spawn/ProcessHandle: add completion callback

This allows callers to not only wait for completion, but also receive
error details.  This will replace lots of weird (protocol/socket)
error messages (because the child has disappeared and has reset a
socket connection to the caller) with spawner error messages.

src/spawn/Client.cxx

@@ -4,6 +4,7 @@
 
 #include "Client.hxx"
 #include "ProcessHandle.hxx"
+#include "CompletionHandler.hxx"
 #include "IProtocol.hxx"
 #include "Builder.hxx"
 #include "Parser.hxx"
@@ -60,6 +61,8 @@ struct SpawnServerClient::ChildProcess final
 
 	const unsigned pid;
 
+	SpawnCompletionHandler *completion_handler = nullptr;
+
 	ExitListener *listener = nullptr;
 
 	ChildProcess(SpawnServerClient &_client, unsigned _pid) noexcept
@@ -70,6 +73,13 @@ struct SpawnServerClient::ChildProcess final
 			Kill(SIGTERM);
 	}
 
+	/* virtual methods from class ChildProcessHandle */
+	void SetCompletionHandler(SpawnCompletionHandler &_completion_handler) noexcept override {
+		assert(!completion_handler);
+
+		completion_handler = &_completion_handler;
+	}
+
 	void SetExitListener(ExitListener &_listener) noexcept override {
 		assert(is_linked());
 
@@ -512,7 +522,21 @@ SpawnServerClient::HandleExecCompleteMessage(SpawnPayload payload)
 		payload.ReadUnsigned(pid);
 		const char *error = payload.ReadString();
 
-		// TODO pass error condition to completion handler
+		if (const auto i = processes.find(pid); i != processes.end()) {
+			// TODO forward errors
+			if (i->completion_handler) {
+				if (*error == 0)
+					i->completion_handler->OnSpawnSuccess();
+				else
+					i->completion_handler->OnSpawnError(std::make_exception_ptr(std::runtime_error{error}));
+
+				/* if there is a completion handler,
+				   don't log error message to
+				   stderr */
+				continue;
+			}
+		}
+
 		if (*error != 0)
 			fmt::print(stderr, "Failed to spawn child process {}: {}\n",
 					   pid, error);

src/spawn/CompletionHandler.hxx

@@ -0,0 +1,18 @@
+// SPDX-License-Identifier: BSD-2-Clause
+// Copyright CM4all GmbH
+// author: Max Kellermann <[email protected]>
+
+#pragma once
+
+#include <exception>
+
+/**
+ * This interface gets notified when spawning a child process
+ * completes.  Pass a reference to an instance to
+ * ChildProcessHandle::SetCompletionHandler().
+ */
+class SpawnCompletionHandler {
+public:
+	virtual void OnSpawnSuccess() noexcept = 0;
+	virtual void OnSpawnError(std::exception_ptr error) noexcept = 0;
+};

src/spawn/Local.cxx

@@ -4,6 +4,7 @@
 
 #include "Local.hxx"
 #include "ProcessHandle.hxx"
+#include "CompletionHandler.hxx"
 #include "PidfdEvent.hxx"
 #include "ExitListener.hxx"
 #include "Config.hxx"
@@ -50,6 +51,12 @@ class LocalChildProcess final : public ChildProcessHandle, ExitListener {
 	void OnChildProcessExit(int status) noexcept override;
 
 	/* virtual methods from class ChildProcessHandle */
+	void SetCompletionHandler(SpawnCompletionHandler &handler) noexcept override {
+		assert(pidfd);
+
+		handler.OnSpawnSuccess();
+	}
+
 	void SetExitListener(ExitListener &listener) noexcept override {
 		assert(pidfd);
 

src/spawn/ProcessHandle.hxx

@@ -4,6 +4,7 @@
 
 #pragma once
 
+class SpawnCompletionHandler;
 class ExitListener;
 
 /**
@@ -18,6 +19,7 @@ public:
 	 */
 	virtual ~ChildProcessHandle() noexcept = default;
 
+	virtual void SetCompletionHandler(SpawnCompletionHandler &handler) noexcept = 0;
 	virtual void SetExitListener(ExitListener &listener) noexcept = 0;
 
 	/**