feat(mcp): embed Python engine and extract to user cache
U2 of the Claude Desktop .mcpb bundle plan. - internal/engine/embed.go embeds the vendored Python tree at build time via //go:embed all:vendored. The engine package owns the embed because Go's directive cannot reach outside its own package directory; sync and gitignore paths are updated to match (internal/engine/vendored/ in place of mcp/vendored/). - internal/engine/extract.go materializes the embed into <cache>/last30days-pp-mcp/<version>/ with a .version sentinel that short-circuits re-extraction. Atomic rename from a .tmp sibling means a partial extraction can never be mistaken for complete. Concurrent first-call extractions serialize behind a per-cache-dir sync.Once. - EnsureUserCache honors a LAST30DAYS_CACHE_DIR env override for locked-down filesystems; the override is named in extract errors. - internal/engine/extract_test.go covers happy path, sentinel skip, version bump, 10-goroutine race, empty-version rejection, unwritable cache parent, and the env override (7 tests, all passing). - A tracked vendored/.gitkeep anchors the embed path so the directive matches even before scripts/sync-engine.sh runs.
This commit is contained in:
Matt Van Horn
@@ -0,0 +1,168 @@
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package engine
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import (
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"errors"
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"fmt"
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"io"
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"io/fs"
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"os"
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"path/filepath"
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"sync"
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)
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// SentinelFilename names the file Ensure writes inside the cache directory
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// after a successful extraction. Its contents are compared to the requested
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// version; a match short-circuits re-extraction on subsequent calls.
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const SentinelFilename = ".version"
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// cacheSubdir namespaces our cache under the OS user cache directory so
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// multiple printing-press-style bundles can coexist.
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const cacheSubdir = "last30days-pp-mcp"
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// CacheEnvOverride lets users redirect the cache directory when the default
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// OS cache location is read-only (locked-down corp images, ephemeral CI
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// containers). Pointed at by extract errors via the documented escape hatch.
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const CacheEnvOverride = "LAST30DAYS_CACHE_DIR"
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// Ensure extracts src into baseDir/last30days-pp-mcp/<version> and returns
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// the cache path. If the sentinel file already records the same version the
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// directory is reused without rewriting. version must be non-empty so the
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// cache layout always namespaces by version.
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//
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// Extraction writes to a sibling .tmp directory and renames it on success
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// so a partial extraction can never be mistaken for a complete one. Concurrent
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// callers within the same process serialize behind a per-cache-dir sync.Once
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// so the rename happens exactly once.
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func Ensure(src fs.FS, baseDir, version string) (string, error) {
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if version == "" {
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return "", errors.New("engine: version is required")
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}
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cacheDir := filepath.Join(baseDir, cacheSubdir, version)
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once := getOnce(cacheDir)
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var extractErr error
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once.Do(func() {
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extractErr = ensureLocked(src, cacheDir, version)
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})
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if extractErr != nil {
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// Reset the sync.Once so a follow-up call can retry rather than
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// permanently caching the error. Retry is the right default when
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// the failure is transient (e.g., disk full, parent dir restored).
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resetOnce(cacheDir)
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return "", extractErr
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}
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return cacheDir, nil
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}
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// EnsureUserCache wraps Ensure with the OS user cache dir (or the
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// LAST30DAYS_CACHE_DIR override) as base. Production callers use this; tests
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// use Ensure with an explicit temp dir.
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func EnsureUserCache(src fs.FS, version string) (string, error) {
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if override := os.Getenv(CacheEnvOverride); override != "" {
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return Ensure(src, override, version)
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}
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base, err := os.UserCacheDir()
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if err != nil {
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return "", fmt.Errorf("engine: resolve user cache dir (set %s to override): %w", CacheEnvOverride, err)
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}
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return Ensure(src, base, version)
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}
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func ensureLocked(src fs.FS, cacheDir, version string) error {
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if sentinelMatches(cacheDir, version) {
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return nil
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}
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tmpDir := cacheDir + ".tmp"
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if err := os.RemoveAll(tmpDir); err != nil {
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return fmt.Errorf("engine: clean tmp cache: %w", err)
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}
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if err := os.MkdirAll(tmpDir, 0o755); err != nil {
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return fmt.Errorf("engine: create tmp cache (%s, set %s to override): %w", tmpDir, CacheEnvOverride, err)
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}
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if err := extractAll(src, tmpDir); err != nil {
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_ = os.RemoveAll(tmpDir)
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return err
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}
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sentinel := filepath.Join(tmpDir, SentinelFilename)
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if err := os.WriteFile(sentinel, []byte(version), 0o644); err != nil {
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_ = os.RemoveAll(tmpDir)
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return fmt.Errorf("engine: write sentinel: %w", err)
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}
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if err := os.RemoveAll(cacheDir); err != nil {
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_ = os.RemoveAll(tmpDir)
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return fmt.Errorf("engine: clean old cache: %w", err)
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}
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if err := os.Rename(tmpDir, cacheDir); err != nil {
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_ = os.RemoveAll(tmpDir)
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return fmt.Errorf("engine: promote tmp cache: %w", err)
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}
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return nil
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}
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func sentinelMatches(cacheDir, version string) bool {
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data, err := os.ReadFile(filepath.Join(cacheDir, SentinelFilename))
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if err != nil {
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return false
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}
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return string(data) == version
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}
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func extractAll(src fs.FS, dst string) error {
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return fs.WalkDir(src, ".", func(path string, d fs.DirEntry, err error) error {
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if err != nil {
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return err
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}
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if path == "." {
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return nil
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}
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target := filepath.Join(dst, path)
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if d.IsDir() {
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return os.MkdirAll(target, 0o755)
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}
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return copyEmbeddedFile(src, path, target)
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})
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}
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func copyEmbeddedFile(src fs.FS, srcPath, dst string) error {
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in, err := src.Open(srcPath)
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if err != nil {
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return fmt.Errorf("engine: open %s: %w", srcPath, err)
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}
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defer func() { _ = in.Close() }()
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if err := os.MkdirAll(filepath.Dir(dst), 0o755); err != nil {
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return fmt.Errorf("engine: ensure parent of %s: %w", dst, err)
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}
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out, err := os.OpenFile(dst, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0o644)
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if err != nil {
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return fmt.Errorf("engine: create %s: %w", dst, err)
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}
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defer func() { _ = out.Close() }()
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if _, err := io.Copy(out, in); err != nil {
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return fmt.Errorf("engine: write %s: %w", dst, err)
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}
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return nil
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}
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// onceRegistry serializes first-call extraction per cache directory so the
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// rename in ensureLocked happens exactly once across goroutines.
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var (
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onceMu sync.Mutex
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onceRegistry = map[string]*sync.Once{}
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)
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func getOnce(cacheDir string) *sync.Once {
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onceMu.Lock()
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defer onceMu.Unlock()
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if o, ok := onceRegistry[cacheDir]; ok {
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return o
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}
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o := &sync.Once{}
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onceRegistry[cacheDir] = o
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return o
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}
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func resetOnce(cacheDir string) {
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onceMu.Lock()
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defer onceMu.Unlock()
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delete(onceRegistry, cacheDir)
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}
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