cmd/gitannex: Add support for different layouts

This commit adds support for the same repo layouts supported by
git-annex-remote-rclone. This should enable git-annex users with remotes
of type "rclone" to switch to a "rclone-builtin" without needing to
retransfer content.

Issue #7625

cmd/gitannex/layout.go

@@ -0,0 +1,72 @@
+package gitannex
+
+import (
+	"fmt"
+	"strings"
+)
+
+type layoutMode string
+
+// All layout modes from git-annex-remote-rclone are supported.
+const (
+	layoutModeLower       layoutMode = "lower"
+	layoutModeDirectory   layoutMode = "directory"
+	layoutModeNodir       layoutMode = "nodir"
+	layoutModeMixed       layoutMode = "mixed"
+	layoutModeFrankencase layoutMode = "frankencase"
+	layoutModeUnknown     layoutMode = ""
+)
+
+func allLayoutModes() []layoutMode {
+	return []layoutMode{
+		layoutModeLower,
+		layoutModeDirectory,
+		layoutModeNodir,
+		layoutModeMixed,
+		layoutModeFrankencase,
+	}
+}
+
+func parseLayoutMode(mode string) layoutMode {
+	for _, knownMode := range allLayoutModes() {
+		if mode == string(knownMode) {
+			return knownMode
+		}
+	}
+	return layoutModeUnknown
+}
+
+type queryDirhashFunc func(msg string) (string, error)
+
+func buildFsString(queryDirhash queryDirhashFunc, mode layoutMode, key, remoteName, prefix string) (string, error) {
+	if mode == layoutModeNodir {
+		return fmt.Sprintf("%s:%s", remoteName, prefix), nil
+	}
+
+	var dirhash string
+	var err error
+	switch mode {
+	case layoutModeLower, layoutModeDirectory:
+		dirhash, err = queryDirhash("DIRHASH-LOWER " + key)
+	case layoutModeMixed, layoutModeFrankencase:
+		dirhash, err = queryDirhash("DIRHASH " + key)
+	default:
+		panic("unreachable")
+	}
+	if err != nil {
+		return "", fmt.Errorf("buildFsString failed to query dirhash: %w", err)
+	}
+
+	switch mode {
+	case layoutModeLower:
+		return fmt.Sprintf("%s:%s/%s", remoteName, prefix, dirhash), nil
+	case layoutModeDirectory:
+		return fmt.Sprintf("%s:%s/%s%s", remoteName, prefix, dirhash, key), nil
+	case layoutModeMixed:
+		return fmt.Sprintf("%s:%s/%s", remoteName, prefix, dirhash), nil
+	case layoutModeFrankencase:
+		return fmt.Sprintf("%s:%s/%s", remoteName, prefix, strings.ToLower(dirhash)), nil
+	default:
+		panic("unreachable")
+	}
+}