Mark the option EnableSignalHandling as immutable.

Add documentation to RuntimeOptionKeyFlag.

Author: christianhaeubl

substratevm/src/com.oracle.svm.core.posix/src/com/oracle/svm/core/posix/PosixUtils.java

@@ -302,7 +302,7 @@ public static void installSignalHandler(int signum, Signal.AdvancedSignalDispatc
      * signals.
      */
     private static int sigaction(int signum, Signal.sigaction structSigAction, Signal.sigaction old) {
-        VMError.guarantee(SubstrateOptions.EnableSignalHandling.getValue(), "Trying to install a signal handler while signal handling.");
+        VMError.guarantee(SubstrateOptions.EnableSignalHandling.getValue(), "Trying to install a signal handler while signal handling is disabled.");
 
         if (VMOperation.isInProgress()) {
             /*

substratevm/src/com.oracle.svm.core/src/com/oracle/svm/core/SubstrateOptions.java

@@ -790,7 +790,7 @@ protected void onValueUpdate(EconomicMap<OptionKey<?>, Object> values, Integer o
     public static final HostedOptionKey<Boolean> DisableTypeIdResultVerification = new HostedOptionKey<>(true);
 
     @Option(help = "Enables signal handling", stability = OptionStability.EXPERIMENTAL, type = Expert)//
-    public static final RuntimeOptionKey<Boolean> EnableSignalHandling = new RuntimeOptionKey<>(null) {
+    public static final RuntimeOptionKey<Boolean> EnableSignalHandling = new RuntimeOptionKey<>(null, Immutable) {
         @Override
         public Boolean getValueOrDefault(UnmodifiableEconomicMap<OptionKey<?>, Object> values) {
             if (values.containsKey(this)) {

substratevm/src/com.oracle.svm.core/src/com/oracle/svm/core/jdk/RuntimeSupport.java

@@ -104,7 +104,7 @@ public void initialize() {
 
     /**
      * Adds a hook which will execute during the shutdown process. Note it is possible for the
-     * {@link #shutdownHooks} to called without the {@link #startupHooks} executing first.
+     * {@link #shutdownHooks} to be called without the {@link #startupHooks} executing first.
      */
     @Platforms(Platform.HOSTED_ONLY.class)
     public void addShutdownHook(Hook hook) {

substratevm/src/com.oracle.svm.core/src/com/oracle/svm/core/option/RuntimeOptionKey.java

@@ -24,8 +24,9 @@
  */
 package com.oracle.svm.core.option;
 
-import com.oracle.svm.core.SubstrateUtil;
-import com.oracle.svm.core.jdk.RuntimeSupport;
+import java.util.Objects;
+import java.util.function.Consumer;
+
 import org.graalvm.collections.EconomicMap;
 import org.graalvm.compiler.api.replacements.Fold;
 import org.graalvm.compiler.options.Option;
@@ -34,8 +35,8 @@
 import org.graalvm.nativeimage.Platform;
 import org.graalvm.nativeimage.Platforms;
 
-import java.util.Objects;
-import java.util.function.Consumer;
+import com.oracle.svm.core.SubstrateUtil;
+import com.oracle.svm.core.jdk.RuntimeSupport;
 
 /**
  * Defines a runtime {@link Option}, in contrast to a {@link HostedOptionKey hosted option}.
@@ -133,7 +134,14 @@ private static int computeFlags(RuntimeOptionKeyFlag[] flags) {
     }
 
     public enum RuntimeOptionKeyFlag {
+        /** If this flag is set, then option value is propagated to all compilation isolates. */
         RelevantForCompilationIsolates,
+
+        /**
+         * If this flag is set, then the option value can only be changed during startup, i.e.,
+         * before the startup hooks are executed (see {@link RuntimeSupport#initialize()}). This
+         * flag should be used for runtime options that are accessed in startup hooks.
+         */
         Immutable,
     }
 }