Port Facebook PR8370 to Stardog: GetFreeSpace wrong if not root (#28)

env/fs_posix.cc

@@ -870,7 +870,17 @@ class PosixFileSystem : public FileSystem {
       return IOError("While doing statvfs", fname, errno);
     }
 
-    *free_space = ((uint64_t)sbuf.f_bsize * sbuf.f_bfree);
+    // sbuf.bfree is total free space available to root
+    // sbuf.bavail is total free space available to unprivileged user
+    //  sbuf.bavail <= sbuf.bfree ... pick correct based upon effective user id
+    if (geteuid()) {
+      // non-zero user is unprivileged, or -1 if error.  take more conservative
+      // size
+      *free_space = ((uint64_t)sbuf.f_bsize * sbuf.f_bavail);
+    } else {
+      // root user can access all disk space
+      *free_space = ((uint64_t)sbuf.f_bsize * sbuf.f_bfree);
+    }
     return IOStatus::OK();
   }