Make max parallel native image builds configurable

docs/src/docs/asciidoc/gradle-plugin.adoc

@@ -196,6 +196,14 @@ include::../snippets/gradle/kotlin/build.gradle.kts[tags=all-config-options]
 
 NOTE: For options that can be set using command-line, if both DSL and command-line options are present, command-line options take precedence.
 
+[[max_parallel_builds]]
+==== Max parallel builds
+
+When using Gradle parallel builds, the plugin will automatically limit the number of native images which can be built concurrently, in order to limit CPU and memory usage.
+By default, it's limited to the number of CPU cores / 16, but you can change this limit either by setting the `org.graalvm.buildtools.max.parallel.builds` gradle property (e.g in your `gradle.properties` file) or by setting the `GRAALVM_BUILDTOOLS_MAX_PARALLEL_BUILDS` environment variable.
+
+[[configuration-advanced]]
+
 [[long_classpath_and_fat_jar_support]]
 ==== Long classpath, @argument file and fat jar support
 

docs/src/docs/asciidoc/index.adoc

@@ -24,6 +24,7 @@ If you are using alternative build systems, see <<alternative-build-systems.adoc
 ==== Gradle plugin
 
 - Fix `collectReachabilityMetadata` not being thread-safe
+- Add an option to configure the maximum number of images which can be built in parallel
 
 === Release 0.9.19
 

native-gradle-plugin/src/main/java/org/graalvm/buildtools/gradle/NativeImageService.java

@@ -44,18 +44,31 @@
 import org.graalvm.buildtools.gradle.internal.GraalVMLogger;
 import org.gradle.api.Project;
 import org.gradle.api.provider.Provider;
+import org.gradle.api.provider.ProviderFactory;
 import org.gradle.api.services.BuildService;
 import org.gradle.api.services.BuildServiceParameters;
 
 @SuppressWarnings({"UnstableApiUsage", "unused"})
 public abstract class NativeImageService implements BuildService<BuildServiceParameters.None> {
+
+    public static final String MAX_PARALLEL_SYSTEM_PROPERTY = "org.graalvm.buildtools.max.parallel.builds";
+    public static final String MAX_PARALLEL_ENV_VAR = "GRAALVM_BUILDTOOLS_MAX_PARALLEL_BUILDS";
+
     public static Provider<NativeImageService> registerOn(Project project) {
         return project.getGradle()
                 .getSharedServices()
                 .registerIfAbsent("nativeImage", NativeImageService.class,
                         spec -> {
                             GraalVMLogger.newBuild();
-                            spec.getMaxParallelUsages().set(1 + Runtime.getRuntime().availableProcessors() / 16);
+                            spec.getMaxParallelUsages().set(maxParallelUsagesOf(project.getProviders()));
                         });
     }
+
+    private static Provider<Integer> maxParallelUsagesOf(ProviderFactory providers) {
+        return providers.gradleProperty(MAX_PARALLEL_SYSTEM_PROPERTY)
+                .forUseAtConfigurationTime()
+                .orElse(providers.environmentVariable(MAX_PARALLEL_ENV_VAR).forUseAtConfigurationTime())
+                .map(Integer::parseInt)
+                .orElse(1 + Runtime.getRuntime().availableProcessors() / 16);
+    }
 }