fix(server): support caching absence of version artifacts (#2183)

Part of
#2160.

The goal is to reduce the number of calls to GitHub and hopefully make
the server more stable when lots of requests come.

Author: krzema12

jit-binding-server/src/main/kotlin/io/github/typesafegithub/workflows/jitbindingserver/ArtifactRoutes.kt

@@ -25,10 +25,12 @@ import io.micrometer.prometheusmetrics.PrometheusMeterRegistry
 import kotlinx.coroutines.CoroutineScope
 import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.launch
+import java.util.Optional
+import kotlin.jvm.optionals.getOrNull
 
 private val logger = logger { }
 
-typealias CachedVersionArtifact = VersionArtifacts?
+typealias CachedVersionArtifact = Optional<VersionArtifacts>
 
 private val prefetchScope = CoroutineScope(Dispatchers.IO)
 
@@ -120,7 +122,7 @@ private suspend fun ApplicationCall.toBindingArtifacts(
     if (refresh) {
         bindingsCache.invalidate(actionCoords)
     }
-    return bindingsCache.get(actionCoords)
+    return bindingsCache.get(actionCoords).getOrNull()
 }
 
 private fun PrometheusMeterRegistry.incrementArtifactCounter(

jit-binding-server/src/main/kotlin/io/github/typesafegithub/workflows/jitbindingserver/Main.kt

@@ -26,6 +26,7 @@ import io.micrometer.core.instrument.Tag
 import io.micrometer.prometheusmetrics.PrometheusConfig
 import io.micrometer.prometheusmetrics.PrometheusMeterRegistry
 import java.time.Duration
+import java.util.Optional
 import kotlin.time.Duration.Companion.hours
 
 private val logger =
@@ -109,7 +110,7 @@ private fun buildBindingsCache(
         .newBuilder()
         .refreshAfterWrite(1.hours)
         .recordStats()
-        .asLoadingCache<ActionCoords, CachedVersionArtifact> { buildVersionArtifacts(it, httpClient) }
+        .asLoadingCache { Optional.ofNullable(buildVersionArtifacts(it, httpClient)) }
 
 @Suppress("ktlint:standard:function-signature") // Conflict with detekt.
 private fun buildMetadataCache(

jit-binding-server/src/test/kotlin/io/github/typesafegithub/workflows/jitbindingserver/ArtifactRoutesTest.kt

@@ -88,11 +88,9 @@ class ArtifactRoutesTest :
                     // Then
                     response2.status shouldBe HttpStatusCode.NotFound
 
-                    // This test shows the current behavior where requesting a resource
-                    // that doesn't exist twice in a row causes calling the version artifact
-                    // twice.
-                    // Fix in scope of https://github.com/typesafegithub/github-workflows-kt/issues/2160
-                    verify(exactly = 2) { mockBuildVersionArtifacts(any(), any()) }
+                    // The fact that the resource doesn't exist is cached, and the
+                    // resource generation logic isn't called in the second request.
+                    verify(exactly = 1) { mockBuildVersionArtifacts(any(), any()) }
                 }
             }