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+---
+layout: post
+title:  OH NIO! What is this system property?
+header: header-eng
+---
+
+AndroidX wanted to take a dip into a beautiful world of `build.gradle.kts` after
+years of using good-ole-speedy-Groovy `build.gradle`. The smallest step towards
+that was [converting a single `build.gradle` file](https://r.android.com/2915203)
+and see how that affects our builds, especially the time before the task
+execution phase. It all looked good locally.
+
+Enter our CI failing with:
+```
+The new output:
+  Calculating task graph as configuration cache cannot be reused because system property 'idea.io.use.nio2' has changed.
+```
+
+Our [log creep detection system](https://medium.com/@mathjeff/preventing-log-creep-968b9d55720c)
+spotted a brand-new warning indicating that the task up-to-date validation run
+(androidx runs our build twice to make sure that tasks do nothing the second
+time they run). This warning means that we did not have a configuration cache
+hit the second time Gradle is invoked, and the reason was that there was a
+change to `idea.io.use.nio2` system property. This is bad as it means we
+unexpectedly rerun the configuration phase in the second invocation making
+overall build CI slower, but also slowing down AndroidX devs locally in the
+same way too.
+
+There are two questions that arise: 1. why is AndroidX build reading this system
+property at configuration time? 2. why is the value of the system property
+changing?
+
+For where it is being read, Gradle gives you a breadcrumb in configuration cache report:
+![Configuration cache report showing idea.io.use.nio2 being read by PomResolver class](/assets/2024-01-18-cache-report.png)
+
+Sadly, this does not give you a stacktrace, so you have to use a debugger
+with a breakpoint on `ConfigurationCacheFingerprintWriter#systemPropertyRead`
+in Gradle to find that [PomResolver#resolveEffectivePom](https://github.com/spdx/spdx-gradle-plugin/blob/ce133bafb1faea5a85846a2784bac1a645360b55/src/main/java/org/spdx/sbom/gradle/maven/PomResolver.java#L203C5-L203C66)
+from SPDX gradle plugin gets every system property through
+`.putAll(System.getProperties())` call.
+
+![JVM Debugger stacktrace showing PomResolver reading nio2 property](/assets/2024-01-18-read-property.png){:width="600px"}
+
+The second part, on what sets it and why it only happens in the first build
+Gradle does not really help you. I quickly did a search through Gradle,
+Android Gradle Plugin, and Kotlin codebases and spotted [`setIdeaIoUseFallback`](https://cs.android.com/android-studio/kotlin/+/master:compiler/cli/cli-common/src/org/jetbrains/kotlin/cli/common/environment/util.kt;l=20;drc=bffe94e48a4bf6dfe6d959ae840010745bbc7077)
+and [`configureKotlinCompilerIoForWindowsSupport`](https://cs.android.com/android-studio/gradle/+/master:build-logic-commons/basics/src/main/kotlin/gradlebuild/basics/util/KotlinSourceParser.kt;l=97;drc=3bd08a5483b20d08d2853a532f5b34767d8ca0a4)
+that seemed suspect. To confirm my hunch, I attached a debugger with a
+breakpoint on `System.setProperty`:
+
+![JVM Debugger stacktrace showing setIdeaIoUseFallback setting nio2 property](/assets/2024-01-18-set-property.png){:width="600px"}
+
+Gradle only sets this value when it detects usage of `build.gradle.kts` and it
+is compiling these scripts. On the second run, the scripts are already compiled,
+so it never calls this code.
+
+To get out of this situation, we have created [an issue for SPDX gradle plugin](https://github.com/spdx/spdx-gradle-plugin/issues/94)
+to remove reading of all properties at configuration time. Ideally, Gradle also
+would not go toggling properties, as this specific one is a workaround for
+Windows but is run on other OSes as well.
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