internal/pkg: move dependency flooding to cache

This imposes a hard upper limit to concurrency during dependency satisfaction and moves all dependency-related code out of individual implementations of Artifact. This change also includes ctx and msg as part of Cache.

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

internal/pkg/testdata/main.go

@@ -12,6 +12,7 @@ import (
 	"strings"
 	"syscall"
 
+	"hakurei.app/container/check"
 	"hakurei.app/container/fhs"
 	"hakurei.app/container/vfs"
 )
@@ -20,14 +21,24 @@ func main() {
 	log.SetFlags(0)
 	log.SetPrefix("testtool: ")
 
-	var hostNet bool
-	wantArgs := []string{"testtool"}
-	if len(os.Args) == 2 {
-		hostNet = true
-		log.SetPrefix("testtool(net): ")
-		wantArgs = []string{"testtool", "net"}
-	}
-	if !slices.Equal(os.Args, wantArgs) {
+	var hostNet, layers bool
+	if len(os.Args) == 2 && os.Args[0] == "testtool" {
+		switch os.Args[1] {
+		case "net":
+			hostNet = true
+			log.SetPrefix("testtool(net): ")
+			break
+
+		case "layers":
+			layers = true
+			log.SetPrefix("testtool(layers): ")
+			break
+
+		default:
+			log.Fatalf("Args: %q", os.Args)
+			return
+		}
+	} else if wantArgs := []string{"testtool"}; !slices.Equal(os.Args, wantArgs) {
 		log.Fatalf("Args: %q, want %q", os.Args, wantArgs)
 	}
 
@@ -35,7 +46,9 @@ func main() {
 	wantEnv := []string{"HAKUREI_TEST=1"}
 	if len(os.Environ()) == 2 {
 		overlayRoot = true
-		log.SetPrefix("testtool(overlay root): ")
+		if !layers {
+			log.SetPrefix("testtool(overlay root): ")
+		}
 		wantEnv = []string{"HAKUREI_TEST=1", "HAKUREI_ROOT=1"}
 	}
 	if !slices.Equal(wantEnv, os.Environ()) {
@@ -116,8 +129,37 @@ func main() {
 				lowerdir = o[len(lowerdirKey):]
 			}
 		}
-		if path.Base(lowerdir) != checksumEmptyDir {
-			log.Fatal("unexpected artifact checksum")
+		if !layers {
+			if path.Base(lowerdir) != checksumEmptyDir {
+				log.Fatal("unexpected artifact checksum")
+			}
+		} else {
+			ident = "o4z73RXEjkF5BBoXYg4WZ3c4VCjwvfJtPKFrstFHcEQxkVDoWTtdc5TMXigFvTsF"
+
+			lowerdirsEscaped := strings.Split(lowerdir, ":")
+			lowerdirs := lowerdirsEscaped[:0]
+			// ignore the option separator since it does not appear in ident
+			for i, e := range lowerdirsEscaped {
+				if len(e) > 0 &&
+					e[len(e)-1] == check.SpecialOverlayEscape[0] &&
+					(len(e) == 1 || e[len(e)-2] != check.SpecialOverlayEscape[0]) {
+					// ignore escaped pathname separator since it does not
+					// appear in ident
+
+					e = e[:len(e)-1]
+					if len(lowerdirsEscaped) != i {
+						lowerdirsEscaped[i+1] = e + lowerdirsEscaped[i+1]
+						continue
+					}
+				}
+				lowerdirs = append(lowerdirs, e)
+			}
+
+			if len(lowerdirs) != 2 ||
+				path.Base(lowerdirs[0]) != "MGWmEfjut2QE2xPJwTsmUzpff4BN_FEnQ7T0j7gvUCCiugJQNwqt9m151fm9D1yU" ||
+				path.Base(lowerdirs[1]) != "nY_CUdiaUM1OL4cPr5TS92FCJ3rCRV7Hm5oVTzAvMXwC03_QnTRfQ5PPs7mOU9fK" {
+				log.Fatalf("unexpected lowerdirs %s", strings.Join(lowerdirs, ", "))
+			}
 		}
 	} else {
 		if hostNet {