gopls/internal/debug: KillTraceViewers: kill process group on UNIX

The "go tool trace" child process begets a "cmd/trace" grandchild,
and os.Process.Kill only gets the child. Use kill(-pid) on UNIX
to terminate the whole process group. This isn't very portable,
but this is only needed for a test that only runs on UNIX.

Fixes golang/go#74668

Change-Id: Ic500ef45f7f7a78cd681ff995cefba6b1925cb8a
Reviewed-on: https://go-review.googlesource.com/c/tools/+/689476
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>

Author: adonovan

gopls/internal/debug/flight.go

@@ -17,20 +17,24 @@ import (
 	"runtime/trace"
 	"strings"
 	"sync"
+	"syscall"
 	"time"
 )
 
 var (
 	traceviewersMu sync.Mutex
 	traceviewers   []*os.Process
+
+	kill        = (*os.Process).Kill // windows, plan9; UNIX impl kills whole process group
+	sysProcAttr syscall.SysProcAttr  // UNIX configuration to create process group
 )
 
 // KillTraceViewers kills all "go tool trace" processes started by
 // /flightrecorder requests, for use in tests (see #74668).
 func KillTraceViewers() {
 	traceviewersMu.Lock()
 	for _, p := range traceviewers {
-		p.Kill() // ignore error
+		kill(p) // ignore error
 	}
 	traceviewers = nil
 	traceviewersMu.Unlock()
@@ -87,6 +91,7 @@ func startFlightRecorder() (http.HandlerFunc, error) {
 		// web server process. It will run until gopls terminates.
 		// (It would be nicer if we could just link it in; see #66843.)
 		cmd := exec.Command("go", "tool", "trace", tracefile)
+		cmd.SysProcAttr = &sysProcAttr
 
 		// Don't connect trace's std{out,err} to our os.Stderr directly,
 		// otherwise the child may outlive the parent in tests,

gopls/internal/debug/flight_unix.go

@@ -0,0 +1,23 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build go1.25 && unix
+
+package debug
+
+import (
+	"os"
+	"syscall"
+)
+
+func init() {
+	// UNIX: kill the whole process group, since
+	// "go tool trace" starts a cmd/trace child.
+	kill = killGroup
+	sysProcAttr.Setpgid = true
+}
+
+func killGroup(p *os.Process) error {
+	return syscall.Kill(-p.Pid, syscall.SIGKILL)
+}