From d8839f52dc736b0260489f44fc01dc39a1a8e204 Mon Sep 17 00:00:00 2001
From: Christiaan <anotherhero@gmail.com>
Date: Tue, 18 Aug 2026 12:38:49 +0200
Subject: [PATCH] fix: synchronize stdin writes to prevent corrupted LSP frames

Concurrent calls to Client.Call, Client.Notify, and the server-request
response path could interleave their writes to the LSP subprocess's
stdin, since WriteMessage's header and body writes were not atomic
across goroutines. Against phpactor, the workspace watcher's
registration-triggered preopen scans fire concurrent
textDocument/didOpen notifications on startup, and the interleaved
writes corrupt the Content-Length-prefixed stream. Once desynced, a
length-prefixed stream can't be resynchronized, so phpactor's LSP
process crashes within about a second of connecting, on every session.

Adding a mutex around the three write sites serializes full frames so
concurrent notifications/requests can no longer interleave.
---
 internal/lsp/client.go    |  9 +++++----
 internal/lsp/transport.go | 15 ++++++++++++---
 2 files changed, 17 insertions(+), 7 deletions(-)

diff --git a/internal/lsp/client.go b/internal/lsp/client.go
index fc07059d..b66fb8e8 100644
--- a/internal/lsp/client.go
+++ b/internal/lsp/client.go
@@ -17,10 +17,11 @@ import (
 )
 
 type Client struct {
-	Cmd    *exec.Cmd
-	stdin  io.WriteCloser
-	stdout *bufio.Reader
-	stderr io.ReadCloser
+	Cmd     *exec.Cmd
+	stdin   io.WriteCloser
+	stdinMu sync.Mutex
+	stdout  *bufio.Reader
+	stderr  io.ReadCloser
 
 	// Request ID counter
 	nextID atomic.Int32
diff --git a/internal/lsp/transport.go b/internal/lsp/transport.go
index 3cc9107a..4f8204d3 100644
--- a/internal/lsp/transport.go
+++ b/internal/lsp/transport.go
@@ -150,7 +150,10 @@ func (c *Client) handleMessages() {
 			}
 
 			// Send response back to server
-			if err := WriteMessage(c.stdin, response); err != nil {
+			c.stdinMu.Lock()
+			err := WriteMessage(c.stdin, response)
+			c.stdinMu.Unlock()
+			if err != nil {
 				lspLogger.Error("Error sending response to server: %v", err)
 			}
 
@@ -217,7 +220,10 @@ func (c *Client) Call(ctx context.Context, method string, params any, result any
 	}()
 
 	// Send request
-	if err := WriteMessage(c.stdin, msg); err != nil {
+	c.stdinMu.Lock()
+	err = WriteMessage(c.stdin, msg)
+	c.stdinMu.Unlock()
+	if err != nil {
 		return fmt.Errorf("failed to send request: %w", err)
 	}
 
@@ -258,7 +264,10 @@ func (c *Client) Notify(ctx context.Context, method string, params any) error {
 		return fmt.Errorf("failed to create notification: %w", err)
 	}
 
-	if err := WriteMessage(c.stdin, msg); err != nil {
+	c.stdinMu.Lock()
+	err = WriteMessage(c.stdin, msg)
+	c.stdinMu.Unlock()
+	if err != nil {
 		return fmt.Errorf("failed to send notification: %w", err)
 	}