[WasmFS] Support file resizing in JSFiles (#22275)

Fixes #22262

src/library_sigs.js

@@ -404,6 +404,7 @@ sigs = {
   _wasmfs_jsimpl_free_file__sig: 'vpp',
   _wasmfs_jsimpl_get_size__sig: 'ipp',
   _wasmfs_jsimpl_read__sig: 'ippppj',
+  _wasmfs_jsimpl_set_size__sig: 'ippj',
   _wasmfs_jsimpl_write__sig: 'ippppj',
   _wasmfs_node_close__sig: 'ii',
   _wasmfs_node_fstat_size__sig: 'iip',

src/library_wasmfs.js

@@ -375,6 +375,8 @@ FS.init();
           wasmFSDeviceStreams[file] = undefined;
         },
         getSize: (file) => {},
+        // Devices cannot be resized.
+        setSize: (file, size) => 0,
         read: (file, buffer, length, offset) => {
           var bufferArray = Module.HEAP8.subarray(buffer, buffer + length);
           try {

src/library_wasmfs_js_file.js

@@ -44,7 +44,17 @@ addToLibrary({
         HEAPU8.set(fileData.subarray(offset, offset + length), buffer);
         return length;
       },
-      getSize: (file) => wasmFS$JSMemoryFiles[file] ? wasmFS$JSMemoryFiles[file].length : 0,
+      getSize: (file) => wasmFS$JSMemoryFiles[file]?.length || 0,
+      setSize: (file, size) => {
+        // Allocate a new array of the proper size, and copy as much data as
+        // possible.
+        var old = wasmFS$JSMemoryFiles[file];
+        var new_ = wasmFS$JSMemoryFiles[file] = new Uint8Array(size);
+        if (old) {
+          new_.set(old.subarray(0, size));
+        }
+        return 0;
+      }
     };
   },
 });

src/library_wasmfs_jsimpl.js

@@ -53,6 +53,13 @@ addToLibrary({
     return wasmFS$backends[backend].getSize(file);
   },
 
+  _wasmfs_jsimpl_set_size: (backend, file, size) => {
+#if ASSERTIONS
+    assert(wasmFS$backends[backend]);
+#endif
+    return wasmFS$backends[backend].setSize(file, size);
+  },
+
   // ProxiedAsyncJSImpl. Each function receives a function pointer and a
   // parameter. We convert those into a convenient Promise API for the
   // implementors of backends: the hooks we call should return Promises, which

system/lib/wasmfs/js_impl_backend.h

@@ -65,6 +65,7 @@ int _wasmfs_jsimpl_read(js_index_t backend,
                         size_t length,
                         off_t offset);
 int _wasmfs_jsimpl_get_size(js_index_t backend, js_index_t index);
+int _wasmfs_jsimpl_set_size(js_index_t backend, js_index_t index, off_t size);
 }
 
 namespace wasmfs {
@@ -101,7 +102,7 @@ class JSImplFile : public DataFile {
   }
 
   int setSize(off_t size) override {
-    WASMFS_UNREACHABLE("TODO: JSImpl setSize");
+    return _wasmfs_jsimpl_set_size(getBackendIndex(), getFileIndex(), size);
   }
 
 public:

test/fs/test_writev_partial_write.c

@@ -68,13 +68,7 @@ void test_writev_direct(void) {
 // and is included here as this code most closely matches the original bug
 // report
 void test_via_stdio(void) {
-  // XXX: We open in append mode because truncating JS based files is not
-  // supported yet. See #22262
-#ifdef WASMFS
-  FILE* f = fopen("/device", "a");
-#else
   FILE* f = fopen("/device", "w");
-#endif
   assert(f);
   // Use line buffering. The bug is exposed with line buffering because with
   // line buffering two entries in __stdio_write's iovs are used.

test/wasmfs/wasmfs_jsfile.c

@@ -110,7 +110,31 @@ int main() {
   lseek(fd4, 0, SEEK_SET);
   assert(errno == 0);
   int bytesRead = read(fd4, buf, sizeof(buf));
+  for (ssize_t i = 0; i < bytesRead; i++) {
+    if (buf[i] == 0) {
+      printf("\\0");
+    } else {
+      printf("%c", buf[i]);
+    }
+  }
+  printf("\n");
+
+  // Check the size.
+  int result = lseek(fd4, 0, SEEK_END);
+  assert(result != -1);
+  printf("size: %lld\n", lseek(fd4, 0, SEEK_CUR));
 
+  // Resize.
+  result = ftruncate(fd4, 42);
+  assert(!result);
+  result = lseek(fd4, 0, SEEK_END);
+  assert(result != -1);
+  printf("resize: %lld\n", lseek(fd4, 0, SEEK_CUR));
+
+  // Check the contents remain after the resize (but are a tad shorter).
+  lseek(fd4, 0, SEEK_SET);
+  assert(errno == 0);
+  bytesRead = read(fd4, buf, sizeof(buf));
   for (ssize_t i = 0; i < bytesRead; i++) {
     if (buf[i] == 0) {
       printf("\\0");
@@ -120,5 +144,6 @@ int main() {
   }
   printf("\n");
 
+  puts("\ndone.");
   return 0;
 }

test/wasmfs/wasmfs_jsfile.out

@@ -1 +1,7 @@
 \0\0\0\0\0\0\0\0\0\0Test with a backend file created under root
+
+size: 54
+resize: 42
+\0\0\0\0\0\0\0\0\0\0Test with a backend file created
+
+done.