internal/pkg: implement caching for directories

This works on any directories and should be robust against any bad state the artifact curing process might have failed at.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/pkg/pkg.go

@@ -8,7 +8,10 @@ import (
 	"encoding/binary"
 	"errors"
 	"io"
+	"io/fs"
 	"os"
+	"path"
+	"path/filepath"
 	"slices"
 	"sync"
 
@@ -190,6 +193,100 @@ func (c *Cache) pathnameIdent(id *ID) *check.Absolute {
 	)
 }
 
+// Store looks up an identifier, and if it is not present, calls makeArtifact
+// with a private working directory and stores its result instead. An optional
+// checksum can be passed via the result buffer which is used to validate the
+// produced directory.
+func (c *Cache) Store(
+	id ID,
+	makeArtifact func(work *check.Absolute) error,
+	buf *Checksum,
+	validate bool,
+) (
+	pathname *check.Absolute,
+	store bool,
+	err error,
+) {
+	pathname = c.pathnameIdent(&id)
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	_, err = os.Lstat(pathname.String())
+	if err == nil || !errors.Is(err, os.ErrNotExist) {
+		return
+	}
+	store = true
+
+	var (
+		workPathname    *check.Absolute
+		workPathnameRaw string
+	)
+	if workPathnameRaw, err = os.MkdirTemp(
+		c.base.Append(dirWork).String(),
+		path.Base(pathname.String()+".*"),
+	); err != nil {
+		return
+	} else if workPathname, err = check.NewAbs(workPathnameRaw); err != nil {
+		return
+	}
+	defer func() {
+		if err != nil {
+			chmodErr := filepath.WalkDir(workPathname.String(), func(path string, d fs.DirEntry, err error) error {
+				if err != nil {
+					return err
+				}
+				if d.IsDir() {
+					return os.Chmod(path, 0700)
+				}
+				return nil
+			})
+			removeErr := os.RemoveAll(workPathname.String())
+			if chmodErr != nil || removeErr != nil {
+				err = errors.Join(err, chmodErr, removeErr)
+			} else if errors.Is(err, os.ErrExist) {
+				// two artifacts may be backed by the same file
+				err = nil
+			}
+		}
+	}()
+	if err = os.Chmod(workPathname.String(), 0700); err != nil {
+		return
+	}
+
+	if err = makeArtifact(workPathname); err != nil {
+		return
+	}
+	var checksum Checksum
+	if checksum, err = HashDir(workPathname); err != nil {
+		return
+	}
+	if validate {
+		if checksum != *buf {
+			err = &ChecksumMismatchError{checksum, *buf}
+			return
+		}
+	} else {
+		*buf = checksum
+	}
+
+	checksumPathname := c.pathname(&checksum)
+	if err = os.Rename(
+		workPathname.String(),
+		checksumPathname.String(),
+	); err != nil {
+		if !errors.Is(err, os.ErrExist) {
+			return
+		}
+	}
+	if linkErr := os.Symlink(
+		"../"+dirChecksum+"/"+path.Base(checksumPathname.String()),
+		pathname.String(),
+	); linkErr != nil {
+		err = linkErr
+	}
+	return
+}
+
 // storeFile stores the contents of a [File]. An optional checksum can be
 // passed via the result buffer which is used to validate the submitted data.
 //