Pass the TestKit coverage agent on the command line, move the shared TestKit directory

.github/workflows/publish.yml

@@ -30,7 +30,7 @@ on:
 jobs:
   publish:
     name: Publish to Maven repositories
-    uses: SpineEventEngine/config/.github/workflows/publishing.yml@5ddff23286f7f6353d4714b0399ab89109c56c79
+    uses: SpineEventEngine/config/.github/workflows/publishing.yml@3cc29d4e51eb6de7da4234d2677973cead2cea84
     permissions:
       contents: read
       packages: write

buildSrc/src/main/kotlin/io/spine/dependency/local/Compiler.kt

@@ -60,7 +60,7 @@ object Compiler : Dependency() {
      * The version of the Compiler dependencies.
      */
     override val version: String
-    private const val fallbackVersion = "2.0.0-SNAPSHOT.081"
+    private const val fallbackVersion = "2.0.0-SNAPSHOT.083"
 
     /**
      * The distinct version of the Compiler used by other build tools.
@@ -69,7 +69,7 @@ object Compiler : Dependency() {
      * transitive dependencies, this is the version used to build the project itself.
      */
     val dogfoodingVersion: String
-    private const val fallbackDfVersion = "2.0.0-SNAPSHOT.081"
+    private const val fallbackDfVersion = "2.0.0-SNAPSHOT.083"
 
     /**
      * The artifact for the Compiler Gradle plugin.

buildSrc/src/main/kotlin/io/spine/dependency/local/CoreJvm.kt

@@ -30,7 +30,7 @@ typealias CoreJava = CoreJvm
 )
 object CoreJvm {
     const val group = Spine.group
-    const val version = "2.0.0-SNAPSHOT.577"
+    const val version = "2.0.0-SNAPSHOT.611"
 
     const val coreArtifact = "spine-core"
     const val clientArtifact = "spine-client"

buildSrc/src/main/kotlin/io/spine/dependency/local/CoreJvmCompiler.kt

@@ -34,12 +34,12 @@ object CoreJvmCompiler {
     /**
      * The version used in the build classpath.
      */
-    const val dogfoodingVersion = "2.0.0-SNAPSHOT.099"
+    const val dogfoodingVersion = "2.0.0-SNAPSHOT.104"
 
     /**
      * The version to be used for integration tests.
      */
-    const val version = "2.0.0-SNAPSHOT.099"
+    const val version = "2.0.0-SNAPSHOT.104"
 
     /**
      * The ID of the Gradle plugin.

buildSrc/src/main/kotlin/io/spine/dependency/local/ProtoTap.kt

@@ -26,7 +26,7 @@ package io.spine.dependency.local
 )
 object ProtoTap {
     const val group = Spine.toolsGroup
-    const val version = "0.17.4"
+    const val version = "0.17.5"
     const val gradlePluginId = "io.spine.prototap"
     const val api = "$group:prototap-api:$version"
     const val gradlePlugin = "$group:prototap-gradle-plugin:$version"

buildSrc/src/main/kotlin/io/spine/dependency/local/ToolBase.kt

@@ -32,8 +32,8 @@ package io.spine.dependency.local
 )
 object ToolBase {
     const val group = Spine.toolsGroup
-    const val version = "2.0.0-SNAPSHOT.428"
-    const val dogfoodingVersion = "2.0.0-SNAPSHOT.428"
+    const val version = "2.0.0-SNAPSHOT.451"
+    const val dogfoodingVersion = "2.0.0-SNAPSHOT.451"
 
     const val archive = "$group:archive:$version"
     const val code = "$group:code:$version"

buildSrc/src/main/kotlin/io/spine/dependency/local/Validation.kt

@@ -27,7 +27,7 @@ object Validation {
     /**
      * The version of the Validation library artifacts.
      */
-    const val version = "2.0.0-SNAPSHOT.469"
+    const val version = "2.0.0-SNAPSHOT.470"
 
     const val group = Spine.toolsGroup
     private const val prefix = "validation"

buildSrc/src/main/kotlin/io/spine/gradle/testing/TestKitCoverage.kt

@@ -35,8 +35,8 @@ import org.gradle.kotlin.dsl.withType
  *     dedicated [AGENT_CONFIGURATION] configuration.
  *  2. Passes the agent JAR path and a per-module exec directory
  *     (`build/`[TESTKIT_COVERAGE_DIR]) to the test JVM as system properties.
- *     The `plugin-testlib` harness reads these and writes a `gradle.properties`
- *     into the worker's Gradle user home that adds `-javaagent:…` to the worker JVM.
+ *     The `plugin-testlib` harness reads these and attaches `-javaagent:…` to the
+ *     worker JVM.
  *  3. Wipes the exec directory at most once per build invocation, from the
  *     `doFirst` of the first `Test` task that actually executes, so stale worker
  *     coverage from a previous run does not accumulate. Two failure modes are
@@ -58,6 +58,11 @@ import org.gradle.kotlin.dsl.withType
  *     coverage. An up-to-date (non-cache) run is unaffected: the previous run's
  *     files remain on disk and the guarded `doFirst` never deletes them.
  *
+ * The method does not choose the TestKit directory of the worker builds.
+ * The `module-testing` and `kmp-module` script plugins point the `Test` tasks
+ * of a module at a shared one (see [useSharedTestKitDir]), which a harness
+ * that passes its own directory to `withTestKitDir(...)` does not use.
+ *
  * The produced `.exec` files are merged into the Kover reports by
  * [io.spine.gradle.report.coverage.KoverConfig]. The agent emits binary
  * execution data rather than an XML report because that is its only file output,

buildSrc/src/main/kotlin/io/spine/gradle/testing/TestKitDir.kt

@@ -0,0 +1,84 @@
+/*
+ * Copyright 2026 CodeMatters, Lda.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file
+ * except in compliance with the License. You may obtain a copy of the License at
+ *
+ * https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software distributed under
+ * the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND,
+ * either express or implied. See the License for the specific language governing permissions
+ * and limitations under the License.
+ */
+
+package io.spine.gradle.testing
+
+import org.gradle.api.Project
+import org.gradle.api.tasks.testing.Test
+import org.gradle.kotlin.dsl.withType
+
+/**
+ * Configures the `Test` tasks of this project so that the Gradle TestKit builds
+ * they run use the TestKit directory [shared by all modules][SHARED_TESTKIT_DIR].
+ *
+ * A TestKit directory is the Gradle user home of the builds that
+ * [GradleRunner][org.gradle.testkit.runner.GradleRunner] runs. It holds mostly
+ * Gradle's instrumented copies of the JARs on the classpath of the plugin under
+ * test, and the compiled build scripts of the test projects. When the directory
+ * is empty, the TestKit builds recreate them, which is slow on a CI runner.
+ *
+ * Without the [`org.gradle.testkit.dir`][TESTKIT_DIR_PROPERTY] system property,
+ * TestKit keeps the directory at `build/tmp/<task>/work/.gradle-test-kit`, in
+ * the temporary directory of the `Test` task. So each `Test` task of each module
+ * has its own, which is empty in a fresh checkout and after `clean`. This
+ * function passes the shared directory to the test JVM as that property, which
+ * `GradleRunner.create()` reads. A runner given a directory with
+ * `withTestKitDir(...)` still uses that one.
+ *
+ * The property is set from a `doFirst` action rather than at configuration time,
+ * so it is not a task input. Where TestKit builds keep their caches does not
+ * affect the test results, while the absolute path differs between machines.
+ * As an input, it would only break the reuse of the `Test` task results from
+ * the build cache across machines. A `Test` task that already has the property
+ * keeps its value, so a repeated call has no further effect.
+ *
+ * The function applies to all the `Test` tasks of the project, whether they
+ * run TestKit builds or not, and whether the project collects the coverage of
+ * those builds or not (see [enableTestKitCoverage]).
+ */
+fun Project.useSharedTestKitDir() {
+    val testKitDir = gradle.gradleUserHomeDir.resolve(SHARED_TESTKIT_DIR)
+    tasks.withType<Test>().configureEach {
+        doFirst {
+            if (TESTKIT_DIR_PROPERTY !in systemProperties) {
+                systemProperty(TESTKIT_DIR_PROPERTY, testKitDir.absolutePath)
+            }
+        }
+    }
+}
+
+/**
+ * The name of the system property carrying the absolute path to the TestKit
+ * directory, which `GradleRunner.create()` reads.
+ *
+ * A runner given a directory with `withTestKitDir(...)` uses that one instead.
+ *
+ * @see org.gradle.testkit.runner.GradleRunner.withTestKitDir
+ */
+private const val TESTKIT_DIR_PROPERTY: String = "org.gradle.testkit.dir"
+
+/**
+ * The path, relative to the Gradle user home of the build, of the TestKit
+ * directory shared by the TestKit builds of all modules.
+ *
+ * The directory is placed under `caches/`, which `gradle/actions/setup-gradle`
+ * restores on every run but saves only on the default branch. So a pull request
+ * run starts with a warm directory once a run on the default branch has saved it.
+ * Locally, the TestKit builds of all the repositories that use this `buildSrc`
+ * share it.
+ *
+ * Unlike a directory under `build/`, it is not reset by `clean`. Delete it to make
+ * the TestKit builds start from an empty one.
+ */
+private const val SHARED_TESTKIT_DIR: String = "caches/spine-testkit"

buildSrc/src/main/kotlin/kmp-module.gradle.kts

@@ -27,6 +27,7 @@ import io.spine.gradle.kotlin.setFreeCompilerArgs
 import io.spine.gradle.publish.IncrementGuard
 import io.spine.gradle.report.license.LicenseReporter
 import io.spine.gradle.testing.configureLogging
+import io.spine.gradle.testing.useSharedTestKitDir
 
 /**
  * Configures this [Project] as a Kotlin Multiplatform module.
@@ -167,6 +168,12 @@ tasks {
     }
 }
 
+/**
+ * Gives all the `Test` tasks the TestKit directory shared by all modules,
+ * as `module-testing` does (see [useSharedTestKitDir]).
+ */
+project.useSharedTestKitDir()
+
 /**
  * Overrides the default location of Kotlin sources.
  *

buildSrc/src/main/kotlin/module-testing.gradle.kts

@@ -20,10 +20,14 @@ import io.spine.dependency.test.Kotest
 import io.spine.dependency.test.Truth
 import io.spine.gradle.testing.configureLogging
 import io.spine.gradle.testing.registerTestTasks
+import io.spine.gradle.testing.useSharedTestKitDir
 
 /**
  * This convention plugin applies test dependencies and configures test-related tasks.
  *
+ * All the `Test` tasks of the module get the TestKit directory shared by all
+ * modules (see [useSharedTestKitDir]).
+ *
  * The version of the [JUnit] platform must be applied via the [BomsPlugin][io.spine.dependency.boms.BomsPlugin]:
  *
  * ```kotlin
@@ -40,6 +44,7 @@ plugins {
 project.run {
     setupTests()
     forceTestDependencies()
+    useSharedTestKitDir()
 }
 
 dependencies {