fix(parse_env_file): support env-file lines larger than 64 KiB (#974)

My builds kept flaking out with errors like `invalid format delimiter 'ghadelimiter_...' not found before end of file` or just strange failures in the complete job. After some digging I found an issue in `parseEnvFile` and have tested this fix against the test case presented.

  - `parseEnvFile` reads `$GITHUB_ENV` / `$GITHUB_OUTPUT` with a `bufio.Scanner` using the default 64 KiB token size, and never checks `s.Err()`.
  - Any action that writes a multi-line value with a single line >64 KiB silently aborts the scan with `bufio.ErrTooLong`, which surfaces as the misleading `"invalid format delimiter
  'ghadelimiter_…' not found before end of file"`.
  - Real-world trigger: `docker/build-push-action`'s `metadata` output embeds the full `GITHUB_EVENT_PATH` payload via buildx provenance; a long PR description (e.g. a Renovate dependency
  table) puts the body field on one JSON-escaped line well past 64 KiB.
  - Raise the scanner buffer to 1 MiB so realistic outputs parse.

### Reproduction
Test this in an action. This removes the `docker/build-push-action` aspect and reproduces it directly.
```yaml
  jobs:
    repro:
      runs-on: ubuntu-latest
      steps:
        - id: big
          run: |
            {
              echo 'value<<EOF'
              head -c 70000 /dev/urandom | base64 -w0
              echo
              echo 'EOF'
            } >> "$GITHUB_OUTPUT"
```

---------

Co-authored-by: Nicolas <bircni@icloud.com>
Co-authored-by: silverwind <me@silverwind.io>
Reviewed-on: https://gitea.com/gitea/runner/pulls/974
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Reviewed-by: silverwind <2021+silverwind@noreply.gitea.com>
Co-authored-by: Jacob Alberty <jacob.alberty@gmail.com>
Co-committed-by: Jacob Alberty <jacob.alberty@gmail.com>

act/container/parse_env_file.go

@@ -29,6 +29,8 @@ func parseEnvFile(e Container, srcPath string, env *map[string]string) common.Ex
 			return err
 		}
 		s := bufio.NewScanner(reader)
+		// Default 64 KiB max token size is too small for realistic env-file lines; allow up to 16 MiB.
+		s.Buffer(make([]byte, 0, 64*1024), 16*1024*1024)
 		for s.Scan() {
 			line := s.Text()
 			singleLineEnv := strings.Index(line, "=")
@@ -50,6 +52,9 @@ func parseEnvFile(e Container, srcPath string, env *map[string]string) common.Ex
 					}
 					multiLineEnvContent += content
 				}
+				if err := s.Err(); err != nil {
+					return fmt.Errorf("reading env file: %w", err)
+				}
 				if !delimiterFound {
 					return fmt.Errorf("invalid format delimiter '%v' not found before end of file", multiLineEnvDelimiter)
 				}
@@ -58,6 +63,9 @@ func parseEnvFile(e Container, srcPath string, env *map[string]string) common.Ex
 				return fmt.Errorf("invalid format '%v', expected a line with '=' or '<<'", line)
 			}
 		}
+		if err := s.Err(); err != nil {
+			return fmt.Errorf("reading env file: %w", err)
+		}
 		env = &localEnv
 		return nil
 	}

act/container/parse_env_file_test.go

@@ -0,0 +1,75 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package container
+
+import (
+	"bufio"
+	"context"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func newTestHostEnv(t *testing.T) (*HostEnvironment, string) {
+	t.Helper()
+	e := &HostEnvironment{Path: t.TempDir()}
+	return e, filepath.Join(e.Path, "envfile")
+}
+
+func TestParseEnvFileSingleLine(t *testing.T) {
+	e, envPath := newTestHostEnv(t)
+	require.NoError(t, os.WriteFile(envPath, []byte("FOO=bar\nBAZ=qux\n"), 0o600))
+
+	env := map[string]string{}
+	require.NoError(t, parseEnvFile(e, envPath, &env)(context.Background()))
+	assert.Equal(t, "bar", env["FOO"])
+	assert.Equal(t, "qux", env["BAZ"])
+}
+
+func TestParseEnvFileMultiLine(t *testing.T) {
+	e, envPath := newTestHostEnv(t)
+	content := "FOO<<EOF\nline1\nline2\nEOF\n"
+	require.NoError(t, os.WriteFile(envPath, []byte(content), 0o600))
+
+	env := map[string]string{}
+	require.NoError(t, parseEnvFile(e, envPath, &env)(context.Background()))
+	assert.Equal(t, "line1\nline2", env["FOO"])
+}
+
+func TestParseEnvFileLargeValueWithinLimit(t *testing.T) {
+	e, envPath := newTestHostEnv(t)
+	big := strings.Repeat("x", 2*1024*1024)
+	content := "FOO<<EOF\n" + big + "\nEOF\n"
+	require.NoError(t, os.WriteFile(envPath, []byte(content), 0o600))
+
+	env := map[string]string{}
+	require.NoError(t, parseEnvFile(e, envPath, &env)(context.Background()))
+	assert.Equal(t, big, env["FOO"])
+}
+
+func TestParseEnvFileLineExceedsBufferReportsScannerError(t *testing.T) {
+	e, envPath := newTestHostEnv(t)
+	tooBig := strings.Repeat("x", 17*1024*1024) // over the 16 MiB cap
+	content := "FOO<<EOF\n" + tooBig + "\nEOF\n"
+	require.NoError(t, os.WriteFile(envPath, []byte(content), 0o600))
+
+	env := map[string]string{}
+	err := parseEnvFile(e, envPath, &env)(context.Background())
+	require.ErrorIs(t, err, bufio.ErrTooLong)
+	assert.Contains(t, err.Error(), "reading env file")
+}
+
+func TestParseEnvFileMissingDelimiter(t *testing.T) {
+	e, envPath := newTestHostEnv(t)
+	require.NoError(t, os.WriteFile(envPath, []byte("FOO<<EOF\nline1\nline2\n"), 0o600))
+
+	env := map[string]string{}
+	err := parseEnvFile(e, envPath, &env)(context.Background())
+	require.Error(t, err)
+	assert.Contains(t, err.Error(), "delimiter")
+}