Duplicate the directory fd in fml::VisitFiles (flutter#13448)

The fd passed to fdopendir will be unusable afterward.  Using a duplicate
preserves the validity of the original directory fd passed to VisitFiles.

Fixes flutter#43844

fml/platform/posix/file_posix.cc

@@ -223,27 +223,34 @@ bool WriteAtomically(const fml::UniqueFD& base_directory,
 }
 
 bool VisitFiles(const fml::UniqueFD& directory, FileVisitor visitor) {
-  // We cannot call closedir(dir) because it will also close the corresponding
-  // UniqueFD, and later reference to that UniqueFD will fail. Also, we don't
-  // have to call closedir because UniqueFD will call close on its destructor.
-  DIR* dir = ::fdopendir(directory.get());
-  if (dir == nullptr) {
+  fml::UniqueFD dup_fd(dup(directory.get()));
+  if (!dup_fd.is_valid()) {
+    FML_DLOG(ERROR) << "Can't dup the directory fd. Error: " << strerror(errno);
+    return true;  // continue to visit other files
+  }
+
+  fml::UniqueDir dir(::fdopendir(dup_fd.get()));
+  if (!dir.is_valid()) {
     FML_DLOG(ERROR) << "Can't open the directory. Error: " << strerror(errno);
     return true;  // continue to visit other files
   }
 
+  // The directory fd will be closed by `closedir`.
+  (void)dup_fd.release();
+
   // Without `rewinddir`, `readir` will directly return NULL (end of dir is
   // reached) after a previuos `VisitFiles` call for the same `const
   // fml::UniqueFd& directory`.
-  rewinddir(dir);
-  while (dirent* ent = readdir(dir)) {
+  rewinddir(dir.get());
+  while (dirent* ent = readdir(dir.get())) {
     std::string filename = ent->d_name;
     if (filename != "." && filename != "..") {
       if (!visitor(directory, filename)) {
         return false;
       }
     }
   }
+
   return true;
 }
 

fml/unique_fd.cc

@@ -27,6 +27,10 @@ void UniqueFDTraits::Free(int fd) {
   close(fd);
 }
 
+void UniqueDirTraits::Free(DIR* dir) {
+  closedir(dir);
+}
+
 }  // namespace os_unix
 
 #endif  // OS_WIN

fml/unique_fd.h

@@ -14,6 +14,7 @@
 
 #else  // OS_WIN
 
+#include <dirent.h>
 #include <unistd.h>
 
 #endif  // OS_WIN
@@ -43,6 +44,12 @@ struct UniqueFDTraits {
   static void Free(int fd);
 };
 
+struct UniqueDirTraits {
+  static DIR* InvalidValue() { return nullptr; }
+  static bool IsValid(DIR* value) { return value != nullptr; }
+  static void Free(DIR* dir);
+};
+
 }  // namespace os_unix
 
 #endif  // OS_WIN
@@ -56,6 +63,7 @@ using UniqueFD = UniqueObject<HANDLE, internal::os_win::UniqueFDTraits>;
 #else  // OS_WIN
 
 using UniqueFD = UniqueObject<int, internal::os_unix::UniqueFDTraits>;
+using UniqueDir = UniqueObject<DIR*, internal::os_unix::UniqueDirTraits>;
 
 #endif  // OS_WIN