feat: 接入 Instance 版本与扩展可用性观测

docs/database/README.md

@@ -46,7 +46,8 @@ Secret metadata 和无关字段变化不重建连接。观测后再次读取 Sec
 不把旧连接的成功作为新凭据有效的证据；这不构成跨 Kubernetes/PostgreSQL 的原子事务。
 Instance UID、endpoint 或凭据引用变化也会释放旧连接；Forget/Close 只释放本地资源。
 
-当前只有 `ObserveVersion` 版本查询用例，不能产生完整 CapabilityObservation 或 Ready。
+当前通过 `ObserveMetadata` 读取服务器版本和可用扩展，`ObserveVersion` 只是其版本读取便捷入口，
+不能产生完整 CapabilityObservation 或 Ready。
 controller 接入、Secret watch、finalizer、registry 观测装配与真实权限检查仍待后续切片；并发 CR 更新
 必须由调用者通过 resourceVersion 校验。应用层沿用源实现的串行处理，本阶段未引入新的调度框架。
 
@@ -58,6 +59,31 @@ namespace 边界、有效值轮换、metadata 无关变化、中途轮换、重
 integration 标签代码。领域单测、真实 API 行为与真实 PostgreSQL 行为分别验收，不以本切片
 替代整个 Instance controller 的集成验收。
 
+## 服务器 metadata 与扩展观测
+
+SQL adapter 通过一条只读语句读取 `pg_catalog.current_setting('server_version')` 和
+`pg_catalog.pg_available_extensions`，避免从已安装列表推断可用列表，且不依赖可修改的
+`search_path`。查询失败丢弃整份结果；成功返回空列表与尚未观察严格区分。
+参考 PostgreSQL 的 [pg_available_extensions](https://www.postgresql.org/docs/18/view-pg-available-extensions.html)
+与 [CREATE EXTENSION](https://www.postgresql.org/docs/18/sql-createextension.html) 合同：可用列表
+表示服务器提供的扩展，不证明管理账号有安装权限，也不保证依赖和其他安装前提满足。
+
+`InstanceService` 保留原有 CredentialReader → Connector → Database 边界，复用同一个
+凭据读取、连接刷新和串行释放流程，不新增连接池封装或任意查询回调。每次重新查询 metadata，
+并在 Secret 有效值回读一致后生成不可变的 `MetadataObservation`，绑定本次 target（含当前
+generation），不绑定建池时的旧 target。结果不包含凭据，扩展集合不与 driver 的可变 slice 共享。
+凭据中途变化、读取失败或查询失败时，返回零值观察并释放连接，不复用旧的扩展列表。
+
+调用方可将 `Target()` 与 `Extensions()` 交给 Instance 的 `ObserveExtensions`；应用调用链
+仍负责同轮次使用，不能持久化或跨轮缓存这份证据。metadata 读取不安装扩展、不初始化 registry、
+不设置 Ready，也不授予 Tenant 写权限。管理权限矩阵、registry 完整回读及 controller 的
+checkpoint/status/finalizer 链路仍是后续切片。
+
+真实 API server + PostgreSQL 测试验证未安装扩展可被观察、名称保持大小写、search_path 遮蔽
+不改变查询来源、低权限账号读取、权限撤回失败与恢复、Secret 中途变化丢弃扩展结果。
+单元测试补充成功空列表、查询附带部分数据时丢弃、结果与可变 slice 隔离、每轮重新读取和
+generation 变化时的目标绑定；原凭据/TLS/超时/并发测试沿同一 metadata 路径继续运行。
+
 ## Registry 所有权存储切片
 
 `adapter/postgresql/registry` 直接迁入上述固定基线的 `internal/postgresql/registry`，

docs/database/development.md

@@ -3,7 +3,7 @@
 > 本页迁入作为 Database 模块的测试分层与 fixture 合同。旧项目的 Make target、devcontainer
 > 和脚手架版本尚未适配 Ayatori；实现时应复用 Ayatori 现有工具链，并保持这里定义的测试边界。
 
-## Ayatori 已接入的凭据与 registry 切片测试
+## Ayatori 已接入的凭据、metadata 与 registry 切片测试
 
 本节命令已在 Ayatori 接入；以下历史 Compose/Kind 操作仍属于迁入的目标合同。
 
@@ -28,6 +28,11 @@ runner 后端修复上线，不能从本地测试通过推断远端已经可用
 fixture 启动失败会保留退出错误与 stderr，并遮蔽测试密码，
 以区分缺少命令、daemon 不可达、权限和镜像拉取失败。
 
+metadata 测试验证版本与可用扩展的只读查询，包括未安装扩展、大小写保持、search_path 遮蔽、
+低权限账号读取、catalog 访问被撤回后的失败与恢复，以及凭据中途变化时同时丢弃版本和扩展。
+权限撤回只修改每个场景自建 PostgreSQL 容器的 ACL；不连接现有服务。
+可用列表不等于安装权限，这些检查不替代后续的完整管理权限矩阵或 Instance Ready 验收。
+
 registry 测试独立使用一次性 PostgreSQL，不启动 Kubernetes API server。覆盖重复和并发迁移、
 所有权唯一约束、并发占用、Retain 墓碑与 Delete 幂等、连接重建、取消恢复、未知 schema 与
 超前版本拒绝。通过在真实 COMMIT 成功后注入客户端错误，验证结果不确定时的重试；这不替代

internal/database/adapter/postgresql/connector.go

@@ -47,14 +47,6 @@ func (d *database) Close() {
 	d.pool.Close()
 }
 
-func (d *database) Version(ctx context.Context) (string, error) {
-	var version string
-	if err := d.pool.QueryRow(ctx, "SHOW server_version").Scan(&version); err != nil {
-		return "", safeError(err, application.ErrObservation)
-	}
-	return version, nil
-}
-
 func (c Connector) Connect(ctx context.Context, endpoint instance.Endpoint, credentials application.Credentials) (application.Database, error) {
 	if err := endpoint.Validate(); err != nil {
 		return nil, err

internal/database/adapter/postgresql/credentials_integration_test.go

@@ -131,13 +131,17 @@ func TestObservationDiscardsResultWhenCredentialsChange(t *testing.T) {
 		}
 	}
 
-	version, err := fixture.service.ObserveVersion(fixture.ctx, fixture.target)
+	observation, err := fixture.service.ObserveMetadata(fixture.ctx, fixture.target)
 	if !errors.Is(err, application.ErrCredentialsChanged) {
 		t.Fatal("in-flight rotation was not detected")
 	}
-	if version != "" {
+	if observation.Version() != "" {
 		t.Fatal("observation returned data obtained with stale credentials")
 	}
+	requested := instance.NewExtensionSet([]string{fixtureExtension})
+	if observation.Extensions().Check(requested).Decision != instance.ExtensionSupportUnobserved {
+		t.Fatal("observation returned extension support obtained with stale credentials")
+	}
 	if fixture.backendIDs(t) != "" {
 		t.Fatal("stale connection was retained after rotation")
 	}

internal/database/adapter/postgresql/fixture_integration_test.go

@@ -42,6 +42,7 @@ import (
 const (
 	fixtureHost         = "fixture.invalid"
 	fixtureUser         = "postgres"
+	fixtureExtension    = "plpgsql"
 	dockerExec          = "exec"
 	fixtureImage        = "postgres@sha256:18cfe3ef5e6815560c98237d6216d1e5119702fb0f3894c8785dd58b8bbe5d73"
 	fixturePassword     = "AYATORI-TEST-ONLY-initial-password"

internal/database/adapter/postgresql/metadata.go

@@ -0,0 +1,48 @@
+/*
+Copyright 2026.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package postgresql
+
+import (
+	"context"
+
+	"git.ddupan.top/panxiao81/ayatori/internal/database/application"
+)
+
+// 使用 pg_catalog 限定名称，避免管理账号的 search_path 改变查询来源。
+// 一个语句读取版本和可用列表；ARRAY 子查询在无行时返回空数组，而非 NULL。
+// 不查询 pg_extension：已安装集合不能代表服务器提供的全部扩展。
+const inspectMetadataStatement = `
+SELECT
+    pg_catalog.current_setting('server_version'),
+    ARRAY(
+        SELECT name::text
+        FROM pg_catalog.pg_available_extensions
+        ORDER BY name
+    )`
+
+func (d *database) InspectMetadata(ctx context.Context) (application.DatabaseMetadata, error) {
+	var metadata application.DatabaseMetadata
+	err := d.pool.QueryRow(ctx, inspectMetadataStatement).Scan(
+		&metadata.Version,
+		&metadata.AvailableExtensions,
+	)
+	if err != nil {
+		// 不返回部分结果，也不把查询失败转换为“成功观察到空列表”。
+		return application.DatabaseMetadata{}, safeError(err, application.ErrObservation)
+	}
+	return metadata, nil
+}

internal/database/adapter/postgresql/metadata_integration_test.go

@@ -0,0 +1,115 @@
+//go:build integration
+
+/*
+Copyright 2026.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package postgresql_test
+
+import (
+	"errors"
+	"testing"
+
+	corev1 "k8s.io/api/core/v1"
+
+	"git.ddupan.top/panxiao81/ayatori/internal/database/application"
+	"git.ddupan.top/panxiao81/ayatori/internal/database/domain/instance"
+)
+
+func TestMetadataObservesAvailableExtensionsWithoutInstalling(t *testing.T) {
+	f := newCredentialFixture(t)
+	f.createSecret(t, controllerNamespace)
+	if installed := f.queryPostgres(t, "SELECT count(*) FROM pg_catalog.pg_extension WHERE extname = 'hstore'"); installed != "0" {
+		t.Fatal("fixture unexpectedly has hstore installed")
+	}
+	// 提供同名遮蔽对象，验证 adapter 不依赖管理账号可修改的 search_path。
+	f.queryPostgres(t, "CREATE VIEW public.pg_available_extensions AS SELECT 'fake_extension'::name AS name")
+	f.queryPostgres(t, "ALTER ROLE postgres SET search_path = public, pg_catalog")
+	observed, err := f.service.ObserveMetadata(f.ctx, f.target)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !observed.Target().Matches(f.target) || observed.Version() == "" {
+		t.Fatal("metadata was not bound to the current target")
+	}
+	requested := instance.NewExtensionSet([]string{"hstore", fixtureExtension})
+	if observed.Extensions().Check(requested).Decision != instance.ExtensionsAccepted {
+		t.Fatal("available but uninstalled extension was omitted")
+	}
+	unsupported := instance.NewExtensionSet([]string{"fake_extension", "HSTORE"})
+	check := observed.Extensions().Check(unsupported)
+	if check.Decision != instance.ExtensionsUnsupported || len(check.Unsupported) != 2 {
+		t.Fatal("metadata accepted shadowed or case-normalized extension names")
+	}
+	if installed := f.queryPostgres(t, "SELECT count(*) FROM pg_catalog.pg_extension WHERE extname = 'hstore'"); installed != "0" {
+		t.Fatal("metadata observation installed an extension")
+	}
+	if schemas := f.queryPostgres(t, "SELECT count(*) FROM pg_catalog.pg_namespace WHERE nspname = 'postgresql_tenant_operator'"); schemas != "0" {
+		t.Fatal("metadata observation initialized the registry")
+	}
+
+	aggregate, err := instance.Reconstitute(f.target, instance.Snapshot{}, false)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := aggregate.ObserveExtensions(observed.Target(), observed.Extensions()); err != nil {
+		t.Fatal(err)
+	}
+	if aggregate.CheckExtensions(requested).Decision != instance.ExtensionsAccepted {
+		t.Fatal("domain rejected observed extension availability")
+	}
+	if err := aggregate.RequireProvisioningReady(); err == nil {
+		t.Fatal("extension availability incorrectly authorized provisioning")
+	}
+}
+
+func TestMetadataPermissionFailureAndRecovery(t *testing.T) {
+	f := newCredentialFixture(t)
+	f.createSecret(t, controllerNamespace)
+	// 低权限账号也能读取可用列表；这不能证明具备 role/database/extension 管理权限。
+	f.queryPostgres(t, "CREATE ROLE metadata_reader LOGIN PASSWORD '"+fixturePassword+"'")
+	f.updateSecret(t, func(secret *corev1.Secret) {
+		secret.Data["login"] = []byte("metadata_reader")
+	})
+	if flags := f.queryPostgres(t, "SELECT rolsuper, rolcreaterole, rolcreatedb FROM pg_catalog.pg_roles WHERE rolname = 'metadata_reader'"); flags != "f|f|f" {
+		t.Fatal("metadata reader unexpectedly has management privileges")
+	}
+	requested := instance.NewExtensionSet([]string{fixtureExtension})
+	observed, err := f.service.ObserveMetadata(f.ctx, f.target)
+	if err != nil || observed.Extensions().Check(requested).Decision != instance.ExtensionsAccepted {
+		t.Fatalf("read-only account could not observe metadata: %v", err)
+	}
+
+	// 仅操作本测试独占容器的 catalog ACL；失败不能转换成“不支持任何扩展”。
+	f.queryPostgres(t, "REVOKE SELECT ON pg_catalog.pg_available_extensions FROM PUBLIC")
+	failed, err := f.service.ObserveMetadata(f.ctx, f.target)
+	if !errors.Is(err, application.ErrObservation) {
+		t.Fatalf("metadata permission failure was not reported safely: %v", err)
+	}
+	if failed.Version() != "" || failed.Target().Validate() == nil {
+		t.Fatal("permission failure returned partial metadata")
+	}
+	if failed.Extensions().Check(requested).Decision != instance.ExtensionSupportUnobserved {
+		t.Fatal("permission failure returned an observed empty set")
+	}
+	if f.backendIDs(t) != "" {
+		t.Fatal("failed metadata connection was retained")
+	}
+	f.queryPostgres(t, "GRANT SELECT ON pg_catalog.pg_available_extensions TO PUBLIC")
+	recovered, err := f.service.ObserveMetadata(f.ctx, f.target)
+	if err != nil || recovered.Extensions().Check(requested).Decision != instance.ExtensionsAccepted {
+		t.Fatalf("metadata observation did not recover: %v", err)
+	}
+}

internal/database/adapter/postgresql/tls_integration_test.go

@@ -107,7 +107,7 @@ func TestPostgreSQLTLSHostIdentity(t *testing.T) {
 	if err != nil {
 		t.Fatal("trusted DNS SAN connection failed", err)
 	}
-	if version, err := db.Version(ctx); err != nil || version == "" {
+	if metadata, err := db.InspectMetadata(ctx); err != nil || metadata.Version == "" {
 		db.Close()
 		t.Fatal("TLS metadata read failed", err)
 	}

internal/database/application/instance_service.go

@@ -33,9 +33,9 @@ var (
 )
 
 // Database 与 Connector 复用原项目 internal/instance/service.go 的能力边界。
-// 版本查询只是本切片的连通性观察，不能产生领域 Ready。
+// Metadata 只查询版本与可用扩展，不能产生领域 Ready。
 type Database interface {
-	Version(context.Context) (string, error)
+	InspectMetadata(context.Context) (DatabaseMetadata, error)
 	Close()
 }
 
@@ -74,19 +74,25 @@ func NewInstanceService(source CredentialReader, connector Connector) (*Instance
 func (s *InstanceService) String() string   { return "[redacted instance service]" }
 func (s *InstanceService) GoString() string { return s.String() }
 
-// ObserveVersion 返回当前目标和凭据下的版本；任何失败均返回空结果。
+// ObserveVersion 是完整 metadata 读取的便捷入口，不再维护另一条连接或查询路径。
-// 调用者仍需使用 CR resourceVersion 保存前提防止 spec 并发修改；本方法不建立跨系统事务。
 func (s *InstanceService) ObserveVersion(ctx context.Context, target instance.ObservationTarget) (string, error) {
+	observation, err := s.ObserveMetadata(ctx, target)
+	return observation.Version(), err
+}
+
+// ObserveMetadata 返回当前目标和凭据下的版本与扩展；任何失败均丢弃全部结果。
+// 调用者仍需使用 CR resourceVersion 保存前提防止 spec 并发修改；本方法不建立跨系统事务。
+func (s *InstanceService) ObserveMetadata(ctx context.Context, target instance.ObservationTarget) (MetadataObservation, error) {
 	if err := target.Validate(); err != nil {
-		return "", err
+		return MetadataObservation{}, err
 	}
 	s.mu.Lock()
 	defer s.mu.Unlock()
 	if s.closed {
-		return "", ErrClosed
+		return MetadataObservation{}, ErrClosed
 	}
 	if err := ctx.Err(); err != nil {
-		return "", err
+		return MetadataObservation{}, err
 	}
 
 	// 先读取有效凭据。读取失败时不得继续使用缓存中的旧连接。
@@ -94,11 +100,11 @@ func (s *InstanceService) ObserveVersion(ctx context.Context, target instance.Ob
 	credentials, err := s.source.Read(ctx, target.Definition().AdminCredential())
 	if err != nil {
 		s.release(name)
-		return "", credentialError(err)
+		return MetadataObservation{}, credentialError(err)
 	}
 	if credentials.username == "" || credentials.password == "" {
 		s.release(name)
-		return "", ErrCredentialsInvalid
+		return MetadataObservation{}, ErrCredentialsInvalid
 	}
 
 	// 连接身份与有效值均未变化时复用 pgxpool；generation 本身不要求换池。
@@ -111,7 +117,7 @@ func (s *InstanceService) ObserveVersion(ctx context.Context, target instance.Ob
 	if current == nil {
 		database, err := s.connector.Connect(ctx, target.Definition().Endpoint(), credentials)
 		if err != nil {
-			return "", err
+			return MetadataObservation{}, err
 		}
 		current = &entry{
 			target:      target,
@@ -121,23 +127,31 @@ func (s *InstanceService) ObserveVersion(ctx context.Context, target instance.Ob
 		s.entries[name] = current
 	}
 
-	version, err := current.database.Version(ctx)
+	metadata, err := current.database.InspectMetadata(ctx)
 	if err != nil {
 		s.release(name)
-		return "", err
+		return MetadataObservation{}, err
+	}
+	if metadata.Version == "" {
+		s.release(name)
+		return MetadataObservation{}, ErrObservation
 	}
 
 	// 回读后再检查凭据，避免把轮换前取得的结果交给新凭据的调用链。
 	latest, err := s.source.Read(ctx, target.Definition().AdminCredential())
 	if err != nil {
 		s.release(name)
-		return "", credentialError(err)
+		return MetadataObservation{}, credentialError(err)
 	}
 	if latest != credentials {
 		s.release(name)
-		return "", ErrCredentialsChanged
+		return MetadataObservation{}, ErrCredentialsChanged
 	}
-	return version, nil
+	return MetadataObservation{
+		target:     target,
+		version:    metadata.Version,
+		extensions: instance.ObserveExtensionSupport(metadata.AvailableExtensions),
+	}, nil
 }
 
 func credentialError(err error) error {

internal/database/application/instance_service_test.go

@@ -24,6 +24,8 @@ import (
 	"git.ddupan.top/panxiao81/ayatori/internal/database/domain/instance"
 )
 
+const serviceTestPassword = "test-only"
+
 // 延续源项目 Service 测试，用于穷举身份与装配失败；真实行为由 adapter 集成测试验证。
 type sourceStub struct {
 	credentials Credentials
@@ -35,11 +37,14 @@ func (s *sourceStub) Read(context.Context, instance.CredentialReference) (Creden
 }
 
 type databaseStub struct {
-	closes int
+	closes   int
-	err    error
+	err      error
+	metadata DatabaseMetadata
 }
 
-func (d *databaseStub) Version(context.Context) (string, error) { return "17", d.err }
+func (d *databaseStub) InspectMetadata(context.Context) (DatabaseMetadata, error) {
+	return d.metadata, d.err
+}
 func (d *databaseStub) Close() {
 	d.closes++
 }
@@ -53,7 +58,12 @@ func (c *connectorStub) Connect(context.Context, instance.Endpoint, Credentials)
 	if c.err != nil {
 		return nil, c.err
 	}
-	db := &databaseStub{}
+	db := &databaseStub{
+		metadata: DatabaseMetadata{
+			Version:             "17",
+			AvailableExtensions: []string{"plpgsql"},
+		},
+	}
 	c.databases = append(c.databases, db)
 	return db, nil
 }
@@ -98,7 +108,7 @@ func serviceTarget(t *testing.T, uid, host, secret string, generation int64) ins
 }
 
 func TestInstanceConnectionIdentity(t *testing.T) {
-	source := &sourceStub{credentials: Credentials{username: testUsername, password: "test-only"}}
+	source := &sourceStub{credentials: Credentials{username: testUsername, password: serviceTestPassword}}
 	connector := &connectorStub{}
 	service, err := NewInstanceService(source, connector)
 	if err != nil {
@@ -118,9 +128,13 @@ func TestInstanceConnectionIdentity(t *testing.T) {
 		{"same name with new UID", serviceTarget(t, "uid-2", "second", "replacement", 1), 4},
 	}
 	for _, testCase := range cases {
-		if _, err := service.ObserveVersion(ctx, testCase.target); err != nil {
+		observation, err := service.ObserveMetadata(ctx, testCase.target)
+		if err != nil {
 			t.Fatal(err)
 		}
+		if !observation.Target().Matches(testCase.target) {
+			t.Fatalf("%s: observation was bound to a previous target", testCase.name)
+		}
 		if len(connector.databases) != testCase.wantConnections {
 			t.Fatalf("%s: got %d connections, want %d", testCase.name, len(connector.databases), testCase.wantConnections)
 		}
@@ -137,11 +151,64 @@ func TestInstanceConnectionIdentity(t *testing.T) {
 	}
 }
 
+func TestMetadataObservationFreshnessAndFailure(t *testing.T) {
+	source := &sourceStub{credentials: Credentials{username: testUsername, password: serviceTestPassword}}
+	connector := &connectorStub{}
+	service, err := NewInstanceService(source, connector)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer service.Close()
+	ctx := context.Background()
+	target := serviceTarget(t, "metadata-uid", "first", "admin", 1)
+	observed, err := service.ObserveMetadata(ctx, target)
+	if err != nil || observed.Version() != "17" || !observed.Target().Matches(target) {
+		t.Fatalf("metadata observation: %v", err)
+	}
+	requested := instance.NewExtensionSet([]string{"plpgsql"})
+	if observed.Extensions().Check(requested).Decision != instance.ExtensionsAccepted {
+		t.Fatal("extension list was not observed")
+	}
+
+	// 连接可以复用，但每轮必须重新查询；旧观察还必须与 adapter 的可变 slice 脱离。
+	database := connector.databases[0]
+	database.metadata.AvailableExtensions[0] = "replacement"
+	if observed.Extensions().Check(requested).Decision != instance.ExtensionsAccepted {
+		t.Fatal("adapter mutation changed a completed observation")
+	}
+	refreshed, err := service.ObserveMetadata(ctx, target)
+	if err != nil || refreshed.Extensions().Check(requested).Decision != instance.ExtensionsUnsupported {
+		t.Fatalf("extension list was cached across observations: %v", err)
+	}
+	database.metadata.AvailableExtensions = nil
+	empty, err := service.ObserveMetadata(ctx, target)
+	if err != nil || empty.Extensions().Check(requested).Decision != instance.ExtensionsUnsupported {
+		t.Fatalf("successful empty list was treated as unobserved: %v", err)
+	}
+
+	// 即使 adapter 附带部分数据，错误仍使整个观察失效。
+	database.err = ErrObservation
+	failed, err := service.ObserveMetadata(ctx, target)
+	if !errors.Is(err, ErrObservation) || failed.Version() != "" || failed.Target().Validate() == nil {
+		t.Fatal("failed query returned a bound observation")
+	}
+	if failed.Extensions().Check(requested).Decision != instance.ExtensionSupportUnobserved {
+		t.Fatal("failed query was interpreted as an empty extension list")
+	}
+	if _, err := service.ObserveMetadata(ctx, target); err != nil {
+		t.Fatalf("retry after query failure: %v", err)
+	}
+	connector.databases[1].metadata.Version = ""
+	if _, err := service.ObserveMetadata(ctx, target); !errors.Is(err, ErrObservation) {
+		t.Fatal("missing server version was accepted as complete metadata")
+	}
+}
+
 func TestInstanceAssemblyFailureRecovery(t *testing.T) {
 	ctx := context.Background()
 	target := serviceTarget(t, "uid-1", "first", "admin", 1)
 	source := &sourceStub{
-		credentials: Credentials{username: testUsername, password: "test-only"},
+		credentials: Credentials{username: testUsername, password: serviceTestPassword},
 		err:         errors.New("unsafe source error"),
 	}
 	connector := &connectorStub{err: ErrConnection}

internal/database/application/metadata.go

@@ -0,0 +1,40 @@
+/*
+Copyright 2026.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package application
+
+import "git.ddupan.top/panxiao81/ayatori/internal/database/domain/instance"
+
+// DatabaseMetadata 是一次只读查询的事实，不包含管理权限或 registry 就绪结论。
+// AvailableExtensions 是服务器提供的可用列表，不是已安装列表或安装授权。
+type DatabaseMetadata struct {
+	Version             string
+	AvailableExtensions []string
+}
+
+// MetadataObservation 只在查询成功且有效凭据再次核对一致后产生。
+// target 绑定本次调用，而非连接最初创建时的 generation；零值表示没有观察。
+type MetadataObservation struct {
+	target     instance.ObservationTarget
+	version    string
+	extensions instance.ExtensionSupport
+}
+
+func (o MetadataObservation) Target() instance.ObservationTarget { return o.target }
+func (o MetadataObservation) Version() string                    { return o.version }
+func (o MetadataObservation) Extensions() instance.ExtensionSupport {
+	return o.extensions
+}