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

This fixes fstat on "anonymous" file descriptors in node rawfs.
It is split off from emscripten-core#23058.

Author: hoodmane

src/library_fs.js

@@ -956,6 +956,9 @@ FS.staticInit();
       }
       return node.node_ops.getattr(node);
     },
+    fstat(fd) {
+      return FS.stat(FS.getStreamChecked(fd).node);
+    },
     lstat(path) {
       return FS.stat(path, true);
     },

src/library_noderawfs.js

@@ -79,6 +79,10 @@ addToLibrary({
       }
       return stat;
     },
+    fstat(fd) {
+      var stream = FS.getStreamChecked(fd);
+      return fs.fstatSync(stream.nfd);
+    },
     chmod(path, mode, dontFollow) {
       mode &= {{{ cDefs.S_IALLUGO }}};
       if (NODEFS.isWindows) {

src/library_syscall.js

@@ -678,8 +678,7 @@ 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 +5847,16 @@ def test_fs_64bit(self):
       self.set_setting('FORCE_FILESYSTEM')
     self.do_runf('fs/test_64bit.c', 'success')
 
+  @crossplatform
+  @also_with_noderawfs
+  def test_fs_stat_unnamed_file_descriptor(self):
+    nodefs = '-DNODEFS' in self.emcc_args or '-DNODERAWFS' in self.emcc_args
+    if self.get_setting('WASMFS'):
+      if nodefs:
+        self.skipTest('NODEFS in WasmFS')
+      self.set_setting('FORCE_FILESYSTEM')
+    self.do_runf('fs/test_stat_unnamed_file_descriptor.c', 'success')
+
   @requires_node
   @crossplatform
   @with_all_fs