From d08ed7d1e93e2b40d2de03ee73081a61b26031bd Mon Sep 17 00:00:00 2001
From: Nick Craig-Wood <nick@craig-wood.com>
Date: Thu, 11 Aug 2022 16:45:21 +0100
Subject: [PATCH] ftp: add notes on how to avoid deadlocks with concurrency -
fixes #6370
---
backend/ftp/ftp.go | 18 ++++++++++++++++--
1 file changed, 16 insertions(+), 2 deletions(-)
diff --git a/backend/ftp/ftp.go b/backend/ftp/ftp.go
index 24916d161..c5c885e00 100644
--- a/backend/ftp/ftp.go
+++ b/backend/ftp/ftp.go
@@ -81,8 +81,22 @@ security from the server in order to upgrade a plain text connection
to an encrypted one. Cannot be used in combination with implicit FTP.`,
Default: false,
}, {
- Name: "concurrency",
- Help: "Maximum number of FTP simultaneous connections, 0 for unlimited.",
+ Name: "concurrency",
+ Help: strings.Replace(`Maximum number of FTP simultaneous connections, 0 for unlimited.
+
+Note that setting this is very likely to cause deadlocks so it should
+be used with care.
+
+If you are doing a sync or copy then make sure concurrency is one more
+than the sum of |--transfers| and |--checkers|.
+
+If you use |--check-first| then it just needs to be one more than the
+maximum of |--checkers| and |--transfers|.
+
+So for |concurrency 3| you'd use |--checkers 2 --transfers 2
+--check-first| or |--checkers 1 --transfers 1|.
+
+`, "|", "`", -1),
Default: 0,
Advanced: true,
}, {