[WasmFS] Make the memory backend public (#18250)

Add `wasmfs_create_memory_backend` to the public header, use it in tests, and
update the internal name of the backend to be `MemoryBackend` rather than
`MemoryFileBackend`.

system/include/emscripten/wasmfs.h

@@ -40,6 +40,8 @@ backend_t wasmfs_create_js_file_backend(void);
 // A function that receives a void* and returns a backend.
 typedef backend_t (*backend_constructor_t)(void*);
 
+backend_t wasmfs_create_memory_backend(void);
+
 backend_t wasmfs_create_fetch_backend(const char* base_url);
 
 backend_t wasmfs_create_node_backend(const char* root);

system/lib/wasmfs/backends/memory_backend.cpp

@@ -13,15 +13,15 @@
 
 namespace wasmfs {
 
-ssize_t MemoryFile::write(const uint8_t* buf, size_t len, off_t offset) {
+ssize_t MemoryDataFile::write(const uint8_t* buf, size_t len, off_t offset) {
   if (offset + len > buffer.size()) {
     buffer.resize(offset + len);
   }
   std::memcpy(&buffer[offset], buf, len);
   return len;
 }
 
-ssize_t MemoryFile::read(uint8_t* buf, size_t len, off_t offset) {
+ssize_t MemoryDataFile::read(uint8_t* buf, size_t len, off_t offset) {
   if (offset >= buffer.size()) {
     len = 0;
   } else if (offset + len >= buffer.size()) {
@@ -90,10 +90,10 @@ std::string MemoryDirectory::getName(std::shared_ptr<File> file) {
   return "";
 }
 
-class MemoryFileBackend : public Backend {
+class MemoryBackend : public Backend {
 public:
   std::shared_ptr<DataFile> createFile(mode_t mode) override {
-    return std::make_shared<MemoryFile>(mode, this);
+    return std::make_shared<MemoryDataFile>(mode, this);
   }
   std::shared_ptr<Directory> createDirectory(mode_t mode) override {
     return std::make_shared<MemoryDirectory>(mode, this);
@@ -103,8 +103,14 @@ class MemoryFileBackend : public Backend {
   }
 };
 
-backend_t createMemoryFileBackend() {
-  return wasmFS.addBackend(std::make_unique<MemoryFileBackend>());
+backend_t createMemoryBackend() {
+  return wasmFS.addBackend(std::make_unique<MemoryBackend>());
 }
 
+extern "C" {
+
+backend_t wasmfs_create_memory_backend() { return createMemoryBackend(); }
+
+} // extern "C"
+
 } // namespace wasmfs

system/lib/wasmfs/memory_backend.h

@@ -16,7 +16,7 @@
 
 namespace wasmfs {
 // This class describes a file that lives in Wasm Memory.
-class MemoryFile : public DataFile {
+class MemoryDataFile : public DataFile {
   std::vector<uint8_t> buffer;
 
   int open(oflags_t) override { return 0; }
@@ -31,11 +31,13 @@ class MemoryFile : public DataFile {
   }
 
 public:
-  MemoryFile(mode_t mode, backend_t backend) : DataFile(mode, backend) {}
+  MemoryDataFile(mode_t mode, backend_t backend) : DataFile(mode, backend) {}
 
   class Handle : public DataFile::Handle {
 
-    std::shared_ptr<MemoryFile> getFile() { return file->cast<MemoryFile>(); }
+    std::shared_ptr<MemoryDataFile> getFile() {
+      return file->cast<MemoryDataFile>();
+    }
 
   public:
     Handle(std::shared_ptr<File> dataFile) : DataFile::Handle(dataFile) {}
@@ -111,6 +113,6 @@ class MemorySymlink : public Symlink {
     : Symlink(backend), target(target) {}
 };
 
-backend_t createMemoryFileBackend();
+backend_t createMemoryBackend();
 
 } // namespace wasmfs

system/lib/wasmfs/wasmfs.cpp

@@ -71,12 +71,12 @@ WasmFS::~WasmFS() {
 }
 
 std::shared_ptr<Directory> WasmFS::initRootDirectory() {
-  
+
 #ifdef WASMFS_CASE_INSENSITIVE
   auto rootBackend =
-    createIgnoreCaseBackend([]() { return createMemoryFileBackend(); });
+    createIgnoreCaseBackend([]() { return createMemoryBackend(); });
 #else
-  auto rootBackend = createMemoryFileBackend();
+  auto rootBackend = createMemoryBackend();
 #endif
   auto rootDirectory =
     rootBackend->createDirectory(S_IRUGO | S_IXUGO | S_IWUGO);

test/wasmfs/get_backend.h

@@ -15,7 +15,7 @@
 
 static backend_t get_backend() {
 #ifdef WASMFS_MEMORY_BACKEND
-  return NULL;
+  return wasmfs_create_memory_backend();
 #else
 #ifdef WASMFS_NODE_BACKEND
   return wasmfs_create_node_backend(".");