1 files changed, 145 insertions, 5 deletions
diff --git a/internal/pkg/pkg.go b/internal/pkg/pkg.go
index e94cf9c4..767c35a7 100644
--- a/internal/pkg/pkg.go
+++ b/internal/pkg/pkg.go
@@ -18,6 +18,7 @@ import (
 	"path/filepath"
 	"runtime"
 	"slices"
+	"strconv"
 	"strings"
 	"sync"
 	"sync/atomic"
@@ -70,6 +71,64 @@ func MustDecode(s string) (checksum Checksum) {
 	return
 }
 
+var (
+	// extension is a string uniquely identifying a set of custom [Artifact]
+	// implementations registered by calling [Register].
+	extension string
+
+	// openMu synchronises access to global state for initialisation.
+	openMu sync.Mutex
+	// opened is false if [Open] was never called.
+	opened bool
+)
+
+// Extension returns a string uniquely identifying the currently registered set
+// of custom [Artifact], or the zero value if none was registered.
+func Extension() string { return extension }
+
+// ValidExtension returns whether s is valid for use in a call to SetExtension.
+func ValidExtension(s string) bool {
+	if l := len(s); l == 0 || l > 128 {
+		return false
+	}
+	for _, v := range s {
+		if v < 'a' || v > 'z' {
+			return false
+		}
+	}
+	return true
+}
+
+// ErrInvalidExtension is returned for a variant identification string for which
+// [ValidExtension] returns false.
+var ErrInvalidExtension = errors.New("invalid extension variant identification string")
+
+// SetExtension sets the extension variant identification string. SetExtension
+// must be called before [Open] if custom [Artifact] implementations had been
+// recorded by calling [Register].
+//
+// The variant identification string must be between 1 and 128 bytes long and
+// consists of only bytes between 'a' and 'z'.
+//
+// SetExtension is not safe for concurrent use. SetExtension is called at most
+// once and must not be called after the first instance of Cache has been opened.
+func SetExtension(s string) {
+	openMu.Lock()
+	defer openMu.Unlock()
+
+	if opened {
+		panic("attempting to set extension after open")
+	}
+	if extension != "" {
+		panic("attempting to set extension twice")
+	}
+	if !ValidExtension(s) {
+		panic(ErrInvalidExtension)
+	}
+	extension = s
+	statusHeader = makeStatusHeader(s)
+}
+
 // common holds elements and receives methods shared between different contexts.
 type common struct {
 	// Context specific to this [Artifact]. The toplevel context in [Cache] must
@@ -102,19 +161,27 @@ type TContext struct {
 	common
 }
 
-// statusHeader is the header written to all status files in dirStatus.
-var statusHeader = func() string {
+// makeStatusHeader creates the header written to every status file. This should
+// not be called directly, its result is stored in statusHeader and will not
+// change after the first [Cache] is opened.
+func makeStatusHeader(extension string) string {
 	s := programName
 	if v := info.Version(); v != info.FallbackVersion {
 		s += " " + v
 	}
+	if extension != "" {
+		s += " with " + extension + " extensions"
+	}
 	s += " (" + runtime.GOARCH + ")"
 	if name, err := os.Hostname(); err == nil {
 		s += " on " + name
 	}
 	s += "\n\n"
 	return s
-}()
+}
+
+// statusHeader is the header written to all status files in dirStatus.
+var statusHeader = makeStatusHeader("")
 
 // prepareStatus initialises the status file once.
 func (t *TContext) prepareStatus() error {
@@ -427,6 +494,9 @@ const (
 	// KindFile is the kind of [Artifact] returned by [NewFile].
 	KindFile
 
+	// _kindEnd is the total number of kinds and does not denote a kind.
+	_kindEnd
+
 	// KindCustomOffset is the first [Kind] value reserved for implementations
 	// not from this package.
 	KindCustomOffset = 1 << 31
@@ -441,6 +511,9 @@ const (
 	// fileLock is the file name appended to Cache.base for guaranteeing
 	// exclusive access to the cache directory.
 	fileLock = "lock"
+	// fileVariant is the file name appended to Cache.base holding the variant
+	// identification string set by a prior call to [SetExtension].
+	fileVariant = "variant"
 
 	// dirIdentifier is the directory name appended to Cache.base for storing
 	// artifacts named after their [ID].
@@ -540,6 +613,10 @@ const (
 	// impurity due to [KindExecNet] being [KnownChecksum]. This flag exists
 	// to support kernels without Landlock LSM enabled.
 	CHostAbstract
+
+	// CPromoteVariant allows [pkg.Open] to promote an unextended on-disk cache
+	// to the current extension variant. This is a one-way operation.
+	CPromoteVariant
 )
 
 // toplevel holds [context.WithCancel] over caller-supplied context, where all
@@ -1930,6 +2007,20 @@ func (c *Cache) Close() {
 	c.unlock()
 }
 
+// UnsupportedVariantError describes an on-disk cache with an extension variant
+// identification string that differs from the value returned by [Extension].
+type UnsupportedVariantError string
+
+func (e UnsupportedVariantError) Error() string {
+	return "unsupported variant " + strconv.Quote(string(e))
+}
+
+var (
+	// ErrWouldPromote is returned by [Open] if the [CPromoteVariant] bit is not
+	// set and the on-disk cache requires variant promotion.
+	ErrWouldPromote = errors.New("operation would promote unextended cache")
+)
+
 // Open returns the address of a newly opened instance of [Cache].
 //
 // Concurrent cures of a [FloodArtifact] dependency graph is limited to the
@@ -1961,6 +2052,14 @@ func open(
 	base *check.Absolute,
 	lock bool,
 ) (*Cache, error) {
+	openMu.Lock()
+	defer openMu.Unlock()
+	opened = true
+
+	if extension == "" && len(irArtifact) != int(_kindEnd) {
+		panic("attempting to open cache with incomplete variant setup")
+	}
+
 	if cures < 1 {
 		cures = runtime.NumCPU()
 	}
@@ -1974,8 +2073,10 @@ func open(
 		dirStatus,
 		dirWork,
 	} {
-		if err := os.MkdirAll(base.Append(name).String(), 0700); err != nil &&
-			!errors.Is(err, os.ErrExist) {
+		if err := os.MkdirAll(
+			base.Append(name).String(),
+			0700,
+		); err != nil && !errors.Is(err, os.ErrExist) {
 			return nil, err
 		}
 	}
@@ -2013,6 +2114,45 @@ func open(
 		c.unlock = func() {}
 	}
 
+	variantPath := base.Append(fileVariant).String()
+	if p, err := os.ReadFile(variantPath); err != nil {
+		if !errors.Is(err, os.ErrNotExist) {
+			c.unlock()
+			return nil, err
+		}
+		// nonexistence implies newly created cache, or a cache predating
+		// variant identification strings, in which case it is silently promoted
+		if err = os.WriteFile(
+			variantPath,
+			[]byte(extension),
+			0400,
+		); err != nil {
+			c.unlock()
+			return nil, err
+		}
+	} else if s := string(p); s == "" {
+		if extension != "" {
+			if flags&CPromoteVariant == 0 {
+				c.unlock()
+				return nil, ErrWouldPromote
+			}
+			if err = os.WriteFile(
+				variantPath,
+				[]byte(extension),
+				0400,
+			); err != nil {
+				c.unlock()
+				return nil, err
+			}
+		}
+	} else if !ValidExtension(s) {
+		c.unlock()
+		return nil, ErrInvalidExtension
+	} else if s != extension {
+		c.unlock()
+		return nil, UnsupportedVariantError(s)
+	}
+
 	return &c, nil
 }