[bugfix] Relax outgoing http request queue (#760)v0.4.0

* add request queue trace logging

* fix  misleading wording

* implement request slots per host per method

* undo formatting change (?)

* remove gratuitous trace logging

* rename done -> release
avoids confusion with ctx.Done

1 files changed, 68 insertions, 0 deletions

diff --git a/internal/httpclient/queue.go b/internal/httpclient/queue.go
new file mode 100644
index 000000000..8cb1274be
--- /dev/null
+++ b/internal/httpclient/queue.go
@@ -0,0 +1,68 @@
+/*
+   GoToSocial
+   Copyright (C) 2021-2022 GoToSocial Authors admin@gotosocial.org
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU Affero General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU Affero General Public License for more details.
+
+   You should have received a copy of the GNU Affero General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+package httpclient
+
+import (
+	"strings"
+	"sync"
+
+	"github.com/superseriousbusiness/gotosocial/internal/log"
+)
+
+type requestQueue struct {
+	hostQueues   sync.Map // map of `hostQueue`
+	maxOpenConns int      // max open conns per host per request method
+}
+
+type hostQueue struct {
+	slotsByMethod sync.Map
+}
+
+// getWaitSpot returns a wait channel and release function for http clients
+// that want to do requests politely: that is, wait for their turn.
+//
+// To wait, a caller should do a select on an attempted insert into the
+// returned wait channel. Once the insert succeeds, then the caller should
+// proceed with the http request that pertains to the given host + method.
+// It doesn't matter what's put into the wait channel, just any interface{}.
+//
+// When the caller is finished with their http request, they should free up the
+// slot they were occupying in the wait queue, by calling the release function.
+//
+// The reason for the caller needing to provide host and method, is that each
+// remote host has a separate wait queue, and there's a separate wait queue
+// per method for that host as well. This ensures that outgoing requests can still
+// proceed for others hosts and methods while other requests are undergoing,
+// while also preventing one host from being spammed with, for example, a
+// shitload of GET requests all at once.
+func (rc *requestQueue) getWaitSpot(host string, method string) (wait chan<- interface{}, release func()) {
+	hostQueueI, _ := rc.hostQueues.LoadOrStore(host, new(hostQueue))
+	hostQueue, ok := hostQueueI.(*hostQueue)
+	if !ok {
+		log.Panic("hostQueueI was not a *hostQueue")
+	}
+
+	waitSlotI, _ := hostQueue.slotsByMethod.LoadOrStore(strings.ToUpper(method), make(chan interface{}, rc.maxOpenConns))
+	methodQueue, ok := waitSlotI.(chan interface{})
+	if !ok {
+		log.Panic("waitSlotI was not a chan interface{}")
+	}
+
+	return methodQueue, func() { <-methodQueue }
+}