feat: add allowSymlinks option to File transport for symlink handling

lib/winston/transports/file.js

@@ -80,6 +80,7 @@ module.exports = class File extends TransportStream {
     this.eol = (typeof options.eol === 'string') ? options.eol : os.EOL;
     this.tailable = options.tailable || false;
     this.lazy = options.lazy || false;
+    this.allowSymlinks = (typeof options.allowSymlinks === 'boolean') ? options.allowSymlinks : true;
 
     // Internal state variables representing the number of files this instance
     // has created and the current size (in bytes) of the current logfile.
@@ -610,7 +611,21 @@ module.exports = class File extends TransportStream {
     const fullpath = path.join(this.dirname, this.filename);
 
     debug('create stream start', fullpath, this.options);
-    const dest = fs.createWriteStream(fullpath, this.options)
+
+    const streamOptions = Object.assign({}, this.options);
+    if (!this.allowSymlinks) {
+      /* eslint-disable no-bitwise */
+      if (streamOptions.flags === 'a') {
+        streamOptions.flags = fs.constants.O_APPEND | fs.constants.O_CREAT | fs.constants.O_WRONLY | fs.constants.O_NOFOLLOW;
+      } else if (streamOptions.flags === 'w') {
+        streamOptions.flags = fs.constants.O_CREAT | fs.constants.O_TRUNC | fs.constants.O_WRONLY | fs.constants.O_NOFOLLOW;
+      } else if (typeof streamOptions.flags === 'number') {
+        streamOptions.flags |= fs.constants.O_NOFOLLOW;
+      }
+      /* eslint-enable no-bitwise */
+    }
+
+    const dest = fs.createWriteStream(fullpath, streamOptions)
       // TODO: What should we do with errors here?
       .on('error', err => debug(err))
       .on('close', () => debug('close', dest.path, dest.bytesWritten))

test/unit/winston/transports/file.test.js

@@ -478,6 +478,79 @@ describe('File Transport', function () {
     });
   });
 
+  describe('Symlink Option', function () {
+    it('should not write to a symlink when allowSymlinks is false', async function () {
+      const targetFilename = 'target.log';
+      const symlinkFilename = 'symlink.log';
+      const targetPath = getFilePath(targetFilename);
+      const symlinkPath = getFilePath(symlinkFilename);
+
+      // Create target file
+      fs.writeFileSync(targetPath, 'initial content');
+
+      // Create symlink
+      try {
+        fs.symlinkSync(targetPath, symlinkPath);
+      } catch (err) {
+        if (err.code !== 'EEXIST') throw err;
+      }
+
+      const transport = new winston.transports.File({
+        dirname: testLogFixturesPath,
+        filename: symlinkFilename,
+        allowSymlinks: false
+      });
+
+      // We expect this to fail silently or emit an error, but definitely NOT write to target
+      // Using a small amount of data to avoid filling the buffer and hanging
+      await logToTransport(transport, { kbytes: 1 });
+
+      // Give it a moment to potentially flush (or fail)
+      await new Promise(resolve => setTimeout(resolve, 100));
+
+      const content = fs.readFileSync(targetPath, 'utf8');
+      assert.strictEqual(
+        content,
+        'initial content',
+        'Target file should not be modified'
+      );
+    });
+
+    it('should write to a symlink when allowSymlinks is true (default)', async function () {
+      const targetFilename = 'target_default.log';
+      const symlinkFilename = 'symlink_default.log';
+      const targetPath = getFilePath(targetFilename);
+      const symlinkPath = getFilePath(symlinkFilename);
+
+      // Create target file
+      fs.writeFileSync(targetPath, 'initial content');
+
+      // Create symlink
+      try {
+        fs.symlinkSync(targetPath, symlinkPath);
+      } catch (err) {
+        if (err.code !== 'EEXIST') throw err;
+      }
+
+      const transport = new winston.transports.File({
+        dirname: testLogFixturesPath,
+        filename: symlinkFilename
+        // allowSymlinks: true is default
+      });
+
+      await logToTransport(transport, { kbytes: 1 });
+
+      // Wait for write
+      await new Promise(resolve => setTimeout(resolve, 500));
+
+      const content = fs.readFileSync(targetPath, 'utf8');
+      assert.notStrictEqual(
+        content,
+        'initial content',
+        'Target file should be modified'
+      );
+    });
+  });
 
   // TODO: Reintroduce these tests
   //