Effectively nobody uses it, it cost more than it was worth, the code
doesn't carry its weight. Remove it and let any similar mechanism in the
future be an internal implementation detail.
Closes#10812
Signed-off-by: Jakob Borg <jakob@kastelo.net>
We've had weak/rolling hashing in the code for quite a while. It was a
popular request for a while, based on the belief that rsync does this
and we should too. However, the benefit is quite small; we save on
average about 0.8% of transferred blocks over the population as a whole:
<img width="974" alt="Screenshot 2025-03-28 at 17 09 02"
src="https://github.com/user-attachments/assets/bbe10dea-f85e-4043-9823-7cef1220b4a2"
/>
This would be fine if the cost was comparably low, however the downside
of attempting rolling hash matching is that we (by default) do a
complete file read on the destination in order to look for matches
before we starting pulling blocks for the file. For any larger file this
means a sometimes long, I/O-intensive pause before the file starts
syncing, for usually no benefit.
I propose we simply rip off the bandaid and save the effort.
This adds a config to enable debug functions on the API server, which is
by default disabled. When enabled, the /rest/debug things become
available and become available without requiring a CSRF token (although
authentication is required if configured).
We also add a new endpoint /rest/debug/cpuprof?duration=15s (with the
duration being configurable, defaulting to 30s). This runs a CPU profile
for the duration and returns it as a file. It sets headers so that a
browser will save the file with an informative name.
The same is done for heap profiles, /rest/debug/heapprof, which does not
take any parameters.
The purpose of this is that any user can enable debugging under
advanced, then point their browser to the endpoint above and get a file
that contains a CPU or heap profile we can use, with the filename
telling us what version and architecture the profile is from.
On the command line, this becomes
curl -O -J http://localhost:8082/rest/debug/cpuprof?duration=5s
curl: Saved to filename
'syncthing-cpu-darwin-amd64-v0.14.3+4-g935bcc0-110307.pprof'
GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3467
Jakob Borg
Simon Frei
Jakob Borg
Audrius Butkevicius