[WasmFS] Remove duplicated Directory code in backends. NFC (#16067)

Author: kripken

system/lib/wasmfs/backend.h

@@ -21,7 +21,13 @@ class Backend {
 
 public:
   virtual std::shared_ptr<DataFile> createFile(mode_t mode) = 0;
-  virtual std::shared_ptr<Directory> createDirectory(mode_t mode) = 0;
+
+  // By default all backends create normal Directory instances for directories.
+  // That is, the default behavior is to keep directory structure in-memory.
+  virtual std::shared_ptr<Directory> createDirectory(mode_t mode) {
+    return std::make_shared<Directory>(mode, this);
+  }
+
   virtual ~Backend() = default;
 };
 

system/lib/wasmfs/js_file_backend.cpp

@@ -67,9 +67,6 @@ class JSFileBackend : public Backend {
   std::shared_ptr<DataFile> createFile(mode_t mode) override {
     return std::make_shared<JSFile>(mode, this);
   }
-  std::shared_ptr<Directory> createDirectory(mode_t mode) override {
-    return std::make_shared<Directory>(mode, this);
-  }
 };
 
 // This function is exposed to users to instantiate a new JSBackend.

system/lib/wasmfs/memory_file_backend.cpp

@@ -18,9 +18,6 @@ class MemoryFileBackend : public Backend {
   std::shared_ptr<DataFile> createFile(mode_t mode) override {
     return std::make_shared<MemoryFile>(mode, this);
   }
-  std::shared_ptr<Directory> createDirectory(mode_t mode) override {
-    return std::make_shared<Directory>(mode, this);
-  }
 };
 
 backend_t createMemoryFileBackend() {

system/lib/wasmfs/proxied_file_backend.cpp

@@ -90,10 +90,6 @@ class ProxiedBackend : public Backend {
     // This creates a file on a thread specified by the proxy member.
     return std::make_shared<ProxiedFile>(mode, this, backend, proxy);
   }
-
-  std::shared_ptr<Directory> createDirectory(mode_t mode) override {
-    return std::make_shared<Directory>(mode, this);
-  }
 };
 
 // Create a proxied backend by supplying another backend.