test: add test that we close stdin on SSH session close (#18711)

closes #18519

Adds a unit test that verifies that we close the stdin to a non-TTY process when the SSH session connected to it exits.

c.f. https://github.com/coder/coder/issues/18519#issuecomment-3027609871

Validates that we match OpenSSH behavior.

agent/agentssh/agentssh.go

@@ -609,7 +609,9 @@ func (s *Server) startNonPTYSession(logger slog.Logger, session ssh.Session, mag
 	// and SSH server close may be delayed.
 	cmd.SysProcAttr = cmdSysProcAttr()
 
-	// to match OpenSSH, we don't actually tear a non-TTY command down, even if the session ends.
+	// to match OpenSSH, we don't actually tear a non-TTY command down, even if the session ends. OpenSSH closes the
+	// pipes to the process when the session ends; which is what happens here since we wire the command up to the
+	// session for I/O.
 	// c.f. https://github.com/coder/coder/issues/18519#issuecomment-3019118271
 	cmd.Cancel = nil
 

agent/agentssh/agentssh_test.go

@@ -8,7 +8,9 @@ import (
 	"context"
 	"fmt"
 	"net"
+	"os"
 	"os/user"
+	"path/filepath"
 	"runtime"
 	"strings"
 	"sync"
@@ -403,6 +405,81 @@ func TestNewServer_Signal(t *testing.T) {
 	})
 }
 
+func TestSSHServer_ClosesStdin(t *testing.T) {
+	t.Parallel()
+	if runtime.GOOS == "windows" {
+		t.Skip("bash doesn't exist on Windows")
+	}
+
+	ctx := testutil.Context(t, testutil.WaitMedium)
+	logger := testutil.Logger(t)
+	s, err := agentssh.NewServer(ctx, logger, prometheus.NewRegistry(), afero.NewMemMapFs(), agentexec.DefaultExecer, nil)
+	require.NoError(t, err)
+	defer s.Close()
+	err = s.UpdateHostSigner(42)
+	assert.NoError(t, err)
+
+	ln, err := net.Listen("tcp", "127.0.0.1:0")
+	require.NoError(t, err)
+
+	done := make(chan struct{})
+	go func() {
+		defer close(done)
+		err := s.Serve(ln)
+		assert.Error(t, err) // Server is closed.
+	}()
+	defer func() {
+		err := s.Close()
+		require.NoError(t, err)
+		<-done
+	}()
+
+	c := sshClient(t, ln.Addr().String())
+
+	sess, err := c.NewSession()
+	require.NoError(t, err)
+	stdout, err := sess.StdoutPipe()
+	require.NoError(t, err)
+	stdin, err := sess.StdinPipe()
+	require.NoError(t, err)
+	defer stdin.Close()
+
+	dir := t.TempDir()
+	err = os.MkdirAll(dir, 0o755)
+	require.NoError(t, err)
+	filePath := filepath.Join(dir, "result.txt")
+
+	// the shell command `read` will block until data is written to stdin, or closed. It will return
+	// exit code 1 if it hits EOF, which is what we want to test.
+	cmdErrCh := make(chan error, 1)
+	go func() {
+		cmdErrCh <- sess.Start(fmt.Sprintf("echo started; read; echo \"read exit code: $?\" > %s", filePath))
+	}()
+
+	cmdErr := testutil.RequireReceive(ctx, t, cmdErrCh)
+	require.NoError(t, cmdErr)
+
+	readCh := make(chan error, 1)
+	go func() {
+		buf := make([]byte, 8)
+		_, err := stdout.Read(buf)
+		assert.Equal(t, "started\n", string(buf))
+		readCh <- err
+	}()
+	err = testutil.RequireReceive(ctx, t, readCh)
+	require.NoError(t, err)
+
+	sess.Close()
+
+	var content []byte
+	testutil.Eventually(ctx, t, func(_ context.Context) bool {
+		content, err = os.ReadFile(filePath)
+		return err == nil
+	}, testutil.IntervalFast)
+	require.NoError(t, err)
+	require.Equal(t, "read exit code: 1\n", string(content))
+}
+
 func sshClient(t *testing.T, addr string) *ssh.Client {
 	conn, err := net.Dial("tcp", addr)
 	require.NoError(t, err)