flutter: move flutter-tools' Gradle build files out of Nix Store

pkgs/development/compilers/flutter/flutter.nix

@@ -82,6 +82,10 @@ let
           "dartSdkVersion": "${dart.version}"
         }
         EOF
+
+        # Suppress a small error now that `.gradle`'s location changed.
+        # Location changed because of the patch "gradle-flutter-tools-wrapper.patch".
+        mkdir -p "$out/packages/flutter_tools/gradle/.gradle"
       '';
 
       installPhase = ''

pkgs/development/compilers/flutter/versions/3_19/patches/gradle-flutter-tools-wrapper.patch

@@ -0,0 +1,44 @@
+This patch introduces an intermediate Gradle build step to alter the behavior
+of flutter_tools' Gradle project, specifically moving the creation of `build`
+and `.gradle` directories from within the Nix Store to somewhere in `$HOME/.cache/flutter/nix-flutter-tools-gradle/$engineShortRev`.
+
+Without this patch, flutter_tools' Gradle project tries to generate `build` and `.gradle`
+directories within the Nix Store. Resulting in read-only errors when trying to build a
+Flutter Android app at runtime.
+
+This patch takes advantage of the fact settings.gradle takes priority over settings.gradle.kts to build the intermediate Gradle project
+when a Flutter app runs `includeBuild("${settings.ext.flutterSdkPath}/packages/flutter_tools/gradle")`
+
+`rootProject.buildFileName = "/dev/null"` so that the intermediate project doesn't use `build.gradle.kts` that's in the same directory.
+
+The intermediate project makes a `settings.gradle` file in `$HOME/.cache/flutter/nix-flutter-tools-gradle/<short engine rev>/` and `includeBuild`s it.
+This Gradle project will build the actual `packages/flutter_tools/gradle` project by setting
+`rootProject.projectDir = new File("$settingsDir")` and `apply from: new File("$settingsDir/settings.gradle.kts")`.
+
+Now the `.gradle` will be built in `$HOME/.cache/flutter/nix-flutter-tools-gradle/<short engine rev>/`, but `build` doesn't.
+To move `build` to `$HOME/.cache/flutter/nix-flutter-tools-gradle/<short engine rev>/` as well, we need to set `buildDirectory`.
+diff --git a/packages/flutter_tools/gradle/settings.gradle b/packages/flutter_tools/gradle/settings.gradle
+new file mode 100644
+index 0000000000..b2485c94b4
+--- /dev/null
++++ b/packages/flutter_tools/gradle/settings.gradle
+@@ -0,0 +1,19 @@
++rootProject.buildFileName = "/dev/null"
++
++def engineShortRev = (new File("$settingsDir/../../../bin/internal/engine.version")).text.take(10)
++def dir = new File("$System.env.HOME/.cache/flutter/nix-flutter-tools-gradle/$engineShortRev")
++dir.mkdirs()
++def file = new File(dir, "settings.gradle")
++
++file.text = """
++rootProject.projectDir = new File("$settingsDir")
++apply from: new File("$settingsDir/settings.gradle.kts")
++
++gradle.allprojects { project ->
++  project.beforeEvaluate {
++    project.layout.buildDirectory = new File("$dir/build")
++  }
++}
++"""
++
++includeBuild(dir)