[WasmFS] Allow backends to report errors from flush (#17781)

And catch and report such errors in the OPFS backend to prevent errors from
causing the async task to hang and create deadlocks.

Author: tlively

src/library_wasmfs_opfs.js

@@ -402,10 +402,14 @@ mergeInto(LibraryManager.library, {
   },
 
   _wasmfs_opfs_flush_access__deps: ['$wasmfsOPFSAccessHandles'],
-  _wasmfs_opfs_flush_access: async function(ctx, accessID) {
+  _wasmfs_opfs_flush_access: async function(ctx, accessID, errPtr) {
     let accessHandle = wasmfsOPFSAccessHandles.get(accessID);
-    // TODO: Error handling
-    await accessHandle.flush();
+    try {
+      await accessHandle.flush();
+    } catch {
+      let err = -{{{ cDefine('EIO') }}};
+      {{{ makeSetValue('errPtr', 0, 'err', 'i32') }}};
+    }
     _emscripten_proxy_finish(ctx);
   }
 });

system/lib/wasmfs/backends/node_backend.cpp

@@ -179,7 +179,7 @@ class NodeFile : public DataFile {
     return nwritten;
   }
 
-  void flush() override {
+  int flush() override {
     WASMFS_UNREACHABLE("TODO: implement NodeFile::flush");
   }
 };

system/lib/wasmfs/backends/opfs_backend.cpp

@@ -107,7 +107,7 @@ void _wasmfs_opfs_set_size_file(em_proxying_ctx* ctx,
                                 uint32_t size,
                                 int* err);
 
-void _wasmfs_opfs_flush_access(em_proxying_ctx* ctx, int access_id);
+void _wasmfs_opfs_flush_access(em_proxying_ctx* ctx, int access_id, int* err);
 
 } // extern "C"
 
@@ -313,18 +313,20 @@ class OPFSFile : public DataFile {
     return nwritten;
   }
 
-  void flush() override {
+  int flush() override {
+    int err = 0;
     switch (state.getKind()) {
       case OpenState::Access:
         proxy([&](auto ctx) {
-          _wasmfs_opfs_flush_access(ctx.ctx, state.getAccessID());
+          _wasmfs_opfs_flush_access(ctx.ctx, state.getAccessID(), &err);
         });
         break;
       case OpenState::Blob:
       case OpenState::None:
       default:
         break;
     }
+    return err;
   }
 };
 

system/lib/wasmfs/backends/proxied_file_backend.cpp

@@ -47,7 +47,7 @@ class ProxiedFile : public DataFile {
     return result;
   }
 
-  void flush() override {}
+  int flush() override { return 0; }
 
   // Querying the size of the Proxied File returns the size of the underlying
   // file given by the proxying mechanism.

system/lib/wasmfs/file.h

@@ -146,8 +146,9 @@ class DataFile : public File {
   // opened. Returns 0 on success or a negative error code.
   virtual int setSize(size_t size) = 0;
 
-  // TODO: Design a proper API for flushing files.
-  virtual void flush() = 0;
+  // Sync the file data to the underlying persistent storage, if any. Returns 0
+  // on success or a negative error code.
+  virtual int flush() = 0;
 
 public:
   static constexpr FileKind expectedKind = File::DataFileKind;
@@ -327,7 +328,7 @@ class DataFile::Handle : public File::Handle {
   [[nodiscard]] int setSize(size_t size) { return getFile()->setSize(size); }
 
   // TODO: Design a proper API for flushing files.
-  void flush() { getFile()->flush(); }
+  [[nodiscard]] int flush() { return getFile()->flush(); }
 
   // This function loads preloaded files from JS Memory into this DataFile.
   // TODO: Make this virtual so specific backends can specialize it for better

system/lib/wasmfs/js_impl_backend.h

@@ -94,7 +94,7 @@ class JSImplFile : public DataFile {
       getBackendIndex(), getFileIndex(), buf, len, offset);
   }
 
-  void flush() override {}
+  int flush() override { return 0; }
 
   size_t getSize() override {
     return _wasmfs_jsimpl_get_size(getBackendIndex(), getFileIndex());

system/lib/wasmfs/memory_backend.h

@@ -23,7 +23,7 @@ class MemoryFile : public DataFile {
   int close() override { return 0; }
   ssize_t write(const uint8_t* buf, size_t len, off_t offset) override;
   ssize_t read(uint8_t* buf, size_t len, off_t offset) override;
-  void flush() override {}
+  int flush() override { return 0; }
   size_t getSize() override { return buffer.size(); }
   int setSize(size_t size) override {
     buffer.resize(size);

system/lib/wasmfs/pipe_backend.h

@@ -44,7 +44,7 @@ class PipeFile : public DataFile {
     return len;
   }
 
-  void flush() override {}
+  int flush() override { return 0; }
 
   size_t getSize() override { return data->size(); }
 

system/lib/wasmfs/proxied_async_js_impl_backend.h

@@ -106,7 +106,7 @@ class ProxiedAsyncJSImplFile : public DataFile {
     return result;
   }
 
-  void flush() override {}
+  int flush() override { return 0; }
 
   size_t getSize() override {
     size_t result;

system/lib/wasmfs/special_files.cpp

@@ -28,7 +28,7 @@ class NullFile : public DataFile {
 
   ssize_t read(uint8_t* buf, size_t len, off_t offset) override { return 0; }
 
-  void flush() override {}
+  int flush() override { return 0; }
   size_t getSize() override { return 0; }
   int setSize(size_t size) override { return -EPERM; }
 
@@ -49,7 +49,7 @@ class StdinFile : public DataFile {
     abort();
   };
 
-  void flush() override {}
+  int flush() override { return 0; }
   size_t getSize() override { return 0; }
   int setSize(size_t size) override { return -EPERM; }
 
@@ -69,12 +69,13 @@ class WritingStdFile : public DataFile {
     return -__WASI_ERRNO_INVAL;
   };
 
-  void flush() override {
+  int flush() override {
     // Write a null to flush the output if we have content.
     if (!writeBuffer.empty()) {
       const uint8_t nothing = '\0';
       write(&nothing, 1, 0);
     }
+    return 0;
   }
 
   size_t getSize() override { return 0; }
@@ -150,7 +151,7 @@ class RandomFile : public DataFile {
     return len;
   };
 
-  void flush() override {}
+  int flush() override { return 0; }
   size_t getSize() override { return 0; }
   int setSize(size_t size) override { return -EPERM; }