mingw: implement readlink()

Implement `readlink()` by reading NTFS reparse points via the
`read_reparse_point()` function that was introduced earlier to determine
the length of symlink targets. Works for symlinks and directory
junctions.

Signed-off-by: Karsten Blees <[email protected]>
Signed-off-by: Johannes Schindelin <[email protected]>

Author: kblees

compat/mingw-posix.h

@@ -121,8 +121,6 @@ struct utsname {
  * trivial stubs
  */
 
-static inline int readlink(const char *path UNUSED, char *buf UNUSED, size_t bufsiz UNUSED)
-{ errno = ENOSYS; return -1; }
 static inline int symlink(const char *oldpath UNUSED, const char *newpath UNUSED)
 { errno = ENOSYS; return -1; }
 static inline int fchmod(int fildes UNUSED, mode_t mode UNUSED)
@@ -197,6 +195,7 @@ int setitimer(int type, struct itimerval *in, struct itimerval *out);
 int sigaction(int sig, struct sigaction *in, struct sigaction *out);
 int link(const char *oldpath, const char *newpath);
 int uname(struct utsname *buf);
+int readlink(const char *path, char *buf, size_t bufsiz);
 
 /*
  * replacements of existing functions

compat/mingw.c

@@ -2698,6 +2698,30 @@ int link(const char *oldpath, const char *newpath)
 	return 0;
 }
 
+int readlink(const char *path, char *buf, size_t bufsiz)
+{
+	WCHAR wpath[MAX_PATH];
+	char tmpbuf[MAX_PATH];
+	int len;
+	DWORD tag;
+
+	if (xutftowcs_path(wpath, path) < 0)
+		return -1;
+
+	if (read_reparse_point(wpath, TRUE, tmpbuf, &len, &tag) < 0)
+		return -1;
+
+	/*
+	 * Adapt to strange readlink() API: Copy up to bufsiz *bytes*, potentially
+	 * cutting off a UTF-8 sequence. Insufficient bufsize is *not* a failure
+	 * condition. There is no conversion function that produces invalid UTF-8,
+	 * so convert to a (hopefully large enough) temporary buffer, then memcpy
+	 * the requested number of bytes (including '\0' for robustness).
+	 */
+	memcpy(buf, tmpbuf, min(bufsiz, len + 1));
+	return min(bufsiz, len);
+}
+
 pid_t waitpid(pid_t pid, int *status, int options)
 {
 	HANDLE h = OpenProcess(SYNCHRONIZE | PROCESS_QUERY_INFORMATION,