fix: skip native write for zero-length callback writeFile

patches/node/.patches

@@ -45,3 +45,4 @@ src_refactor_wasmstreaming_finish_to_accept_a_callback.patch
 src_stop_using_v8_propertycallbackinfo_t_this.patch
 build_restore_macos_deployment_target_to_12_0.patch
 fix_add_externalpointertypetag_to_v8_external_api_calls.patch
+fs_skip_native_write_for_zero-length_callback_writefile.patch

patches/node/fs_skip_native_write_for_zero-length_callback_writefile.patch

@@ -0,0 +1,96 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Caner Altinbasak <cal@brightsign.biz>
+Date: Thu, 25 Jun 2026 10:51:20 +0100
+Subject: fs: skip native write for zero-length callback writeFile()
+
+Callback fs.writeFile()/writeAll() always issued a native
+fs.write(fd, buffer, 0, 0, ...) syscall even when the data was empty.
+On the Electron-based roHtmlWidget Node integration that zero-length
+write returns "EFAULT: bad address in system call argument, write",
+whereas it succeeds under the standalone roNodeJs runtime and under
+fs/promises.writeFile().
+
+Add a zero-length guard to writeAll() that skips the syscall and runs
+the normal completion path (optional fsync, close the fd only when
+writeFile() opened it, invoke callback(null)), matching the early
+return already present in fs/promises.writeFile(). Non-empty writes,
+validation, abort/signal handling and caller-owned fd semantics are
+unchanged.
+
+diff --git a/lib/fs.js b/lib/fs.js
+index 6e688375f48d2d6a12e0af6ac0c94c84b9c3c9f4..05c0b69e68cd26a862153dd3cf10cc94d7c0da32 100644
+--- a/lib/fs.js
++++ b/lib/fs.js
+@@ -2283,6 +2283,41 @@ function writeAll(fd, isUserFd, buffer, offset, length, signal, flush, callback)
+     }
+     return;
+   }
++  // Shared completion path: optionally flush, close the fd if writeFile()
++  // opened it internally, and invoke the callback.
++  const onWriteSucceeded = () => {
++    if (!flush) {
++      if (isUserFd) {
++        callback(null);
++      } else {
++        fs.close(fd, callback);
++      }
++    } else {
++      fs.fsync(fd, (syncErr) => {
++        if (syncErr) {
++          if (isUserFd) {
++            callback(syncErr);
++          } else {
++            fs.close(fd, (err) => {
++              callback(aggregateTwoErrors(err, syncErr));
++            });
++          }
++        } else if (isUserFd) {
++          callback(null);
++        } else {
++          fs.close(fd, callback);
++        }
++      });
++    }
++  };
++  // A zero-length write would still issue a native fs.write(fd, buffer, 0, 0,
++  // ...) syscall, which returns EFAULT on some runtimes (notably the
++  // Electron-based roHtmlWidget Node integration). Skip the syscall and finish
++  // as a successful 0-byte write, matching fs/promises.writeFile().
++  if (length === 0) {
++    onWriteSucceeded();
++    return;
++  }
+   // write(fd, buffer, offset, length, position, callback)
+   fs.write(fd, buffer, offset, length, null, (writeErr, written) => {
+     if (writeErr) {
+@@ -2294,29 +2329,7 @@ function writeAll(fd, isUserFd, buffer, offset, length, signal, flush, callback)
+         });
+       }
+     } else if (written === length) {
+-      if (!flush) {
+-        if (isUserFd) {
+-          callback(null);
+-        } else {
+-          fs.close(fd, callback);
+-        }
+-      } else {
+-        fs.fsync(fd, (syncErr) => {
+-          if (syncErr) {
+-            if (isUserFd) {
+-              callback(syncErr);
+-            } else {
+-              fs.close(fd, (err) => {
+-                callback(aggregateTwoErrors(err, syncErr));
+-              });
+-            }
+-          } else if (isUserFd) {
+-            callback(null);
+-          } else {
+-            fs.close(fd, callback);
+-          }
+-        });
+-      }
++      onWriteSucceeded();
+     } else {
+       offset += written;
+       length -= written;

spec/node-spec.ts

@@ -400,6 +400,115 @@ describe('node feature', () => {
       );
     });
 
+    // Regression test for a zero-length callback fs.writeFile() that failed
+    // with "EFAULT: bad address in system call argument, write" only in the
+    // renderer (nodeIntegration / roHtmlWidget) Node integration, while it
+    // worked in a standalone Node process and via fs/promises.writeFile().
+    // Callback fs.writeFile() now skips the native zero-length write, matching
+    // the early return already present in fs/promises.writeFile().
+    describe('zero-length fs.writeFile in renderer', () => {
+      itremote('fs.writeFile(path, "", cb) succeeds and creates an empty file', async () => {
+        const fs = require('node:fs');
+        const path = require('node:path');
+        const file = path.join(require('node:os').tmpdir(), `electron-fswrite-empty-string-${Date.now()}`);
+        try {
+          await new Promise<void>((resolve, reject) => {
+            fs.writeFile(file, '', (err: Error | null) => (err ? reject(err) : resolve()));
+          });
+          expect(fs.statSync(file).size).to.equal(0);
+        } finally {
+          try {
+            fs.unlinkSync(file);
+          } catch {
+            /* ignore */
+          }
+        }
+      });
+
+      itremote('fs.writeFile(path, Buffer.alloc(0), cb) succeeds', async () => {
+        const fs = require('node:fs');
+        const path = require('node:path');
+        const file = path.join(require('node:os').tmpdir(), `electron-fswrite-empty-buffer-${Date.now()}`);
+        try {
+          await new Promise<void>((resolve, reject) => {
+            fs.writeFile(file, Buffer.alloc(0), (err: Error | null) => (err ? reject(err) : resolve()));
+          });
+          expect(fs.statSync(file).size).to.equal(0);
+        } finally {
+          try {
+            fs.unlinkSync(file);
+          } catch {
+            /* ignore */
+          }
+        }
+      });
+
+      itremote('fs.writeFile(path, "1", cb) still writes non-empty data', async () => {
+        const fs = require('node:fs');
+        const path = require('node:path');
+        const file = path.join(require('node:os').tmpdir(), `electron-fswrite-non-empty-${Date.now()}`);
+        try {
+          await new Promise<void>((resolve, reject) => {
+            fs.writeFile(file, '1', (err: Error | null) => (err ? reject(err) : resolve()));
+          });
+          expect(fs.readFileSync(file, 'utf-8')).to.equal('1');
+        } finally {
+          try {
+            fs.unlinkSync(file);
+          } catch {
+            /* ignore */
+          }
+        }
+      });
+
+      itremote('fs.writeFile(path, "", cb) truncates an existing file', async () => {
+        const fs = require('node:fs');
+        const path = require('node:path');
+        const file = path.join(require('node:os').tmpdir(), `electron-fswrite-truncate-${Date.now()}`);
+        try {
+          fs.writeFileSync(file, 'non-empty contents');
+          expect(fs.statSync(file).size).to.be.greaterThan(0);
+          await new Promise<void>((resolve, reject) => {
+            fs.writeFile(file, '', (err: Error | null) => (err ? reject(err) : resolve()));
+          });
+          expect(fs.statSync(file).size).to.equal(0);
+        } finally {
+          try {
+            fs.unlinkSync(file);
+          } catch {
+            /* ignore */
+          }
+        }
+      });
+
+      itremote('fs.writeFile(fd, Buffer.alloc(0), cb) succeeds without closing a caller-owned fd', async () => {
+        const fs = require('node:fs');
+        const path = require('node:path');
+        const file = path.join(require('node:os').tmpdir(), `electron-fswrite-user-fd-${Date.now()}`);
+        const fd = fs.openSync(file, 'w');
+        try {
+          await new Promise<void>((resolve, reject) => {
+            fs.writeFile(fd, Buffer.alloc(0), (err: Error | null) => (err ? reject(err) : resolve()));
+          });
+          // If writeFile had closed our fd this write would throw EBADF.
+          fs.writeSync(fd, 'still open');
+          fs.closeSync(fd);
+          expect(fs.readFileSync(file, 'utf-8')).to.equal('still open');
+        } finally {
+          try {
+            fs.closeSync(fd);
+          } catch {
+            /* already closed */
+          }
+          try {
+            fs.unlinkSync(file);
+          } catch {
+            /* ignore */
+          }
+        }
+      });
+    });
+
     describe('error thrown in renderer process node context', () => {
       itremote(
         'gets emitted as a process uncaughtException event',