feat: make pseudo-TTY allocation opt-in (#961)

Fixes #956.

Pseudo-TTY allocation is now an explicit, runner-wide opt-in via `runner.allocate_pty`, applied to both host and docker backends. Default is off, matching GitHub `actions/runner`.

```yaml
runner:
  allocate_pty: false  # default
```

**Before:** the host backend hardcoded `if true /* allocate Terminal */` and the docker backend used `term.IsTerminal(os.Stdout.Fd())`. As a result, `docker build` (and other TTY-aware tools) saw a TTY and emitted cursor-control redraw frames that flooded captured logs with thousands of duplicate-looking progress lines — only on host-mode runners in production, and on docker-mode runners when the daemon happened to be launched from a shell rather than a service.

**After:** both backends consult `Config.AllocatePTY`. The `term.IsTerminal` heuristic is gone, so behavior no longer depends on whether the daemon has a controlling terminal.

**Reproduction:** running `docker build` through `HostEnvironment.Exec` with output captured to a buffer:

| | Before (`if true`) | After (`AllocatePTY=false`) |
|---|---:|---:|
| bytes captured | 18,167 | 1,048 |
| ANSI CSI sequences | 556 | 0 |
| cursor-up `\e[1A` | 181 | 0 |

**Side fix:** `ptyWriter.AutoStop` is now `atomic.Bool`. The field is written from the exec goroutine after `cmd.Wait()` and read from the `copyPtyOutput` goroutine via `ptyWriter.Write`; existing tests never tripped the race detector because their commands produced no output before exit. The new host-mode test does.

---
This PR was written with the help of Claude Opus 4.7

---------

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Nicolas <bircni@icloud.com>
Reviewed-on: https://gitea.com/gitea/runner/pulls/961
Reviewed-by: Nicolas <bircni@icloud.com>
Co-authored-by: silverwind <2021+silverwind@noreply.gitea.com>
Co-committed-by: silverwind <2021+silverwind@noreply.gitea.com>

act/container/container_types.go

@@ -47,6 +47,7 @@ type NewContainerInput struct {
 	// Gitea specific
 	AutoRemove   bool
 	ValidVolumes []string
+	AllocatePTY  bool // allocate a pseudo-TTY for the container's exec processes
 }
 
 // FileEntry is a file to copy to a container

act/container/docker_run.go

@@ -41,7 +41,6 @@ import (
 	"github.com/moby/moby/client"
 	specs "github.com/opencontainers/image-spec/specs-go/v1"
 	"github.com/spf13/pflag"
-	"golang.org/x/term"
 )
 
 // NewContainer creates a reference to a container
@@ -450,7 +449,6 @@ func (cr *containerReference) create(capAdd, capDrop []string) common.Executor {
 			return nil
 		}
 		logger := common.Logger(ctx)
-		isTerminal := term.IsTerminal(int(os.Stdout.Fd()))
 		input := cr.input
 		exposedPorts, err := convertPortSet(input.ExposedPorts)
 		if err != nil {
@@ -466,7 +464,7 @@ func (cr *containerReference) create(capAdd, capDrop []string) common.Executor {
 			WorkingDir:   input.WorkingDir,
 			Env:          input.Env,
 			ExposedPorts: exposedPorts,
-			Tty:          isTerminal,
+			Tty:          input.AllocatePTY,
 		}
 		// For Gitea, reduce log noise
 		// logger.Debugf("Common container.Config ==> %+v", config)
@@ -604,7 +602,7 @@ func (cr *containerReference) exec(cmd []string, env map[string]string, user, wo
 		}
 
 		logger.Debugf("Exec command '%s'", cmd)
-		isTerminal := term.IsTerminal(int(os.Stdout.Fd()))
+		isTerminal := cr.input.AllocatePTY
 		envList := make([]string, 0)
 		for k, v := range env {
 			envList = append(envList, fmt.Sprintf("%s=%s", k, v))
@@ -899,7 +897,7 @@ func (cr *containerReference) attach() common.Executor {
 		if err != nil {
 			return fmt.Errorf("failed to attach to container: %w", err)
 		}
-		isTerminal := term.IsTerminal(int(os.Stdout.Fd()))
+		isTerminal := cr.input.AllocatePTY
 
 		var outWriter io.Writer
 		outWriter = cr.input.Stdout

act/container/host_environment.go

@@ -19,6 +19,7 @@ import (
 	"strconv"
 	"strings"
 	"sync"
+	"sync/atomic"
 	"time"
 
 	"gitea.com/gitea/runner/act/common"
@@ -42,6 +43,7 @@ type HostEnvironment struct {
 	ActPath     string
 	CleanUp     func()
 	StdOut      io.Writer
+	AllocatePTY bool // allocate a pseudo-TTY for each step's process
 
 	mu          sync.Mutex
 	runningPIDs map[int]struct{}
@@ -200,12 +202,12 @@ func (e *HostEnvironment) Start(_ bool) common.Executor {
 
 type ptyWriter struct {
 	Out       io.Writer
-	AutoStop  bool
+	AutoStop  atomic.Bool
 	dirtyLine bool
 }
 
 func (w *ptyWriter) Write(buf []byte) (int, error) {
-	if w.AutoStop && len(buf) > 0 && buf[len(buf)-1] == 4 {
+	if w.AutoStop.Load() && len(buf) > 0 && buf[len(buf)-1] == 4 {
 		n, err := w.Out.Write(buf[:len(buf)-1])
 		if err != nil {
 			return n, err
@@ -335,21 +337,20 @@ func (e *HostEnvironment) exec(ctx context.Context, command []string, cmdline st
 			tty.Close()
 		}
 	}()
-	if true /* allocate Terminal */ {
+	if e.AllocatePTY {
 		var err error
 		ppty, tty, err = setupPty(cmd, cmdline)
 		if err != nil {
 			common.Logger(ctx).Debugf("Failed to setup Pty %v\n", err.Error())
 		}
 	}
-	writer := &ptyWriter{Out: e.StdOut}
-	logctx, finishLog := context.WithCancel(context.Background())
+	var writer *ptyWriter
+	var logctx context.Context
 	if ppty != nil {
+		writer = &ptyWriter{Out: e.StdOut}
+		var finishLog context.CancelFunc
+		logctx, finishLog = context.WithCancel(context.Background())
 		go copyPtyOutput(writer, ppty, finishLog)
-	} else {
-		finishLog()
-	}
-	if ppty != nil {
 		go writeKeepAlive(ppty)
 	}
 	// Split Start/Wait so the PID can be registered before the process can exit;
@@ -379,14 +380,11 @@ func (e *HostEnvironment) exec(ctx context.Context, command []string, cmdline st
 		return err
 	}
 	if tty != nil {
-		writer.AutoStop = true
+		writer.AutoStop.Store(true)
 		if _, err := tty.WriteString("\x04"); err != nil {
 			common.Logger(ctx).Debug("Failed to write EOT")
 		}
-	}
-	<-logctx.Done()
-
-	if ppty != nil {
+		<-logctx.Done()
 		ppty.Close()
 		ppty = nil
 	}

act/container/host_environment_test.go

@@ -6,12 +6,14 @@ package container
 
 import (
 	"archive/tar"
+	"bytes"
 	"context"
 	"io"
 	"os"
 	"path"
 	"path/filepath"
 	"runtime"
+	"strings"
 	"testing"
 
 	"gitea.com/gitea/runner/act/common"
@@ -100,6 +102,45 @@ func TestHostEnvironmentExecExitCode(t *testing.T) {
 	assert.Equal(t, "Process completed with exit code 3.", err.Error())
 }
 
+func TestHostEnvironmentAllocatePTY(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skip("uses POSIX shell")
+	}
+	for _, tc := range []struct {
+		name     string
+		allocPTY bool
+		expect   string
+	}{
+		{name: "off", allocPTY: false, expect: "NOTTY"},
+		{name: "on", allocPTY: true, expect: "TTY"},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			dir := t.TempDir()
+			buf := &bytes.Buffer{}
+			e := &HostEnvironment{
+				Path:        filepath.Join(dir, "path"),
+				TmpDir:      filepath.Join(dir, "tmp"),
+				ToolCache:   filepath.Join(dir, "tool_cache"),
+				ActPath:     filepath.Join(dir, "act_path"),
+				StdOut:      buf,
+				Workdir:     filepath.Join(dir, "path"),
+				AllocatePTY: tc.allocPTY,
+			}
+			for _, p := range []string{e.Path, e.TmpDir, e.ToolCache, e.ActPath} {
+				require.NoError(t, os.MkdirAll(p, 0o700))
+			}
+
+			err := e.Exec(
+				[]string{"sh", "-c", "[ -t 1 ] && printf TTY || printf NOTTY"},
+				map[string]string{"PATH": os.Getenv("PATH")}, "", "",
+			)(context.Background())
+			require.NoError(t, err)
+			got := strings.TrimSpace(strings.ReplaceAll(buf.String(), "\r", ""))
+			assert.Equal(t, tc.expect, got)
+		})
+	}
+}
+
 func TestHostEnvironmentRemoveCleansWorkdir(t *testing.T) {
 	logger := logrus.New()
 	ctx := common.WithLogger(context.Background(), logrus.NewEntry(logger))