Add keepalive comment messages, per spec

Makefile

@@ -5,7 +5,7 @@ FIRE_LDLIBS ?= -lgflags -lglog -lpthread
 
 all: firesse.a firesse.o firesse.so example_clock
 
-objects = server.o index.o stream.o
+objects = server.o keepalive.o index.o stream.o
 
 firecgi/firecgi.o:
 	$(MAKE) --directory=firecgi firecgi.o

index.cc

@@ -47,4 +47,48 @@ void Index::Freshen(Stream* stream) {
 	Add(stream);
 }
 
+std::chrono::nanoseconds Index::WithStalest(std::function<void(Stream*)> callback, const std::chrono::nanoseconds& min_stale) {
+	Stream* stalest = nullptr;
+	std::chrono::nanoseconds ret;
+	const auto now = std::chrono::steady_clock::now();
+	const auto latest = now - min_stale;
+
+	{
+		std::lock_guard l(mu_);
+		if (!stalest_) {
+			return min_stale;
+		}
+		if (stalest_->last_message_time_ > latest) {
+			return stalest_->last_message_time_ - latest;
+		}
+
+		// stalest_ is valid for callback
+		stalest = stalest_;
+
+		if (stalest->fresher_) {
+			if (stalest->fresher_->last_message_time_ > latest) {
+				ret = stalest_->fresher_->last_message_time_ - latest;
+			}
+			// Otherwise ret is 0 for immediate cycle
+		} else {
+			ret = min_stale;
+		}
+
+		if (!stalest->mu_.try_lock()) {
+			// We're acquiring mutexes in the wrong order here; normally it's
+			// (Stream, Index), but we're doing (Index, Stream). That means we
+			// may deadlock. We take the lower-priority path and fail in case
+			// of deadlock.
+			return {};
+		}
+
+		Freshen(stalest);
+	}
+
+	callback(stalest);
+	stalest->mu_.unlock();
+
+	return ret;
+}
+
 } // namespace firesse

index.h

@@ -5,12 +5,22 @@
 namespace firesse {
 
 // Track live streams
+// O(1) for Add(), Remove(), and Freshen()
+// Manages this by only supporting insertion of the freshest element,
+// and by requiring an iterator for removal.
+// Avoids allocation and folds in iterators by using an intrusive list
+// inside Stream.
 class Index {
   public:
 	void Add(Stream* stream);
 	void Remove(Stream* stream);
 	void Freshen(Stream* stream);
 
+	// Returns time to sleep until next stalest, or min_stale if none
+	// Only calls callback if stalest is at least min_stale
+	// Handles all locking and marks Stream as fresh after callback
+	std::chrono::nanoseconds WithStalest(std::function<void(Stream*)> callback, const std::chrono::nanoseconds& min_stale);
+
   private:
 	std::recursive_mutex mu_;
 	Stream* freshest_ = nullptr;

keepalive.cc

@@ -0,0 +1,42 @@
+#include <poll.h>
+#include <sys/eventfd.h>
+
+#include "keepalive.h"
+
+namespace firesse {
+
+KeepAlive::KeepAlive(const std::chrono::nanoseconds& max_stale, Index* index)
+		: max_stale_(max_stale),
+		  index_(index),
+		  shutdown_(eventfd(0, 0)) {
+	PCHECK(shutdown_ >= 0) << "eventfd()";
+}
+
+void KeepAlive::Start() {
+	thread_ = std::thread([this]() {
+		int timeout = 0;	
+		constexpr auto num_fds = 1;
+		pollfd fds[num_fds] = {
+			{
+				.fd = shutdown_,
+				.events = POLLIN,
+			},
+		};
+		while (poll(fds, num_fds, timeout) <= 0) {
+			auto sleep = index_->WithStalest([](Stream* stream) {
+				stream->WriteRaw(":\n");
+			}, max_stale_);
+			timeout = std::chrono::duration_cast<std::chrono::milliseconds>(sleep).count();
+		}
+	});
+}
+
+void KeepAlive::Stop() {
+	CHECK(thread_.joinable());
+
+	uint64_t shutdown = 1;
+	PCHECK(write(shutdown_, &shutdown, sizeof(shutdown)) == sizeof(shutdown));
+	thread_.join();
+}
+
+} // namespace firesse

keepalive.h

@@ -0,0 +1,23 @@
+#pragma once
+
+#include <thread>
+
+#include "index.h"
+
+namespace firesse {
+
+class KeepAlive {
+  public:
+	KeepAlive(const std::chrono::nanoseconds& max_stale, Index* index);
+	void Start();
+	void Stop();
+
+  private:
+	const std::chrono::nanoseconds max_stale_;
+	Index* index_;
+	int shutdown_;
+
+	std::thread thread_;
+};
+
+} // namespace firesse

server.cc

@@ -4,12 +4,15 @@ namespace firesse {
 
 Server::Server(int port, const std::function<void(Stream*)>& callback)
 		: callback_(callback),
+		  keep_alive_(std::chrono::seconds(15), &index_),
 		  firecgi_server_(port,
 		                  [this](firecgi::Request* request) { OnRequest(request); },
 						  1) {}
 
 void Server::Serve() {
+	keep_alive_.Start();
 	firecgi_server_.Serve();
+	keep_alive_.Stop();
 }
 
 void Server::Shutdown() {

server.h

@@ -5,6 +5,7 @@
 #include "firecgi/server.h"
 
 #include "index.h"
+#include "keepalive.h"
 #include "stream.h"
 
 namespace firesse {
@@ -21,6 +22,7 @@ class Server {
 
 	std::function<void(Stream*)> callback_;
 	Index index_;
+	KeepAlive keep_alive_;
 	firecgi::Server firecgi_server_;
 };
 

stream.cc

@@ -19,11 +19,10 @@ void Stream::OnClose(const std::function<void()>& callback) {
 	on_close_ = callback;
 }
 
-bool Stream::WriteEvent(const std::string& data, uint64_t id, const std::string& type) {
-	{
-		std::lock_guard l(mu_);
-		index_->Freshen(this);
-	}
+bool Stream::WriteEvent(const std::string_view& data, uint64_t id, const std::string& type) {
+	std::lock_guard l(mu_);
+
+	index_->Freshen(this);
 
 	return request_->InTransaction<bool>([=]() {
 		if (id) {
@@ -37,6 +36,13 @@ bool Stream::WriteEvent(const std::string& data, uint64_t id, const std::string&
 	});
 }
 
+bool Stream::WriteRaw(const std::string_view& data) {
+	std::lock_guard l(mu_);
+
+	request_->WriteBody(data);
+	return request_->Flush();
+}
+
 bool Stream::End() {
 	return request_->End();
 }

stream.h

@@ -15,7 +15,8 @@ class Stream {
 
 	void OnClose(const std::function<void()>& callback);
 
-	bool WriteEvent(const std::string& data, uint64_t id=0, const std::string& type="");
+	bool WriteEvent(const std::string_view& data, uint64_t id=0, const std::string& type="");
+	bool WriteRaw(const std::string_view& data);
 	bool End();
 	void Close();
 
@@ -25,7 +26,8 @@ class Stream {
 
 	std::function<void()> on_close_;
 
-	std::mutex mu_;
+	// TODO: What exactly is this protecting?
+	std::recursive_mutex mu_;
 	std::chrono::steady_clock::time_point last_message_time_;
 	Stream* fresher_;
 	Stream* staler_;