techdocs: fix buildLogTransport-related memory leak

Signed-off-by: zcmander <zcmander@gmail.com>

.changeset/late-forks-fetch.md

@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-techdocs-backend': patch
+---
+
+Fix potential memory leak by not creating a build log transport if not given via `RouterOptions`.

plugins/techdocs-backend/src/service/DocsSynchronizer.ts

@@ -48,7 +48,7 @@ export type DocsSynchronizerSyncOpts = {
 export class DocsSynchronizer {
   private readonly publisher: PublisherBase;
   private readonly logger: winston.Logger;
-  private readonly buildLogTransport: winston.transport;
+  private readonly buildLogTransport?: winston.transport;
   private readonly config: Config;
   private readonly scmIntegrations: ScmIntegrationRegistry;
   private readonly cache: TechDocsCache | undefined;
@@ -64,7 +64,7 @@ export class DocsSynchronizer {
   }: {
     publisher: PublisherBase;
     logger: winston.Logger;
-    buildLogTransport: winston.transport;
+    buildLogTransport?: winston.transport;
     config: Config;
     scmIntegrations: ScmIntegrationRegistry;
     cache: TechDocsCache | undefined;
@@ -109,7 +109,9 @@ export class DocsSynchronizer {
     });
 
     taskLogger.add(new winston.transports.Stream({ stream: logStream }));
-    taskLogger.add(this.buildLogTransport);
+    if (this.buildLogTransport) {
+      taskLogger.add(this.buildLogTransport);
+    }
 
     // check if the last update check was too recent
     if (!shouldCheckForUpdate(entity.metadata.uid!)) {

plugins/techdocs-backend/src/service/router.ts

@@ -39,7 +39,6 @@ import {
   DocsBuildStrategy,
 } from './DocsBuildStrategy';
 import * as winston from 'winston';
-import { PassThrough } from 'stream';
 
 /**
  * Required dependencies for running TechDocs in the "out-of-the-box"
@@ -113,9 +112,7 @@ export async function createRouter(
     options.catalogClient ?? new CatalogClient({ discoveryApi: discovery });
   const docsBuildStrategy =
     options.docsBuildStrategy ?? DefaultDocsBuildStrategy.fromConfig(config);
-  const buildLogTransport =
-    options.buildLogTransport ??
-    new winston.transports.Stream({ stream: new PassThrough() });
+  const buildLogTransport = options.buildLogTransport;
 
   // Entities are cached to optimize the /static/docs request path, which can be called many times
   // when loading a single techdocs page.