fix crash when stopping/restarting with an invalid pidfile

[lpirl]

I use Tahoe as an vehicle for some fault injection experiments.
For a reason I am not interested in, a node ended up with an empty PID file in its directory (maybe due to a forced VM crash?).
When trying to stop or restart the node, it crashed since it did not handle non-numeric values in the PID file.
This PR attempts to fix this issue.
I decided to not remove the broken PID file, so operators can investigate if they wish to do so.

[lpirl]

Oh by the way, a node with an invalid PID file does not start. Is this intended behavior?

[meejah]

Patch looks good to me.

As far as "what to do", it does seem somewhat safer to not-start .. but by the same token, and invalid PID file could mean anything and if you just did tahoe run or tahoe start you must mean "I want it running", so trying to run seems fine to me if the PID-file is bogus. Other thoughts?

[lpirl]

Exactly, that is roughly what I considered.
Introducing a --keep-going/--try-harder/--force seems to be a bit overkill – or are there possible more use cases for such a flag?
For sure, when fully automating Tahoe (as I do), it is sort of annoying to check for and handle stale PID files because Tahoe cannot deal with them in all cases.

[meejah]

@warner any thoughts? I think we should merge this, and then also possibly merge something that just deletes a completely-invalid PID file and then keeps going.

[codecov-io]

Codecov Report

Merging #440 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #440      +/-   ##
==========================================
+ Coverage   87.91%   87.92%   +0.01%     
==========================================
  Files         148      148              
  Lines       27553    27557       +4     
  Branches     3953     3953              
==========================================
+ Hits        24222    24230       +8     
+ Misses       2621     2614       -7     
- Partials      710      713       +3

Impacted Files	Coverage Δ
src/allmydata/scripts/startstop_node.py	34.59% <100%> (+7.49%)	⬆️
src/allmydata/mutable/servermap.py	93.38% <0%> (-0.49%)	⬇️
src/allmydata/immutable/upload.py	95.25% <0%> (-0.28%)	⬇️
src/allmydata/web/status.py	82.97% <0%> (+0.11%)	⬆️

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[exarkun]

@lpirl I've created lpirl#1 which should fix the coverage checks. Can you merge that into your branch and then hopefully we can get a green build here - at which point I'd be happy to see this merged.

Thanks.

[meejah]

\o/ Good job everyone

[meejah]

BTW, we're planning on deprecating tahoe start and encouraging people to use tahoe run alongside their favorite daemonization-tool. Also, tahoe start doesn't do daemonization on Windows (it does what tahoe run does).