support for testing offline via gradle-side reflection hacks and firejail

buildSrc/java-offline

@@ -0,0 +1,2 @@
+#!/bin/bash
+firejail --net=none "$G_ORIGINAL_EXECUTABLE" "$@"

buildSrc/src/main/kotlin/inceptum.java.gradle.kts

@@ -36,4 +36,23 @@ publishing {
             }
         }
     }
+}
+
+afterEvaluate {
+    if (hasProperty("offline")) {
+        tasks.withType(JavaExec::class) {
+            environment("G_ORIGINAL_EXECUTABLE", executable ?: "java")
+            //TODO once we are not using a toolchain, just do executable(rootDir.resolve("buildSrc/java-offline"))
+            val originalMetadata = javaLauncher.get().metadata
+            val field = org.gradle.api.internal.provider.AbstractProperty::class.java.getDeclaredField("value")
+            field.isAccessible = true
+            val customLauncher = object: JavaLauncher {
+                override fun getMetadata(): JavaInstallationMetadata = originalMetadata
+                override fun getExecutablePath(): RegularFile = rootProject.layout.projectDirectory
+                    .dir("buildSrc")
+                    .file("java-offline")
+            }
+            field.set(javaLauncher, org.gradle.api.internal.provider.Providers.of(customLauncher))
+        }
+    }
 }