined(__MINGW64_VERSION_MAJOR) && (!defined(_MSC_VER) || _MSC_VER < 1800)
 #define PRIuMAX "I64u"
 #define PRId64 "I64d"

t/t0060-path-utils.sh

@@ -281,6 +281,14 @@ test_expect_success SYMLINKS 'real path works on symlinks' '
 	test_cmp expect actual
 '
 
+test_expect_success MINGW 'real path works near drive root' '
+	# we need a non-existing path at the drive root; simply skip if C:/xyz exists
+	if test ! -e C:/xyz
+	then
+		test C:/xyz = $(test-tool path-utils real_path C:/xyz)
+	fi
+'
+
 test_expect_success SYMLINKS 'prefix_path works with absolute paths to work tree symlinks' '
 	ln -s target symlink &&
 	echo "symlink" >expect &&

t/t3700-add.sh

@@ -548,7 +548,7 @@ test_expect_success CASE_INSENSITIVE_FS 'path is case-insensitive' '
 	git add "$downcased"
 '
 
-test_expect_failure MINGW 'can add files via NTFS junctions' '
+test_expect_success MINGW 'can add files via NTFS junctions' '
 	test_when_finished "cmd //c rmdir junction && rm -rf target" &&
 	test_create_repo target &&
 	cmd //c "mklink /j junction target" &&

t/t5601-clone.sh

@@ -78,6 +78,13 @@ test_expect_success 'clone respects GIT_WORK_TREE' '
 
 '
 
+test_expect_success CASE_INSENSITIVE_FS 'core.worktree is not added due to path case' '
+
+	mkdir UPPERCASE &&
+	git clone src "$(pwd)/uppercase" &&
+	test "unset" = "$(git -C UPPERCASE config --default unset core.worktree)"
+'
+
 test_expect_success 'clone from hooks' '
 
 	test_create_repo r0 &&