Bug 1440207 - Part 2: Add a cross-platform DuplicateFileHandle method to MFBT, r=glandium

This is convenient in cross-platform code which needs to work with
`UniqueFileHandle` objects. `dup` is not supported by wasi, so the method isn't
available there.

Differential Revision: https://phabricator.services.mozilla.com/D221370

mfbt/UniquePtrExtensions.cpp

@@ -32,4 +32,25 @@ void FileHandleDeleter::operator()(FileHandleHelper aHelper) {
 }
 
 }  // namespace detail
+
+#ifndef __wasm__
+UniqueFileHandle DuplicateFileHandle(detail::FileHandleType aFile) {
+#  ifdef XP_WIN
+  if (aFile != INVALID_HANDLE_VALUE && aFile != NULL) {
+    HANDLE handle;
+    HANDLE currentProcess = ::GetCurrentProcess();
+    if (::DuplicateHandle(currentProcess, aFile, currentProcess, &handle, 0,
+                          false, DUPLICATE_SAME_ACCESS)) {
+      return UniqueFileHandle{handle};
+    }
+  }
+#  else
+  if (aFile != -1) {
+    return UniqueFileHandle{dup(aFile)};
+  }
+#  endif
+  return nullptr;
+}
+#endif
+
 }  // namespace mozilla

mfbt/UniquePtrExtensions.h

@@ -220,6 +220,14 @@ using UniqueFreePtr = UniquePtr<T, detail::FreePolicy<T>>;
 using UniqueFileHandle =
     UniquePtr<detail::FileHandleType, detail::FileHandleDeleter>;
 
+#ifndef __wasm__
+// WASI does not have `dup`
+MFBT_API UniqueFileHandle DuplicateFileHandle(detail::FileHandleType aFile);
+inline UniqueFileHandle DuplicateFileHandle(const UniqueFileHandle& aFile) {
+  return DuplicateFileHandle(aFile.get());
+}
+#endif
+
 #if defined(XP_DARWIN) && !defined(RUST_BINDGEN)
 // A RAII class for a Mach port that names a send right.
 using UniqueMachSendRight =