internal/pkg: garbage collection

This destroys cache entries not referred to by user-specified artifacts and optionally their inputs.

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

internal/pkg/pkg.go

@@ -816,6 +816,28 @@ func (e *ChecksumMismatchError) Error() string {
 		" instead of " + Encode(e.Want)
 }
 
+// LinknamePrefixError describes a malformed linkname to a [Checksum].
+type LinknamePrefixError string
+
+func (e LinknamePrefixError) Error() string {
+	return "linkname " + strconv.Quote(string(e)) + " missing prefix"
+}
+
+// readlinkChecksum reads a symbolic link to a dirChecksum entry and saves the
+// decoded [Checksum] to the value pointed to by buf. The checksumLinknamePrefix
+// is required.
+func readlinkChecksum(a *check.Absolute, buf *Checksum) error {
+	linkname, err := os.Readlink(a.String())
+	if err != nil {
+		return nil
+	}
+
+	if !strings.HasPrefix(linkname, checksumLinknamePrefix) {
+		return LinknamePrefixError(linkname)
+	}
+	return Decode(buf, linkname[len(checksumLinknamePrefix):])
+}
+
 // ScrubError describes the outcome of a [Cache.Scrub] call where errors were
 // found and removed from the underlying storage of [Cache].
 type ScrubError struct {