gopls/internal/cmd: wait for startup log in TestMCPCommandHTTP

This test continues to be flaky. The most recent flake occurred
because after eight connection attempts with exponential backoff,
the server still wasn't ready to accept client connections. Instead
of using retries, we check the server's output logs to verify that
it has started.

Change-Id: I3cae36ea3c240049aff84d2a983a4565230679bb
Reviewed-on: https://go-review.googlesource.com/c/tools/+/687075
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Alan Donovan <[email protected]>

Author: madelinekalil

gopls/internal/cmd/mcp_test.go

@@ -5,6 +5,7 @@
 package cmd_test
 
 import (
+	"bufio"
 	"bytes"
 	"fmt"
 	"net"
@@ -159,7 +160,6 @@ func TestMCPCommandHTTP(t *testing.T) {
 -- go.mod --
 module example.com
 go 1.18
-
 -- a.go --
 package a
 
@@ -176,7 +176,14 @@ func MyFun() {}
 	goplsCmd.Env = append(os.Environ(), "ENTRYPOINT=goplsMain")
 	goplsCmd.Dir = tree
 	goplsCmd.Stdout = os.Stderr
-	goplsCmd.Stderr = os.Stderr
+
+	// Pipe stderr to a scanner, so that we can wait for the log message that
+	// tells us the server has started.
+	stderr, err := goplsCmd.StderrPipe()
+	if err != nil {
+		t.Fatal(err)
+	}
+	// forward stdout to test output
 	if err := goplsCmd.Start(); err != nil {
 		t.Fatalf("starting gopls: %v", err)
 	}
@@ -190,23 +197,30 @@ func MyFun() {}
 		goplsCmd.Wait()
 	}()
 
+	// Wait for the MCP server to start listening. The referenced log occurs
+	// after the connection is opened via net.Listen and the HTTP handlers are
+	// set up.
+	ready := make(chan bool)
+	go func() {
+		// Copy from the pipe to stderr, keeping an eye out for the "mcp http
+		// server listening" string.
+		scan := bufio.NewScanner(stderr)
+		for scan.Scan() {
+			line := scan.Text()
+			if strings.Contains(line, "mcp http server listening") {
+				ready <- true
+			}
+			fmt.Fprintln(os.Stderr, line)
+		}
+		if err := scan.Err(); err != nil {
+			t.Logf("reading from pipe: %v", err)
+		}
+	}()
+
+	<-ready
 	client := mcp.NewClient("client", "v0.0.1", nil)
 	ctx := t.Context()
-	// Wait for http server to start listening.
-	var (
-		mcpSession *mcp.ClientSession
-		err        error
-	)
-	maxRetries := 8
-	for i := range maxRetries {
-		mcpSession, err = client.Connect(ctx, mcp.NewSSEClientTransport("http://"+addr, nil))
-		if err == nil {
-			t.Log("succeeded")
-			break // success
-		}
-		t.Logf("failed to connect on attempt %d: %v, trying again", i, err)
-		time.Sleep(50 * time.Millisecond << i) // retry with exponential backoff
-	}
+	mcpSession, err := client.Connect(ctx, mcp.NewSSEClientTransport("http://"+addr, nil))
 	if err != nil {
 		t.Fatalf("connecting to server: %v", err)
 	}

gopls/internal/mcp/mcp.go

@@ -70,6 +70,7 @@ func Serve(ctx context.Context, address string, sessions Sessions, isDaemon bool
 		<-ctx.Done()
 		svr.Close() // ignore error
 	}()
+	log.Printf("mcp http server listening")
 	return svr.Serve(listener)
 }