fix(net): Avoid a rare panic when a connection is dropped

[teor2345]

Motivation

Zebra sometimes panics when a Connection is dropped and there is no error response:

Message: ClientRequest oneshot sender must not be dropped before send: Canceled
Location: zebra-network/src/peer/client.rs:625
Metadata:
version: 1.0.0-rc.6

https://github.com/ZcashFoundation/zebra/actions/runs/4764031851/jobs/8495315588#step:8:543

This fixes a regression, the original ticket was #1471.

Complex Code or Requirements

This is a rare panic due to concurrency.

Solution

Replace the panic with returning a generic error.

Review

This is a low-priority security fix. It seems like it could be remotely triggerable, but I'm not sure.

Reviewer Checklist

• Will the PR name make sense to users?
  • Does it need extra CHANGELOG info? (new features, breaking changes, large changes)
• Are the PR labels correct?
• Does the code do what the ticket and PR says?
  • Does it change concurrent code, unsafe code, or consensus rules?
• How do you know it works? Does it have tests?

[dconnolly]

lgtm

[codecov]

Codecov Report

Merging #6566 (710812f) into main (fb3fec0) will increase coverage by 0.14%.
The diff coverage is 83.33%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6566      +/-   ##
==========================================
+ Coverage   77.67%   77.81%   +0.14%     
==========================================
  Files         307      307              
  Lines       40283    40287       +4     
==========================================
+ Hits        31288    31348      +60     
+ Misses       8995     8939      -56