feat: check service dependencies on startup

Allows services to optionally define the capabilities they provide to the
rest of libp2p and also the capabilities they require from other services.

This allows, for example, the `WebRTC` transport to require the `CircuitRelay`
transport to be present, or `KAD-DHT` (or anything that uses a topology)
to require the identify protocol.

Fixes #2263
Refs #2135

packages/interface/src/index.ts

@@ -715,6 +715,24 @@ export interface RoutingOptions extends AbortOptions, ProgressOptions {
   useCache?: boolean
 }
 
+/**
+ * This symbol is used by libp2p services to define the capabilities they can
+ * provide to other libp2p services.
+ *
+ * The service should define a property with this symbol as the key and the
+ * value should be a string array of provided capabilities.
+ */
+export const serviceCapabilities = Symbol.for('@libp2p/service-capabilities')
+
+/**
+ * This symbol is used by libp2p services to define the capabilities they
+ * require from other libp2p services.
+ *
+ * The service should define a property with this symbol as the key and the
+ * value should be a string array of required capabilities.
+ */
+export const serviceDependencies = Symbol.for('@libp2p/service-dependencies')
+
 export * from './connection/index.js'
 export * from './connection-encrypter/index.js'
 export * from './connection-gater/index.js'

packages/libp2p/src/components.ts

@@ -1,4 +1,4 @@
-import { CodeError } from '@libp2p/interface'
+import { CodeError, serviceCapabilities, serviceDependencies } from '@libp2p/interface'
 import { isStartable, type Startable, type Libp2pEvents, type ComponentLogger, type NodeInfo, type ConnectionProtector, type ConnectionGater, type ContentRouting, type TypedEventTarget, type Metrics, type PeerId, type PeerRouting, type PeerStore, type PrivateKey, type Upgrader } from '@libp2p/interface'
 import { defaultLogger } from '@libp2p/logger'
 import type { AddressManager, ConnectionManager, RandomWalk, Registrar, TransportManager } from '@libp2p/interface-internal'
@@ -157,3 +157,41 @@ export function defaultComponents (init: ComponentsInit = {}): Components {
   // @ts-expect-error component keys are proxied
   return proxy
 }
+
+export function checkServiceDependencies (components: Components): void {
+  const serviceCapabilities: Record<string, ConstrainBoolean> = {}
+
+  for (const service of Object.values(components.components)) {
+    for (const capability of getServiceCapabilities(service)) {
+      serviceCapabilities[capability] = true
+    }
+  }
+
+  for (const service of Object.values(components.components)) {
+    for (const capability of getServiceDependencies(service)) {
+      if (serviceCapabilities[capability] !== true) {
+        throw new CodeError(`Service "${getServiceName(service)}" required capability "${capability}" but it was not provided by any component, you may need to add additional configuration when creating your node.`, 'ERR_UNMET_SERVICE_DEPENDENCIES')
+      }
+    }
+  }
+}
+
+function getServiceCapabilities (service: any): string[] {
+  if (Array.isArray(service?.[serviceCapabilities])) {
+    return service[serviceCapabilities]
+  }
+
+  return []
+}
+
+function getServiceDependencies (service: any): string[] {
+  if (Array.isArray(service?.[serviceDependencies])) {
+    return service[serviceDependencies]
+  }
+
+  return []
+}
+
+function getServiceName (service: any): string {
+  return service?.[Symbol.toStringTag] ?? service?.toString() ?? 'unknown'
+}

packages/libp2p/src/libp2p.ts

@@ -10,7 +10,7 @@ import { MemoryDatastore } from 'datastore-core/memory'
 import { concat as uint8ArrayConcat } from 'uint8arrays/concat'
 import { fromString as uint8ArrayFromString } from 'uint8arrays/from-string'
 import { DefaultAddressManager } from './address-manager/index.js'
-import { defaultComponents } from './components.js'
+import { checkServiceDependencies, defaultComponents } from './components.js'
 import { connectionGater } from './config/connection-gater.js'
 import { validateConfig } from './config.js'
 import { DefaultConnectionManager } from './connection-manager/index.js'
@@ -187,6 +187,9 @@ export class Libp2pNode<T extends ServiceMap = Record<string, unknown>> extends
         }
       }
     }
+
+    // Ensure all services have their required dependencies
+    checkServiceDependencies(components)
   }
 
   private configureComponent <T> (name: string, component: T): T {

packages/libp2p/test/core/service-dependencies.spec.ts

@@ -0,0 +1,69 @@
+import { serviceCapabilities, serviceDependencies, stop } from '@libp2p/interface'
+import { expect } from 'aegir/chai'
+import { createLibp2p } from '../../src/index.js'
+import type { Libp2p } from '@libp2p/interface'
+
+/**
+ * A service with no dependencies
+ */
+function serviceA () {
+  return () => {
+    return {
+      [serviceCapabilities]: [
+        '@libp2p/service-a'
+      ]
+    }
+  }
+}
+
+/**
+ * A service with a dependency on service A
+ */
+function serviceB () {
+  return () => {
+    return {
+      [Symbol.toStringTag]: 'service-b',
+      [serviceDependencies]: [
+        '@libp2p/service-a'
+      ]
+    }
+  }
+}
+
+describe('service dependencies', () => {
+  let node: Libp2p
+
+  afterEach(async () => {
+    await stop(node)
+  })
+
+  it('should start when services have no dependencies', async () => {
+    node = await createLibp2p({
+      services: {
+        a: serviceA()
+      }
+    })
+
+    expect(node).to.be.ok()
+  })
+
+  it('should error when service dependencies are unmet', async () => {
+    await expect(createLibp2p({
+      services: {
+        b: serviceB()
+      }
+    })).to.eventually.be.rejected
+      .with.property('code', 'ERR_UNMET_SERVICE_DEPENDENCIES')
+  })
+
+  it('should not error when service dependencies are met', async () => {
+    node = await createLibp2p({
+      services: {
+        a: serviceA(),
+        b: serviceB()
+      }
+    })
+
+    expect(node).to.be.ok()
+  })
+})