feat: runtime manager

lobby/internal/domain/engineimage/resolver.go

@@ -0,0 +1,66 @@
+// Package engineimage resolves the Docker reference Lobby publishes on
+// `runtime:start_jobs`. The reference is built from a configurable
+// template that must contain the literal `{engine_version}` placeholder
+// and a per-game `target_engine_version`.
+//
+// The resolver intentionally performs only template substitution and a
+// non-empty-version guard. Semver validation of the engine version
+// itself lives in `lobby/internal/domain/game` and runs at game-record
+// construction time; by the time `startgame.Service.Handle` reads the
+// record the version is already validated.
+package engineimage
+
+import (
+	"errors"
+	"fmt"
+	"strings"
+)
+
+// VersionPlaceholder is the literal token a template must contain. The
+// resolver substitutes it with the per-game engine version verbatim.
+const VersionPlaceholder = "{engine_version}"
+
+// Resolver substitutes a per-game engine version into a pre-validated
+// template. The template is validated once at construction so per-game
+// `Resolve` calls remain pure string substitution.
+type Resolver struct {
+	template string
+}
+
+// NewResolver returns a Resolver that uses template for every Resolve
+// call. It returns an error if template is empty or does not contain
+// VersionPlaceholder.
+func NewResolver(template string) (*Resolver, error) {
+	trimmed := strings.TrimSpace(template)
+	if trimmed == "" {
+		return nil, errors.New("engine image resolver: template must not be empty")
+	}
+	if !strings.Contains(trimmed, VersionPlaceholder) {
+		return nil, fmt.Errorf(
+			"engine image resolver: template %q must contain placeholder %q",
+			template, VersionPlaceholder,
+		)
+	}
+	return &Resolver{template: trimmed}, nil
+}
+
+// Template returns the validated template string the resolver was
+// constructed with. The accessor is intended for diagnostics and tests.
+func (resolver *Resolver) Template() string {
+	if resolver == nil {
+		return ""
+	}
+	return resolver.template
+}
+
+// Resolve substitutes VersionPlaceholder in the validated template with
+// version. It returns an error when version is empty or whitespace.
+func (resolver *Resolver) Resolve(version string) (string, error) {
+	if resolver == nil {
+		return "", errors.New("engine image resolver: nil resolver")
+	}
+	if strings.TrimSpace(version) == "" {
+		return "", errors.New("engine image resolver: engine version must not be empty")
+	}
+	return strings.ReplaceAll(resolver.template, VersionPlaceholder, version), nil
+}