[bugfix] Trim log entries to 1700 chars before they enter syslog (#493)

* start implementing trimming hook

* add test with very long test

* test syslog w/ unix socket + long (trimmed) msg

* trim long entries with trimhook

* trim to 1700 chars instead

internal/log/log.go

@@ -73,7 +73,7 @@ func Initialize() error {
 			return err
 		}
 
-		logrus.AddHook(hook)
+		logrus.AddHook(&trimHook{hook})
 	}
 
 	return nil

internal/log/trimhook.go

@@ -0,0 +1,53 @@
+/*
+   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 log
+
+import (
+	"github.com/sirupsen/logrus"
+)
+
+// trimHook is a wrapper round a logrus hook that trims the *entry.Message
+// to no more than 1700 characters before sending it through to the wrapped hook,
+// to avoid spamming syslog with messages that are too long for it.
+type trimHook struct {
+	wrappedHook logrus.Hook
+}
+
+func (t *trimHook) Fire(e *logrus.Entry) error {
+	// only copy/truncate if we need to
+	if len(e.Message) < 1700 {
+		return t.wrappedHook.Fire(e)
+	}
+
+	// it's too long, truncate + fire a copy of the entry so we don't meddle with the original
+	return t.wrappedHook.Fire(&logrus.Entry{
+		Logger:  e.Logger,
+		Data:    e.Data,
+		Time:    e.Time,
+		Level:   e.Level,
+		Caller:  e.Caller,
+		Message: e.Message[:1696] + "...", // truncate
+		Buffer:  e.Buffer,
+		Context: e.Context,
+	})
+}
+
+func (t *trimHook) Levels() []logrus.Level {
+	return t.wrappedHook.Levels()
+}