Debugging Syncthing

There’s a lot that happens behind the covers, and Syncthing is generally quite silent about it. A number of environment variables can be used to set the logging to verbose for various parts of the program, and to enable profiling.

Under Unix (including Mac) the easiest way to run Syncthing with an environment variable set is to prepend the variable to the command line. I.e:

$ STTRACE=model syncthing

On windows, it needs to be set prior to running Syncthing.

C:\> set STTRACE=model
C:\> syncthing

Environment Variables

STTRACE

Used to increase the debugging verbosity in specific facilities, generally mapping to a Go package. Enter a comma-separated string of facilities to trace: api,beacon. Optionally, a log level can be given per facility to specify something other than DEBUG: api:WARN,beacon:ERR, potentially overriding a global --log-level adjustment.

The valid facility strings are listed below; additionally, syncthing serve --help always outputs the most up-to-date list.

api - REST API beacon - Multicast and broadcast discovery config - Configuration loading and saving connections - Connection handling db/sqlite - SQLite database dialer - Dialing connections discover - Remote device discovery events - Event generation and logging fs - Filesystem access main - Main package model - The root hub nat - NAT discovery and port mapping pmp - NAT-PMP discovery and port mapping protocol - The BEP protocol relay/client - Relay client scanner - File change detection and hashing stun - STUN functionality syncthing - Main run facility upgrade - Binary upgrades upnp - UPnP discovery and port mapping ur - Usage reporting versioner - File versioning watchaggregator - Filesystem event watcher

STLOCKTHRESHOLD

Used for debugging internal deadlocks; sets debug sensitivity. Use only under direction of a developer.

STVERSIONEXTRA

Add extra information to the version string in logs and the version line in the GUI. Can be set to the name of a wrapper or tool controlling syncthing to communicate this to the end user.

GOMAXPROCS

Set the maximum number of CPU cores to use. Defaults to all available CPU cores.

GOGC

Percentage of heap growth at which to trigger GC. Default is 100. Lower numbers keep peak memory usage down, at the price of CPU usage (i.e. performance).

LOGGER_DISCARD

Hack to completely disable logging, for example when running benchmarks. Set to any nonempty value to use it.

Stepping with breakpoints

If you like to step through the running program, build a non-optimized binary and run with https://github.com/derekparker/delve.

Follow these steps:

$ go run build.go -debug-binary build
$ STNODEFAULTFOLDER=1 STNOUPGRADE=1  STNORESTART=1 dlv --listen=:2345 --headless=true --api-version=2 exec ./syncthing -- --home=./_test_config --no-browser

For installing and using delve itself see: