fix(gopher): reject resume when caller supplies Writer

internal/protocol/gopher/gopher.go

@@ -165,6 +165,16 @@ func (p *Protocol) downloadText(ctx context.Context, reader *bufio.Reader, req *
 	// fresh downloads (create) and resume (append). When req.Writer is
 	// set (caller manages the file), we use it as-is.
 	if req.Output != "" && req.Output != "-" {
+		// Resume is meaningful only when we own the output file. If the
+		// caller already supplied a Writer we cannot append to Output
+		// behind their back, so disable resume to keep the partial-state
+		// sidecar consistent with what actually lands on disk.
+		if req.Resume && req.Writer != nil && req.Verbose {
+			fmt.Fprintf(os.Stderr, "[gopher] warning: --resume ignored because a Writer is set; output goes to the Writer only\n")
+		}
+		if req.Resume && req.Writer != nil {
+			req.Resume = false
+		}
 		if req.Resume {
 			canResume, resumeMeta, rerr := output.CanResume(req.Output, req.URL.String())
 			if rerr != nil {

internal/protocol/gopher/gopher_test.go

@@ -438,3 +438,53 @@ func TestGopherResumeStaleMetadata(t *testing.T) {
 		t.Errorf("content = %q, want %q", string(data), "Fresh\n")
 	}
 }
+
+// TestGopherResumeIgnoredWhenWriterSet verifies that --resume is silently
+// disabled when the caller also supplies a Writer: the resume metadata
+// sidecar would otherwise lie about bytes written to Output, since the
+// real data ends up in the Writer.
+func TestGopherResumeIgnoredWhenWriterSet(t *testing.T) {
+	listener, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer listener.Close()
+	port := listener.Addr().(*net.TCPAddr).Port
+
+	go func() {
+		conn, _ := listener.Accept()
+		if conn == nil {
+			return
+		}
+		defer conn.Close()
+		r := bufio.NewReader(conn)
+		r.ReadString('\n')
+		conn.Write([]byte("0Line one\t/foo\thost\t70\r\n.\r\n"))
+	}()
+
+	proto := NewProtocol()
+	u, _ := url.Parse("gopher://127.0.0.1:" + strconv.Itoa(port) + "/foo")
+	outPath := t.TempDir() + "/ignored-output.txt"
+
+	var buf strings.Builder
+	res, dlErr := proto.Download(context.Background(), &core.DownloadRequest{
+		URL:    u,
+		Output: outPath,
+		Writer: &stringWriter{&buf},
+		Resume: true,
+	})
+	if dlErr != nil {
+		t.Fatalf("download: %v", dlErr)
+	}
+	if res.Resumed {
+		t.Error("expected Resumed=false when Writer is set (resume must be ignored)")
+	}
+	if !strings.Contains(buf.String(), "Line one") {
+		t.Errorf("writer should receive the body, got %q", buf.String())
+	}
+	// Output is supplied but a Writer takes precedence; the file must
+	// not be created or touched by resume machinery.
+	if _, statErr := os.Stat(outPath); statErr == nil {
+		t.Error("Output file should not be created when Writer is set")
+	}
+}