swapspace buffers stdout in systemd, which makes SIGUSR1 not show in journalctl

[adamsmd]

Problem

Running swapspace under systemd seems to have stdout (non-line) buffering on. This means that calls to kill -SIGUSR1 do not show in journalctl -u swapspace.service immediately (though repeated calls will eventually fill the buffer causing the buffer to flush to stdout).

Suspected Cause

I am not in a position to test this, but I suspect this is due to the C standard library's default buffering behavior, which is to line buffer on TTYs but (non-line) buffer on anything else, and that under systemd the stdout is not a TTY.

Suggested Change

As far as I can tell, swapspace does not currently provide a direct way to resolve this, so I would suggest a change like one of the two following (though I don't know enough about your design to know which is best).

Option 1: Have swapspace call setlinebuffer or setvbuf explicitly.

Option 2: Have the logging methods in swapspace explicitly call flush() after printing.

I think that swapspace outputs small enough amounts of output that one could have these always on, but you could also have these controlled by a configuration/command-line flag.

Option 1 has the advantage of covering all outputs without having to worry about missing any, but Option 2 gives you the option of controlling which outputs flush the buffer.

Workaround

A wrapper such as expect's unbuffer is working for me in the meantime. Thus, my swapspace.service file contains:

ExecStart=/usr/bin/unbuffer /usr/bin/swapspace ...

My understanding is that unbuffer simulates a TTY on the stdout of swapspace, which makes the C library use line buffering for stdout.

[Tookmund]

Thank you for the investigation and suggested solutions!

I'm likely to implement option 1, since Swapspace doesn't have much output, but will probably do a bit more research myself first to understand the implications of this change.