[WasmFS] Gracefully error when moving OPFS directory (#19320)

The OPFS API does not yet support moving directories. When user code tried to
move a directory using the OPFS backend, we previously crashed because the
backend implementation incorrectly assumed that the moved file was a data file.
Fix the crash by returning EBUSY instead in that case.

Author: tlively

src/library_wasmfs_opfs.js

@@ -189,13 +189,14 @@ mergeInto(LibraryManager.library, {
     wasmfsOPFSProxyFinish(ctx);
   },
 
-  _wasmfs_opfs_move__sig: 'vpiipp',
-  _wasmfs_opfs_move__deps: ['$wasmfsOPFSFileHandles',
-                            '$wasmfsOPFSDirectoryHandles', '$wasmfsOPFSProxyFinish'],
-  _wasmfs_opfs_move: async function(ctx, fileID, newDirID, namePtr, errPtr) {
+  _wasmfs_opfs_move_file__sig: 'vpiipp',
+  _wasmfs_opfs_move_file__deps: ['$wasmfsOPFSFileHandles',
+                                 '$wasmfsOPFSDirectoryHandles',
+                                 '$wasmfsOPFSProxyFinish'],
+  _wasmfs_opfs_move_file: async function(ctx, fileID, newParentID, namePtr, errPtr) {
     let name = UTF8ToString(namePtr);
     let fileHandle = wasmfsOPFSFileHandles.get(fileID);
-    let newDirHandle = wasmfsOPFSDirectoryHandles.get(newDirID);
+    let newDirHandle = wasmfsOPFSDirectoryHandles.get(newParentID);
     try {
       await fileHandle.move(newDirHandle, name);
     } catch {

system/lib/wasmfs/backends/opfs_backend.cpp

@@ -41,11 +41,11 @@ void _wasmfs_opfs_insert_directory(em_proxying_ctx* ctx,
                                    const char* name,
                                    int* child_id);
 
-void _wasmfs_opfs_move(em_proxying_ctx* ctx,
-                       int file_id,
-                       int new_dir_id,
-                       const char* name,
-                       int* err);
+void _wasmfs_opfs_move_file(em_proxying_ctx* ctx,
+                            int file_id,
+                            int new_parent_id,
+                            const char* name,
+                            int* err);
 
 void _wasmfs_opfs_remove_child(em_proxying_ctx* ctx,
                                int dir_id,
@@ -431,11 +431,19 @@ class OPFSDirectory : public Directory {
   }
 
   int insertMove(const std::string& name, std::shared_ptr<File> file) override {
-    auto old_file = std::static_pointer_cast<OPFSFile>(file);
     int err = 0;
-    proxy([&](auto ctx) {
-      _wasmfs_opfs_move(ctx.ctx, old_file->fileID, dirID, name.c_str(), &err);
-    });
+    if (file->is<DataFile>()) {
+      auto opfsFile = std::static_pointer_cast<OPFSFile>(file);
+      proxy([&](auto ctx) {
+        _wasmfs_opfs_move_file(
+          ctx.ctx, opfsFile->fileID, dirID, name.c_str(), &err);
+      });
+    } else {
+      // TODO: Support moving directories once OPFS supports that.
+      // EBUSY can be returned when the directory is "in use by the system,"
+      // which can mean whatever we want.
+      err = -EBUSY;
+    }
     return err;
   }
 

test/wasmfs/wasmfs_opfs_errors.c

@@ -4,6 +4,7 @@
 #include <emscripten/wasmfs.h>
 #include <errno.h>
 #include <fcntl.h>
+#include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <unistd.h>
@@ -121,6 +122,25 @@ int try_oob_write(void) {
   return 2;
 }
 
+EMSCRIPTEN_KEEPALIVE
+int try_rename_dir(void) {
+  int err = mkdir("/opfs/dir1", 0666);
+  if (err != 0) {
+    return 2;
+  }
+  err = rename("/opfs/dir1", "/opfs/dir2");
+  if (err == 0) {
+    return 1;
+  }
+  if (errno == EBUSY) {
+    rmdir("/opfs/dir1");
+    return 0;
+  }
+  emscripten_console_error(strerror(errno));
+  rmdir("/opfs/dir1");
+  return 2;
+}
+
 EMSCRIPTEN_KEEPALIVE
 void report_result(int result) {
   EM_ASM({ console.log(new Error().stack); });

test/wasmfs/wasmfs_opfs_errors_post.js

@@ -66,5 +66,9 @@ async function run_test() {
           "nothing prevents it)";
   }
 
+  if (Module._try_rename_dir() != 0) {
+    throw "Did not get expected EBUSY while renaming directory";
+  }
+
   Module._report_result(0);
 }