fix wrong truncation on fs.writeFileSync with fd argument (#10225)

* fix wrong truncate

* close fd in test

* [autofix.ci] apply automated fixes

---------

Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>

src/bun.js/node/node_fs.zig

@@ -5474,7 +5474,9 @@ pub const NodeFS = struct {
             }
         } else {
             // https://github.com/oven-sh/bun/issues/2931
-            if ((@intFromEnum(args.flag) & std.os.O.APPEND) == 0) {
+            // https://github.com/oven-sh/bun/issues/10222
+            // only truncate if we're not appending and writing to a path
+            if ((@intFromEnum(args.flag) & std.os.O.APPEND) == 0 and args.file != .fd) {
                 _ = ftruncateSync(.{ .fd = fd, .len = @as(JSC.WebCore.Blob.SizeType, @truncate(written)) });
             }
         }

test/js/node/fs/fs.test.ts

@@ -2971,3 +2971,13 @@ describe.if(isWindows)("windows path handling", () => {
     });
   }
 });
+
+it("using writeFile on an fd does not truncate it", () => {
+  const temp = tmpdir();
+  const fd = fs.openSync(join(temp, "file.txt"), "w+");
+  fs.writeFileSync(fd, "x");
+  fs.writeFileSync(fd, "x");
+  fs.closeSync(fd);
+  const content = fs.readFileSync(join(temp, "file.txt"), "utf8");
+  expect(content).toBe("xx");
+});