fix(server): support caching absence of version artifacts

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

@@ -28,8 +28,6 @@ import kotlinx.coroutines.launch
 
 private val logger = logger { }
 
-typealias CachedVersionArtifact = VersionArtifacts?
-
 private val prefetchScope = CoroutineScope(Dispatchers.IO)
 
 fun Routing.artifactRoutes(
@@ -120,7 +118,7 @@ private suspend fun ApplicationCall.toBindingArtifacts(
     if (refresh) {
         bindingsCache.invalidate(actionCoords)
     }
-    return bindingsCache.get(actionCoords)
+    return bindingsCache.get(actionCoords).toNullableVersionArtifacts()
 }
 
 private fun PrometheusMeterRegistry.incrementArtifactCounter(

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

@@ -0,0 +1,27 @@
+package io.github.typesafegithub.workflows.jitbindingserver
+
+import io.github.typesafegithub.workflows.mavenbinding.VersionArtifacts
+
+/**
+ * This wrapper exists because Caffeine/Aedile cache doesn't support caching
+ * null values.
+ */
+sealed interface CachedVersionArtifact {
+    object Absent : CachedVersionArtifact
+
+    class Present(
+        val artifacts: VersionArtifacts,
+    ) : CachedVersionArtifact
+}
+
+fun CachedVersionArtifact.toNullableVersionArtifacts(): VersionArtifacts? =
+    when (this) {
+        is CachedVersionArtifact.Absent -> null
+        is CachedVersionArtifact.Present -> artifacts
+    }
+
+fun VersionArtifacts?.toCachedVersionArtifact(): CachedVersionArtifact =
+    when (this) {
+        null -> CachedVersionArtifact.Absent
+        else -> CachedVersionArtifact.Present(this)
+    }

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

@@ -109,7 +109,7 @@ private fun buildBindingsCache(
         .newBuilder()
         .refreshAfterWrite(1.hours)
         .recordStats()
-        .asLoadingCache<ActionCoords, CachedVersionArtifact> { buildVersionArtifacts(it, httpClient) }
+        .asLoadingCache { buildVersionArtifacts(it, httpClient).toCachedVersionArtifact() }
 
 @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()) }
                 }
             }