internal/pkg: record cure faults

These are useful for troubleshooting. This change records them in a separate directory.

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

internal/pkg/pkg.go

@@ -4,6 +4,7 @@ package pkg
 import (
 	"bufio"
 	"bytes"
+	"cmp"
 	"context"
 	"crypto/sha512"
 	"encoding/base64"
@@ -25,6 +26,7 @@ import (
 	"sync/atomic"
 	"syscall"
 	"testing"
+	"time"
 	"unique"
 	"unsafe"
 
@@ -248,7 +250,14 @@ func (t *TContext) destroy(errP *error) {
 			*errP = errors.Join(*errP, err)
 		}
 		if *errP != nil {
-			*errP = errors.Join(*errP, os.Remove(t.statusPath.String()))
+			*errP = errors.Join(*errP, os.Rename(
+				t.statusPath.String(), t.cache.base.Append(
+					dirFault,
+					t.ids+"."+strconv.FormatUint(uint64(
+						time.Now().UnixNano(),
+					), 10),
+				).String(),
+			))
 		}
 		t.status = nil
 	}
@@ -527,6 +536,8 @@ const (
 	// identifier. For [FloodArtifact], the same file is also available under
 	// its substitute identifier.
 	dirStatus = "status"
+	// dirFault holds status files of faulted cures.
+	dirFault = "fault"
 
 	// dirWork holds working pathnames set up during [Cache.Cure].
 	dirWork = "work"
@@ -2148,6 +2159,52 @@ func (c *Cache) OpenStatus(a Artifact) (r io.ReadSeekCloser, err error) {
 	return
 }
 
+// Fault holds the pathname and termination time of an [Artifact] fault entry.
+type Fault struct {
+	*check.Absolute
+	t uint64
+}
+
+// Time returns the instant in time where the fault occurred.
+func (f Fault) Time() time.Time { return time.Unix(0, int64(f.t)) }
+
+// Open opens the underlying entry for reading.
+func (f Fault) Open() (io.ReadCloser, error) { return os.Open(f.Absolute.String()) }
+
+// Destroy removes the underlying fault entry.
+func (f Fault) Destroy() error { return os.Remove(f.Absolute.String()) }
+
+// ReadFaults returns fault entries for an [Artifact].
+func (c *Cache) ReadFaults(a Artifact) (faults []Fault, err error) {
+	prefix := Encode(c.Ident(a).Value()) + "."
+	var dents []os.DirEntry
+	if dents, err = os.ReadDir(c.base.Append(dirFault).String()); err != nil {
+		return
+	}
+
+	for _, dent := range dents {
+		name := dent.Name()
+		if !strings.HasPrefix(name, prefix) {
+			continue
+		}
+		var t uint64
+		t, err = strconv.ParseUint(name[len(prefix):], 10, 64)
+		if err != nil {
+			return
+		}
+
+		faults = append(faults, Fault{c.base.Append(
+			dirFault,
+			name,
+		), t})
+	}
+
+	slices.SortFunc(faults, func(a, b Fault) int {
+		return cmp.Compare(a.t, b.t)
+	})
+	return
+}
+
 // Abort cancels all pending cures and waits for them to clean up, but does not
 // close the cache.
 func (c *Cache) Abort() {
@@ -2251,6 +2308,7 @@ func open(
 		dirIdentifier,
 		dirChecksum,
 		dirStatus,
+		dirFault,
 		dirWork,
 	} {
 		if err := os.MkdirAll(