[FS] Make fstat work on file descriptors with no name in node rawfs

src/library_fs.js

@@ -965,6 +965,9 @@ FS.staticInit();
       var getattr = FS.checkOpExists(node.node_ops.getattr, {{{ cDefs.EPERM }}});
       return getattr(node);
     },
+    fstat(fd) {
+      return FS.stat(FS.getStreamChecked(fd).path);
+    },
     lstat(path) {
       return FS.stat(path, true);
     },

src/library_noderawfs.js

@@ -80,6 +80,10 @@ addToLibrary({
       }
       return stat;
     },
+    fstat(fd) {
+      var stream = FS.getStreamChecked(fd);
+      return fs.fstatSync(stream.nfd);
+    },
     statfsStream(stream) {
       return fs.statfsSync(stream.path);
     },

src/library_syscall.js

@@ -690,8 +690,7 @@ var SyscallsLibrary = {
     return SYSCALLS.writeStat(buf, FS.lstat(path));
   },
   __syscall_fstat64: (fd, buf) => {
-    var stream = SYSCALLS.getStreamFromFD(fd);
-    return SYSCALLS.writeStat(buf, FS.stat(stream.path));
+    return SYSCALLS.writeStat(buf, FS.fstat(fd));
   },
   __syscall_fchown32: (fd, owner, group) => {
     FS.fchown(fd, owner, group);

test/fs/test_stat_unnamed_file_descriptor.c

@@ -0,0 +1,20 @@
+#include <fcntl.h>
+#include <unistd.h>
+#include <sys/stat.h>
+#include <assert.h>
+#include "stdio.h"
+
+int main() {
+  int fd = open("file.txt", O_RDWR | O_CREAT, 0666);
+  unlink("file.txt");
+  int res;
+  struct stat buf;
+  res = fstat(fd, &buf);
+  assert(res == 0);
+  assert(buf.st_atime > 1000000000);
+  res = fchmod(fd, 0777);
+  assert(res == 0);
+  res = ftruncate(fd, 10);
+  assert(res == 0);
+  printf("success\n");
+}

test/test_core.py

@@ -5847,6 +5847,14 @@ def test_fs_64bit(self):
       self.set_setting('FORCE_FILESYSTEM')
     self.do_runf('fs/test_64bit.c', 'success')
 
+  @crossplatform
+  @also_with_nodefs_both
+  def test_fs_stat_unnamed_file_descriptor(self):
+    noderawfs = '-DNODERAWFS' in self.emcc_args
+    if not noderawfs:
+      self.skipTest('TODO: only works in noderawfs for now')
+    self.do_runf('fs/test_stat_unnamed_file_descriptor.c', 'success')
+
   @requires_node
   @crossplatform
   @with_all_fs