Extend Maven datasource caching

[rarkins]

Describe the proposed change(s).

The Maven datasource does not appear to have caching = true set. When run on a repo like https://github.com/renovate-reproductions/net.twisterrob.gradle caching does not seem to be working or efficient, e.g. repeated runs still take 42 seconds for lookup on my laptop, even though all registries are public ones.

Logs should 0 set cache methods performed. Ideally all results should be cached such that potentially 0 requests to registries are required when the cache is "hot". On the other hand I note a comment // cache non-null responses unless marked as private and I'm not sure why we're being so conservative about not caching null.

We need to:

• Review the Maven datasource to ensure isPrivate is set reliably, and
• Enable caching

[rarkins]

Related: #30420

[rarkins]

If we achieve 100% caching for this repo but it still seems that the lookup is subjectively slow, create a new issue for further investigation.

[rarkins]

I played around with this diff:

diff --git a/lib/modules/datasource/index.ts b/lib/modules/datasource/index.ts
index 006a0023c..4f677016c 100644
--- a/lib/modules/datasource/index.ts
+++ b/lib/modules/datasource/index.ts
@@ -70,7 +70,7 @@ async function getRegistryReleases(
       cacheKey,
     );
     // istanbul ignore if
-    if (cachedResult) {
+    if (cachedResult !== undefined) {
       logger.trace({ cacheKey }, 'Returning cached datasource response');
       return cachedResult;
     }
@@ -80,13 +80,13 @@ async function getRegistryReleases(
     res.registryUrl ??= registryUrl;
   }
   // cache non-null responses unless marked as private
-  if (datasource.caching && res) {
+  if (datasource.caching) {
     const cachePrivatePackages = GlobalConfig.get(
       'cachePrivatePackages',
       false,
     );
-    if (cachePrivatePackages || !res.isPrivate) {
       logger.trace({ cacheKey }, 'Caching datasource response');
+    if (cachePrivatePackages || !res?.isPrivate) {
       const cacheMinutes = 15;
       await packageCache.set(cacheNamespace, cacheKey, res, cacheMinutes);
     }
diff --git a/lib/modules/datasource/maven/index.ts b/lib/modules/datasource/maven/index.ts
index 3476ded21..4d2f18de7 100644
--- a/lib/modules/datasource/maven/index.ts
+++ b/lib/modules/datasource/maven/index.ts
@@ -68,6 +68,8 @@ export class MavenDatasource extends Datasource {
 
   override readonly defaultVersioning: string = mavenVersioning.id;
 
+  override readonly caching = true;
+
   override readonly registryStrategy: RegistryStrategy = 'merge';
 
   override readonly releaseTimestampSupport = true;

[zharinov]

For null values, we can't determine whether it's private or not, so I think it's better not to cache nulls

[rarkins]

You mean private but missing credentials? Why not cache that?

[zharinov]

I mean null results for private packages

[rarkins]

What can go wrong if we cache those?