Write native configuration files as UTF-8 - #36972

Overview

A small consistency/hardening change: make FileNativeConfigurationWriter always write the GraalVM native-image configuration files (reachability-metadata.json, etc.) as UTF-8.

This is a minor edge-case fix rather than a critical bug; the trigger is narrow (see below), so please feel free to decline if you consider it not worth the change.

Problem

writeTo uses a plain FileWriter:

try (FileWriter out = new FileWriter(file)) {
    writer.accept(createJsonWriter(out));
}

FileWriter without an explicit charset encodes using the JVM platform default charset, which is not UTF-8 on every supported configuration (for example a Windows JVM, where the default is a code-page charset such as windows-1252 prior to JDK 18 / JEP 400). BasicJsonWriter passes non-ASCII characters through unescaped, so the on-disk encoding is decided entirely by the writer's charset. GraalVM expects these files to be UTF-8, so on a non-UTF-8 platform non-ASCII characters in resource patterns or bundle names could be written with the wrong encoding.

The conditions are narrow (a non-UTF-8 default charset combined with non-ASCII content), so this is more of a hardening/consistency improvement than a frequently hit bug.

Fix

Specify StandardCharsets.UTF_8 explicitly:

try (FileWriter out = new FileWriter(file, StandardCharsets.UTF_8)) {
    writer.accept(createJsonWriter(out));
}

This is consistent with the explicit UTF-8 usage already present in the aot.generate package.

A test writes a resource pattern containing a non-ASCII character and asserts that the raw file bytes contain its UTF-8 byte sequence. Note that on a JVM whose default charset is already UTF-8 (such as the CI environment) the test passes both before and after the change; it serves as a guard on non-UTF-8 platforms and documents the intended encoding. The correctness of the fix itself rests on the FileWriter(File, Charset) contract rather than on reproducing a non-UTF-8 default in CI.