diff options
Diffstat (limited to 'modules/detect_time_jump')
| -rw-r--r-- | modules/detect_time_jump/.packaging/test.config | 4 | ||||
| -rw-r--r-- | modules/detect_time_jump/README.rst | 22 | ||||
| -rw-r--r-- | modules/detect_time_jump/detect_time_jump.lua | 45 |
3 files changed, 71 insertions, 0 deletions
diff --git a/modules/detect_time_jump/.packaging/test.config b/modules/detect_time_jump/.packaging/test.config new file mode 100644 index 0000000..7ed0e60 --- /dev/null +++ b/modules/detect_time_jump/.packaging/test.config @@ -0,0 +1,4 @@ +-- SPDX-License-Identifier: GPL-3.0-or-later +modules.load('detect_time_jump') +assert(detect_time_jump) +quit() diff --git a/modules/detect_time_jump/README.rst b/modules/detect_time_jump/README.rst new file mode 100644 index 0000000..066f5a3 --- /dev/null +++ b/modules/detect_time_jump/README.rst @@ -0,0 +1,22 @@ +.. SPDX-License-Identifier: GPL-3.0-or-later + +.. _mod-detect_time_jump: + +Detect discontinuous jumps in the system time +============================================= + +This module detect discontinuous jumps in the system time when resolver +is running. It clears cache when a significant backward time jumps occurs. + +Time jumps are usually created by NTP time change or by admin intervention. +These change can affect cache records as they store timestamp and TTL in real +time. + +If you want to preserve cache during time travel you should disable +this module by ``modules.unload('detect_time_jump')``. + +Due to the way monotonic system time works on typical systems, +suspend-resume cycles will be perceived as forward time jumps, +but this direction of shift does not have the risk of using records +beyond their intended TTL, so forward jumps do not cause erasing the cache. + diff --git a/modules/detect_time_jump/detect_time_jump.lua b/modules/detect_time_jump/detect_time_jump.lua new file mode 100644 index 0000000..ab099e8 --- /dev/null +++ b/modules/detect_time_jump/detect_time_jump.lua @@ -0,0 +1,45 @@ +-- SPDX-License-Identifier: GPL-3.0-or-later +-- Module interface +local ffi = require('ffi') + +local mod = {} +mod.threshold = 10 * min +local event_id = nil + +-- Get time of last cache clear. Compute difference between realtime +-- and monotonic time. Compute difference of actual realtime and monotonic +-- time. In ideal case these differences should be almost same. +-- If they differ more than mod.threshold value then clear cache. +local function check_time() + local checkpoint = cache.checkpoint() + local cache_timeshift = checkpoint.walltime.sec * 1000 - checkpoint.monotime + local actual_timeshift = os.time() * 1000 - tonumber(ffi.C.kr_now()) + local jump_backward = cache_timeshift - actual_timeshift + if jump_backward > mod.threshold then + log("Detected backwards time jump, clearing cache.\n" .. + "But what does that mean? It means your future hasn't been written yet." + ) + cache.clear() + elseif -jump_backward > mod.threshold then + -- On Linux 4.17+ this shouldn't happen anymore: https://lwn.net/Articles/751482/ + log("Detected forward time jump. (Suspend-resume, possibly.)") + cache.checkpoint(true) + end +end + +function mod.init() + if event_id then + error("Module is already loaded.") + else + event_id = event.recurrent(1 * min , check_time) + end +end + +function mod.deinit() + if event_id then + event.cancel(event_id) + event_id = nil + end +end + +return mod |