3 files changed, 187 insertions, 6 deletions
diff --git a/internal/httpclient/client.go b/internal/httpclient/client.go
index 56992b915..45994b2ba 100644
--- a/internal/httpclient/client.go
+++ b/internal/httpclient/client.go
@@ -80,7 +80,7 @@ type Config struct {
 //   is available (context channels still respected)
 type Client struct {
 	client http.Client
-	queue  chan struct{}
+	rc     *requestQueue
 	bmax   int64
 }
 
@@ -118,7 +118,9 @@ func New(cfg Config) *Client {
 
 	// Prepare client fields
 	c.bmax = cfg.MaxBodySize
-	c.queue = make(chan struct{}, cfg.MaxOpenConns)
+	c.rc = &requestQueue{
+		maxOpenConns: cfg.MaxOpenConns,
+	}
 	c.client.Timeout = cfg.Timeout
 
 	// Set underlying HTTP client roundtripper
@@ -143,13 +145,18 @@ func New(cfg Config) *Client {
 // as the standard http.Client{}.Do() implementation except that response body will
 // be wrapped by an io.LimitReader() to limit response body sizes.
 func (c *Client) Do(req *http.Request) (*http.Response, error) {
+	// request a spot in the wait queue...
+	wait, release := c.rc.getWaitSpot(req.Host, req.Method)
+
+	// ... and wait our turn
 	select {
-	// Request context cancelled
 	case <-req.Context().Done():
+		// the request was canceled before we
+		// got to our turn: no need to release
 		return nil, req.Context().Err()
+	case wait <- struct{}{}:
+		// it's our turn!
 
-	// Slot in queue acquired
-	case c.queue <- struct{}{}:
 		// NOTE:
 		// Ideally here we would set the slot release to happen either
 		// on error return, or via callback from the response body closer.
@@ -160,7 +167,7 @@ func (c *Client) Do(req *http.Request) (*http.Response, error) {
 		// that connections may not be closed until response body is closed.
 		// The current implementation will reduce the viability of denial of
 		// service attacks, but if there are future issues heed this advice :]
-		defer func() { <-c.queue }()
+		defer release()
 	}
 
 	// Firstly, ensure this is a valid request
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 }
+}
diff --git a/internal/httpclient/queue_test.go b/internal/httpclient/queue_test.go
new file mode 100644
index 000000000..c6d6ad324
--- /dev/null
+++ b/internal/httpclient/queue_test.go
@@ -0,0 +1,106 @@
+/*
+   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 (
+	"net/http"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/suite"
+)
+
+type QueueTestSuite struct {
+	suite.Suite
+}
+
+func (suite *QueueTestSuite) TestQueue() {
+	maxOpenConns := 5
+	waitTimeout := 1 * time.Second
+
+	rc := &requestQueue{
+		maxOpenConns: maxOpenConns,
+	}
+
+	// fill all the open connections
+	var release func()
+	for i, n := range make([]interface{}, maxOpenConns) {
+		w, r := rc.getWaitSpot("example.org", http.MethodPost)
+		w <- n
+		if i == maxOpenConns-1 {
+			// save the last release function
+			release = r
+		}
+	}
+
+	// try to wait again for the same host/method combo, it should timeout
+	waitAgain, _ := rc.getWaitSpot("example.org", "post")
+
+	select {
+	case waitAgain <- struct{}{}:
+		suite.FailNow("first wait did not time out")
+	case <-time.After(waitTimeout):
+		break
+	}
+
+	// now close the final release that we derived earlier
+	release()
+
+	// try waiting again, it should work this time
+	select {
+	case waitAgain <- struct{}{}:
+		break
+	case <-time.After(waitTimeout):
+		suite.FailNow("second wait timed out")
+	}
+
+	// the POST queue is now sitting on full
+	suite.Len(waitAgain, maxOpenConns)
+
+	// we should still be able to make a GET for the same host though
+	getWait, getRelease := rc.getWaitSpot("example.org", http.MethodGet)
+	select {
+	case getWait <- struct{}{}:
+		break
+	case <-time.After(waitTimeout):
+		suite.FailNow("get wait timed out")
+	}
+
+	// the GET queue has one request waiting
+	suite.Len(getWait, 1)
+	// clear it...
+	getRelease()
+	suite.Empty(getWait)
+
+	// even though the POST queue for example.org is full, we
+	// should still be able to make a POST request to another host :)
+	waitForAnotherHost, _ := rc.getWaitSpot("somewhere.else", http.MethodPost)
+	select {
+	case waitForAnotherHost <- struct{}{}:
+		break
+	case <-time.After(waitTimeout):
+		suite.FailNow("get wait timed out")
+	}
+
+	suite.Len(waitForAnotherHost, 1)
+}
+
+func TestQueueTestSuite(t *testing.T) {
+	suite.Run(t, &QueueTestSuite{})
+}