Implemented an partial emulation of fstatat for OS X

fstatat does not exists under OS X and it was replaced by a dummy
stub. This resulted in symlinks being seen as unknown file type
when reading directory entries. Ultimately, this leads to the
inability to glob symlinks under OS X.

--
MOS_MIGRATED_REVID=89304190

src/main/native/unix_jni_darwin.cc

@@ -16,11 +16,16 @@
 
 #include <assert.h>
 #include <errno.h>
+#include <fcntl.h>
+#include <stdlib.h>
 #include <string.h>
 #include <sys/stat.h>
+#include <sys/syslimits.h>
 
 #include <string>
 
+const int PATH_MAX2 = PATH_MAX * 2;
+
 using std::string;
 
 // See unix_jni.h.
@@ -33,9 +38,39 @@ string ErrorMessage(int error_number) {
   return string(buf);
 }
 
+
 int portable_fstatat(int dirfd, char *name, struct stat *statbuf, int flags) {
-  errno = ENOSYS;
-  return -1;
+  char dirPath[PATH_MAX2];  // Have enough room for relative path
+
+  // No fstatat under darwin, simulate it
+  if (flags != 0) {
+    // We don't support any flags
+    errno = ENOSYS;
+    return -1;
+  }
+  if (strlen(name) == 0 || name[0] == '/') {
+    // Absolute path, simply stat
+    return stat(name, statbuf);
+  }
+  // Relative path, construct an absolute path
+  if (fcntl(dirfd, F_GETPATH, dirPath) == -1) {
+    return -1;
+  }
+  int l = strlen(dirPath);
+  if (dirPath[l-1] != '/') {
+    // dirPath is twice the PATH_MAX size, we always have room for the extra /
+    dirPath[l] = '/';
+    dirPath[l+1] = 0;
+    l++;
+  }
+  strncat(dirPath, name, PATH_MAX2-l-1);
+  char *newpath = realpath(dirPath, NULL);  // this resolve the relative path
+  if (newpath == NULL) {
+    return -1;
+  }
+  int r = stat(newpath, statbuf);
+  free(newpath);
+  return r;
 }
 
 int StatSeconds(const struct stat &statbuf, StatTimes t) {