windows: use/accept platform-specific signal for stopping agent (#26780)

On Windows, the `os.Process.Signal` method returns an error when sending
`os.Interrupt` (SIGINT) because it isn't implemented. This causes test servers
in the `testutil` packages to break on Windows. Use the platform specific
syscalls to generate the SIGINT instead.

The agent's signal handler also did not correctly handle the Ctrl-C because we
were masking os.Interrupt instead of SIGINT.

Fixes: https://github.com/hashicorp/nomad/issues/26775

Co-authored-by: Chris Roberts <croberts@hashicorp.com>

api/internal/testutil/server.go

@@ -240,7 +240,9 @@ func NewTestServer(t testing.TB, cb ServerConfigCallback) *TestServer {
 // Stop stops the test Nomad server, and removes the Nomad data
 // directory once we are done.
 func (s *TestServer) Stop() {
-	defer func() { _ = os.RemoveAll(s.Config.DataDir) }()
+	s.t.Cleanup(func() {
+		_ = os.RemoveAll(s.Config.DataDir)
+	})
 
 	// wait for the process to exit to be sure that the data dir can be
 	// deleted on all platforms.
@@ -251,7 +253,7 @@ func (s *TestServer) Stop() {
 	}()
 
 	// kill and wait gracefully
-	err := s.cmd.Process.Signal(os.Interrupt)
+	err := s.gracefulStop()
 	must.NoError(s.t, err)
 
 	select {

api/internal/testutil/server_default.go

@@ -0,0 +1,17 @@
+// Copyright (c) HashiCorp, Inc.
+// SPDX-License-Identifier: MPL-2.0
+
+//go:build !windows
+
+package testutil
+
+import (
+	"os"
+)
+
+// gracefulStop performs a platform-specific graceful stop. On non-Windows this
+// uses the Go API for SIGINT
+func (s *TestServer) gracefulStop() error {
+	err := s.cmd.Process.Signal(os.Interrupt)
+	return err
+}

api/internal/testutil/server_windows.go

@@ -0,0 +1,37 @@
+// Copyright (c) HashiCorp, Inc.
+// SPDX-License-Identifier: MPL-2.0
+
+//go:build windows
+
+package testutil
+
+import (
+	"fmt"
+	"syscall"
+)
+
+var (
+	kernel32           = syscall.NewLazyDLL("kernel32.dll")
+	procSetCtrlHandler = kernel32.NewProc("SetConsoleCtrlHandler")
+	procGenCtrlEvent   = kernel32.NewProc("GenerateConsoleCtrlEvent")
+)
+
+// gracefulStop performs a platform-specific graceful stop. On Windows the Go
+// API does not implement SIGINT even though it's supported on Windows via
+// CTRL_C_EVENT
+func (s *TestServer) gracefulStop() error {
+	// note: err is always non-nil from these proc Call methods because it's
+	// always populated from GetLastError and you need to check the result
+	// returned against the docs.
+	pid := s.cmd.Process.Pid
+	result, _, err := procSetCtrlHandler.Call(0, 1)
+	if result == 0 {
+		return fmt.Errorf("failed to modify handlers for ctrl-c on pid %d: %w", pid, err)
+	}
+
+	result, _, err = procGenCtrlEvent.Call(syscall.CTRL_C_EVENT, uintptr(pid))
+	if result == 0 {
+		return fmt.Errorf("failed to send ctrl-C event to pid %d: %w", pid, err)
+	}
+	return nil
+}