1 files changed, 92 insertions, 0 deletions
diff --git a/internal/lsp/transport_concurrency_test.go b/internal/lsp/transport_concurrency_test.go
new file mode 100644
index 0000000..f2390c7
--- /dev/null
+++ b/internal/lsp/transport_concurrency_test.go
@@ -0,0 +1,92 @@
+package lsp
+
+import (
+	"bytes"
+	"encoding/json"
+	"io"
+	"log"
+	"runtime"
+	"strconv"
+	"strings"
+	"sync"
+	"sync/atomic"
+	"testing"
+)
+
+type raceBuffer struct {
+	buf     bytes.Buffer
+	bufMu   sync.Mutex
+	inWrite atomic.Int32
+	raced   atomic.Bool
+}
+
+func (b *raceBuffer) Write(p []byte) (int, error) {
+	if b.inWrite.Swap(1) != 0 {
+		b.raced.Store(true)
+	}
+	runtime.Gosched()
+	b.bufMu.Lock()
+	n, err := b.buf.Write(p)
+	b.bufMu.Unlock()
+	b.inWrite.Store(0)
+	return n, err
+}
+
+func (b *raceBuffer) Bytes() []byte {
+	b.bufMu.Lock()
+	defer b.bufMu.Unlock()
+	return append([]byte(nil), b.buf.Bytes()...)
+}
+
+func (b *raceBuffer) Raced() bool {
+	return b.raced.Load()
+}
+
+func TestServerReplySerializesWrites(t *testing.T) {
+	t.Parallel()
+
+	writer := &raceBuffer{}
+	srv := NewServer(bytes.NewReader([]byte{}), writer, log.New(io.Discard, "", 0), ServerOptions{})
+
+	const goroutines = 16
+	start := make(chan struct{})
+	var wg sync.WaitGroup
+	for i := 0; i < goroutines; i++ {
+		wg.Add(1)
+		go func(i int) {
+			defer wg.Done()
+			<-start
+			id := json.RawMessage(strconv.Itoa(i + 1))
+			srv.reply(id, map[string]int{"index": i}, nil)
+		}(i)
+	}
+	close(start)
+	wg.Wait()
+
+	if writer.Raced() {
+		t.Fatalf("detected overlapping writes to server output")
+	}
+
+	data := writer.Bytes()
+	for len(data) > 0 {
+		headerEnd := bytes.Index(data, []byte("\r\n\r\n"))
+		if headerEnd < 0 {
+			t.Fatalf("missing header delimiter in %q", string(data))
+		}
+		header := string(data[:headerEnd])
+		if !strings.HasPrefix(header, "Content-Length: ") {
+			t.Fatalf("unexpected header %q", header)
+		}
+		lengthStr := strings.TrimSpace(header[len("Content-Length: "):])
+		length, err := strconv.Atoi(lengthStr)
+		if err != nil {
+			t.Fatalf("invalid content length %q: %v", lengthStr, err)
+		}
+		payloadStart := headerEnd + 4
+		payloadEnd := payloadStart + length
+		if payloadEnd > len(data) {
+			t.Fatalf("payload truncated: need %d bytes, have %d", length, len(data)-payloadStart)
+		}
+		data = data[payloadEnd:]
+	}
+}