spawn/Client: export statistics

src/spawn/Client.cxx

@@ -429,9 +429,11 @@ Serialize(SpawnSerializer &s, const PreparedChildProcess &p)
 std::unique_ptr<ChildProcessHandle>
 SpawnServerClient::SpawnChildProcess(const char *name,
 				     PreparedChildProcess &&p)
-{
+try {
 	assert(!shutting_down);
 
+	++stats.spawned;
+
 	/* this check is performed again on the server (which is obviously
 	   necessary, and the only way to have it secure); this one is
 	   only here for the developer to see the error earlier in the
@@ -467,6 +469,9 @@ SpawnServerClient::SpawnChildProcess(const char *name,
 	auto handle = std::make_unique<ChildProcess>(*this, pid);
 	processes.insert(*handle);
 	return handle;
+} catch (...) {
+	++stats.errors;
+	throw;
 }
 
 void
@@ -526,10 +531,12 @@ SpawnServerClient::HandleExecCompleteMessage(SpawnPayload payload)
 		if (const auto i = processes.find(pid); i != processes.end()) {
 			// TODO forward errors
 			if (i->completion_handler) {
-				if (*error == 0)
+				if (*error == 0) {
 					i->completion_handler->OnSpawnSuccess();
-				else
+				} else {
+					++stats.errors;
 					i->completion_handler->OnSpawnError(std::make_exception_ptr(std::runtime_error{error}));
+				}
 
 				/* if there is a completion handler,
 				   don't log error message to

src/spawn/Client.hxx

@@ -6,6 +6,7 @@
 
 #include "Interface.hxx"
 #include "Config.hxx"
+#include "Stats.hxx"
 #include "event/DeferEvent.hxx"
 #include "event/SocketEvent.hxx"
 #include "net/MultiReceiveMessage.hxx"
@@ -47,7 +48,9 @@ class SpawnServerClient final : public SpawnService {
 				 IntrusiveHashSetOperators<ChildProcess,
 							   ChildProcessGetKey,
 							   std::hash<unsigned>,
-							   std::equal_to<unsigned>>>;
+							   std::equal_to<unsigned>>,
+				 IntrusiveHashSetBaseHookTraits<ChildProcess>,
+				 IntrusiveHashSetOptions{.constant_time_size=true}>;
 
 	ChildProcessSet processes;
 
@@ -63,6 +66,8 @@ class SpawnServerClient final : public SpawnService {
 
 	MultiReceiveMessage receive{16, 1024, CMSG_SPACE(sizeof(int)), 1};
 
+	mutable SpawnStats stats{};
+
 	unsigned last_pid = 0;
 
 	/**
@@ -107,6 +112,11 @@ public:
 		return cgroups;
 	}
 
+	const SpawnStats &GetStats() const noexcept {
+		stats.alive = processes.size();
+		return stats;
+	}
+
 	void Shutdown() noexcept;
 
 	UniqueSocketDescriptor Connect();

src/spawn/Stats.hxx

@@ -0,0 +1,25 @@
+// SPDX-License-Identifier: BSD-2-Clause
+// Copyright CM4all GmbH
+// author: Max Kellermann <[email protected]>
+
+#pragma once
+
+#include <cstdint>
+
+struct SpawnStats {
+	/**
+	 * The total number of SpawnChildProcess() calls (include
+         * failed ones).
+	 */
+	uint_least64_t spawned;
+
+	/**
+	 * The number of failed SpawnChildProcess() calls.
+	 */
+	uint_least64_t errors;
+
+	/**
+	 * How many child processse are alive right now?
+	 */
+	std::size_t alive;
+};