Add generateBuildSubstitutions task to bom projects.

CONTRIBUTING.md

@@ -141,7 +141,7 @@ It does not support all required rules, so you still have to run `spotlessApply`
 
 * Unit tests target Java 8, so language features such as lambda and streams can be used in tests.
 
-## Common tasks
+## Specific tasks
 
 ### Updating OTLP proto dependency version
 
@@ -151,3 +151,44 @@ The OTLP proto dependency version is defined [here](proto/build.gradle). To bump
 2. Download the zip source code archive
 3. Run `shasum -a 256 ~/path/to/downloaded.zip` to compute its checksum
 4. Update `protoVersion` and `protoChecksum` in the build file with the new version and checksum
+
+### Composing builds
+
+Beware that this section is only meant for developers of opentelemetry-java, or closely related projects.
+The steps described here could change at any time and what you do for one version (commit) may break
+with the next one already.
+
+Gradle provides a feature called ["composite builds"](https://docs.gradle.org/current/userguide/composite_builds.html)
+that allows to replace some normally externally provided dependencies with a project that is built
+(included) in the same Gradle invocation. This can be useful to quickly test a new feature or bug fix you are
+developing in opentelmetry-java with the examples or the app or instrumentation library where you
+need the feature or run into the bug. Unfortunately, opentelemetry-java does not work out of the box
+with this feature because Gradle is unable to map the project names to the customized artifact
+coordinates (see e.g. [gradle/gradle#18291](https://github.com/gradle/gradle/issues/18291)
+and related issues. However, gradle supports manually declaring the mapping between ("substitution of")
+artifact coordinates and project names. To ease this tedious task, opentelemetry-java provides a
+gradle task `:generateBuildSubstitutions` that generates a code snippet with these substitutions in
+kts (Kotlin Script) format.
+
+Example usage could be as follows:
+
+1. Run `./gradlew generateBuildSubstitutions`
+2. Two files named `build/substitutions.gradle.kts` are generated in the bom and bom-alpha project's
+   directory, containing substitutions for the stable and alpha projects respectively.
+3. Copy & paste the content of these files to a new `settings.gradle.kts` or the one where you want
+   to include the opentelemetry build into, so that it contains something like the following:
+
+   ```kotlin
+   includeBuild("PATH/TO/OPENTELEMETRY-JAVA/ROOT/DIRECTORY") {
+     // Copy & paste following block from the generated substitutions.gradle.kts, *not* from here!
+     dependencySubstitution {
+       substitute(module("io.opentelemetry:opentelemetry-api")).using(project(":api:all"))
+       substitute(module("io.opentelemetry:opentelemetry-sdk")).using(project(":sdk:all"))
+       // ...
+     }
+   }
+   ```
+
+4. If you now build your project, it will use the included build to supply the opentelemetry-java artifacts,
+   ignoring any version declarations. Use the prefix `:DIRECTORY:` to refer to tasks/projects within
+   the included build, where DIRECTORY is the name of the directory in the included build (only the part after the last `/`).

README.md

@@ -171,6 +171,12 @@ Libraries will usually only need `opentelemetry-api`, while applications
 will want to use the `opentelemetry-sdk` module which contains our standard implementation
 of the APIs.
 
+## Gradle composite builds
+
+For opentelemetry-java developers that need to test the latest source code with another
+project, composite builds can be used as an alternative to `publishToMavenLocal`. This
+requires some setup which is explained [here](CONTRIBUTING.md#composing-builds).
+
 ## Releases
 
 See the [VERSIONING.md](VERSIONING.md) document for our policies for releases and compatibility

bom-alpha/build.gradle.kts

@@ -1,30 +1,9 @@
 plugins {
-  id("java-platform")
-  id("otel.publish-conventions")
+  id("otel.bom-conventions")
 }
 
 description = "OpenTelemetry Bill of Materials (Alpha)"
 group = "io.opentelemetry"
 base.archivesBaseName = "opentelemetry-bom-alpha"
 
-rootProject.subprojects.forEach { subproject ->
-  if (!project.name.startsWith("bom")) {
-    evaluationDependsOn(subproject.path)
-  }
-}
-
-afterEvaluate {
-  dependencies {
-    constraints {
-      rootProject.subprojects
-        .sortedBy { it.findProperty("archivesBaseName") as String? }
-        .filter { !it.name.startsWith("bom") }
-        .filter { it.findProperty("otel.release") == "alpha" }
-        .forEach { project ->
-          project.plugins.withId("maven-publish") {
-            api(project)
-          }
-        }
-    }
-  }
-}
+otelBom.projectFilter.set { it.findProperty("otel.release") == "alpha" }

bom/build.gradle.kts

@@ -1,30 +1,9 @@
 plugins {
-  id("java-platform")
-  id("otel.publish-conventions")
+  id("otel.bom-conventions")
 }
 
 description = "OpenTelemetry Bill of Materials"
 group = "io.opentelemetry"
 base.archivesBaseName = "opentelemetry-bom"
 
-rootProject.subprojects.forEach { subproject ->
-  if (project != subproject) {
-    evaluationDependsOn(subproject.path)
-  }
-}
-
-afterEvaluate {
-  dependencies {
-    constraints {
-      rootProject.subprojects
-        .sortedBy { it.findProperty("archivesBaseName") as String? }
-        .filter { !it.name.startsWith("bom") }
-        .filter { !it.hasProperty("otel.release") }
-        .forEach { project ->
-          project.plugins.withId("maven-publish") {
-            api(project)
-          }
-        }
-    }
-  }
-}
+otelBom.projectFilter.set { !it.hasProperty("otel.release") }

build.gradle.kts

@@ -59,6 +59,12 @@ nexusPublishing {
   }
 }
 
+// The BOM projects register dependent tasks that actually do the generating.
+tasks.register("generateBuildSubstitutions") {
+  group = "publishing"
+  description = "Generate a code snippet that can be copy-pasted for use in composite builds."
+}
+
 subprojects {
   group = "io.opentelemetry"
 }

buildSrc/src/main/kotlin/io/opentelemetry/gradle/OtelBomExtension.kt

@@ -0,0 +1,14 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.gradle
+
+import org.gradle.api.Project
+import org.gradle.api.provider.Property
+import java.util.function.Predicate
+
+abstract class OtelBomExtension {
+  abstract val projectFilter: Property<Predicate<Project>>
+}

buildSrc/src/main/kotlin/otel.bom-conventions.gradle.kts

@@ -0,0 +1,64 @@
+import io.opentelemetry.gradle.OtelBomExtension
+import org.gradle.kotlin.dsl.create
+
+plugins {
+  id("otel.publish-conventions")
+  id("java-platform")
+}
+
+if (!project.name.startsWith("bom")) {
+  throw IllegalStateException("Name of BOM projects must start with 'bom'.")
+}
+
+rootProject.subprojects.forEach { subproject ->
+  if (!subproject.name.startsWith("bom")) {
+    evaluationDependsOn(subproject.path)
+  }
+}
+val otelBom = extensions.create<OtelBomExtension>("otelBom")
+
+val generateBuildSubstitutions by tasks.registering {
+  group = "publishing"
+  description = "Generate a code snippet that can be copy-pasted for use in composite builds."
+}
+
+tasks.named("publish") {
+  dependsOn(generateBuildSubstitutions)
+}
+
+rootProject.tasks.named(generateBuildSubstitutions.name) {
+  dependsOn(generateBuildSubstitutions)
+}
+
+afterEvaluate {
+  otelBom.projectFilter.finalizeValue()
+  val stableProjects = rootProject.subprojects
+    .sortedBy { it.findProperty("archivesBaseName") as String? }
+    .filter { !it.name.startsWith("bom") }
+    .filter(otelBom.projectFilter.get()::test)
+    .filter { it.plugins.hasPlugin("maven-publish") }
+
+  generateBuildSubstitutions {
+    val outputFile = File(buildDir, "substitutions.gradle.kts")
+    outputs.file(outputFile)
+    val substitutionSnippet = stableProjects.joinToString(
+      separator = "\n",
+      prefix = "dependencySubstitution {\n",
+      postfix = "\n}\n"
+    ) { project ->
+      val publication = project.publishing.publications.getByName("mavenPublication") as MavenPublication
+      "  substitute(module(\"${publication.groupId}:${publication.artifactId}\")).using(project(\"${project.path}\"))"
+    }
+    inputs.property("projectPathsAndArtifactCoordinates", substitutionSnippet)
+    doFirst {
+      outputFile.writeText(substitutionSnippet)
+    }
+  }
+  stableProjects.forEach { project ->
+    dependencies {
+      constraints {
+        api(project)
+      }
+    }
+  }
+}