[HotReloadAgent] Handle unsupported platform for PosixSignalRegistration

[jonathanpeppers]

If running on Android, for example, you can get the following exception:

01-14 10:46:37.635 28537 28537 F mono-rt : [ERROR] FATAL UNHANDLED EXCEPTION: System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation.
01-14 10:46:37.635 28537 28537 F mono-rt :  ---> System.PlatformNotSupportedException: Operation is not supported on this platform.
01-14 10:46:37.635 28537 28537 F mono-rt :    at System.Runtime.InteropServices.PosixSignalRegistration.Register(PosixSignal signal, Action`1 handler)
01-14 10:46:37.635 28537 28537 F mono-rt :    at System.Runtime.InteropServices.PosixSignalRegistration.Create(PosixSignal signal, Action`1 handler)
01-14 10:46:37.635 28537 28537 F mono-rt :    at StartupHook.RegisterSignalHandlers()
01-14 10:46:37.635 28537 28537 F mono-rt :    at StartupHook.InitializeWithHttp(String processDir)
01-14 10:46:37.635 28537 28537 F mono-rt :    at StartupHook.Initialize()
01-14 10:46:37.635 28537 28537 F mono-rt :    at System.Reflection.MethodBaseInvoker.InterpretedInvoke_Method(Object obj, IntPtr* args)
01-14 10:46:37.635 28537 28537 F mono-rt :    at System.Reflection.MethodBaseInvoker.InvokeWithNoArgs(Object obj, BindingFlags invokeAttr)
01-14 10:46:37.635 28537 28537 F mono-rt :    --- End of inner exception stack trace ---
01-14 10:46:37.635 28537 28537 F mono-rt :    at System.Reflection.MethodBaseInvoker.InvokeWithNoArgs(Object obj, BindingFlags invokeAttr)
01-14 10:46:37.635 28537 28537 F mono-rt :    at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
01-14 10:46:37.635 28537 28537 F mono-rt :    at System.Reflection.MethodBase.Invoke(Object obj, Object[] parameters)
01-14 10:46:37.635 28537 28537 F mono-rt :    at System.StartupHookProvider.CallStartupHook(StartupHookNameOrPath startupHook)
01-14 10:46:37.635 28537 28537 F mono-rt :    at System.StartupHookProvider.ProcessStartupHooks(String diagnosticStartupHooks)

Just like in 79e5fa6, PosixSignalRegistration APIs are decorated with:

[UnsupportedOSPlatform("android")]
[UnsupportedOSPlatform("browser")]
[UnsupportedOSPlatform("ios")]
[UnsupportedOSPlatform("tvos")]
public static PosixSignalRegistration Create(PosixSignal signal, Action<PosixSignalContext> handler);

This is used for handling Ctrl+C, for example, which we'll likely implement a different way for mobile platforms.

For now, let's not run the PlatformNotSupportedException-throwing code on unsupported platforms.

[jonathanpeppers]

Note that I'll send this to release/10.0.3xx instead, when that branch is available.

[tmat]

LGTM. Do not merge to 2xx though.

[Copilot]

Pull request overview

This PR fixes a PlatformNotSupportedException crash that occurs when running the HotReload agent on platforms where PosixSignalRegistration is not supported (Android, iOS, tvOS, and Browser). The fix adds a platform check before attempting to register POSIX signal handlers.

Changes:

• Introduces a new s_supportsPosixSignals boolean field to track platform support
• Adds a conditional check in RegisterSignalHandlers() to skip POSIX signal registration on unsupported platforms

[jonathanpeppers]

@tmat retargeted to release/10.0.3xx.