feat(oidc): add fine grained access control (#91)

* feat(access-control): fine grain tokens

* fix(docs): clean wording

* test(access-control): add tests for team extraction from access tokens and bucket info permissions

* test(vocabulary): add test for ExpandGlob function to reject non-glob patterns

* feat(helm): add multi-user access control documentation and schema support

backend/internal/authz/authorizer.go

@@ -0,0 +1,129 @@
+package authz
+
+import (
+	"sort"
+	"strings"
+)
+
+// Subject is who is asking: resolved once per request by the TeamResolver.
+type Subject struct {
+	ID           string
+	IsAdmin      bool
+	Bindings     []Binding
+	ClusterPerms PermSet
+}
+
+// Resource is what is being acted on. Empty Bucket means the action is
+// global or unscoped (e.g. the ListBuckets endpoint itself).
+type Resource struct {
+	Bucket string
+}
+
+// Decision is the outcome of an authorization check.
+type Decision struct {
+	Allow  bool
+	Reason string
+}
+
+// Authorizer decides whether a subject may perform an action on a resource.
+// It is an interface so enforcement can later move to an external PDP or
+// Garage-side scoped tokens without touching handlers.
+type Authorizer interface {
+	Decide(subj Subject, action string, res Resource) Decision
+}
+
+type policyAuthorizer struct{}
+
+// NewAuthorizer returns the built-in policy evaluator.
+func NewAuthorizer() Authorizer { return policyAuthorizer{} }
+
+func (policyAuthorizer) Decide(subj Subject, action string, res Resource) Decision {
+	return Decide(subj, action, res)
+}
+
+// Decide is the pure decision function. The synthetic admin subject flows
+// through the same logic as any team, with no IsAdmin shortcut.
+func Decide(subj Subject, action string, res Resource) Decision {
+	spec, ok := Vocabulary[action]
+	if !ok {
+		return Decision{Allow: false, Reason: "unknown_permission"}
+	}
+
+	if spec.Scope == ScopeGlobal {
+		if _, ok := subj.ClusterPerms[action]; ok {
+			return Decision{Allow: true, Reason: "cluster_permission"}
+		}
+		return Decision{Allow: false, Reason: "no_cluster_permission"}
+	}
+
+	// Prefix-scoped.
+	for _, b := range subj.Bindings {
+		if _, ok := b.Permissions[action]; !ok {
+			continue
+		}
+		// Unscoped call (list endpoint): any binding holding the permission
+		// suffices; per-bucket filtering happens on the response.
+		if res.Bucket == "" {
+			return Decision{Allow: true, Reason: "any_binding"}
+		}
+		if prefixesMatch(b.BucketPrefixes, res.Bucket) {
+			return Decision{Allow: true, Reason: "binding_match"}
+		}
+	}
+	return Decision{Allow: false, Reason: "no_matching_binding"}
+}
+
+func prefixesMatch(prefixes []string, bucket string) bool {
+	for _, p := range prefixes {
+		if p == "*" || strings.HasPrefix(bucket, p) {
+			return true
+		}
+	}
+	return false
+}
+
+// AdminSubject builds the synthetic admin team: one wildcard binding holding
+// every prefix-scoped permission (admin-only included) plus every global
+// permission. Same code path as any team.
+func AdminSubject(id string) Subject {
+	prefixPerms := PermSet{}
+	clusterPerms := PermSet{}
+	for name, spec := range Vocabulary {
+		if spec.Scope == ScopePrefix {
+			prefixPerms[name] = struct{}{}
+		} else {
+			clusterPerms[name] = struct{}{}
+		}
+	}
+	return Subject{
+		ID:           id,
+		IsAdmin:      true,
+		Bindings:     []Binding{{BucketPrefixes: []string{"*"}, Permissions: prefixPerms}},
+		ClusterPerms: clusterPerms,
+	}
+}
+
+// EffectivePermissions returns the sorted union of prefix-scoped permissions
+// the subject holds on the named bucket. This is the value served in API responses
+// so the frontend never does prefix matching. Returns nil when nothing
+// matches.
+func EffectivePermissions(subj Subject, bucket string) []string {
+	set := PermSet{}
+	for _, b := range subj.Bindings {
+		if !prefixesMatch(b.BucketPrefixes, bucket) {
+			continue
+		}
+		for perm := range b.Permissions {
+			set[perm] = struct{}{}
+		}
+	}
+	if len(set) == 0 {
+		return nil
+	}
+	out := make([]string, 0, len(set))
+	for perm := range set {
+		out = append(out, perm)
+	}
+	sort.Strings(out)
+	return out
+}