Fix nil pointer panic in Raft-dependent methods during startup (issue #1702)

[Copilot]

After node restart, Dkron panics with "runtime error: invalid memory address or nil pointer dereference" causing 5+ minute connection delays. The HTTP server starts before Raft initialization, creating a race where API requests can trigger nil pointer access.

Changes

Added nil checks to Raft-dependent methods:

• IsLeader() - returns false
• leaderMember() - returns ErrLeaderNotFound
• Leader() - returns empty ServerAddress
• applySetJob() - returns error
• RaftApply() - returns nil
• Stop() - guards raft.Shutdown() call

Added tests:

• Unit test verifying methods handle nil raft gracefully
• Integration test simulating API access during early startup

Example

Before:

func (a *Agent) IsLeader() bool {
    return a.raft.State() == raft.Leader  // panics if a.raft is nil
}

After:

func (a *Agent) IsLeader() bool {
    if a.raft == nil {
        return false
    }
    return a.raft.State() == raft.Leader
}

Fixes #1702

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• 127.0.0.10
  • Triggering command: /tmp/go-build1868015620/b001/dkron.test -test.testlogfile=/tmp/go-build1868015620/b001/testlog.txt -test.paniconexit0 -test.v=true -test.run=TestAgent|TestIsLeader -test.timeout=1m0s (packet block)
  • Triggering command: /tmp/go-build3610477801/b001/dkron.test -test.testlogfile=/tmp/go-build3610477801/b001/testlog.txt -test.paniconexit0 -test.v=true -test.run=API -test.timeout=2m0s (packet block)
• 127.0.0.11
  • Triggering command: /tmp/go-build830898429/b001/dkron.test -test.testlogfile=/tmp/go-build830898429/b001/testlog.txt -test.paniconexit0 -test.v=true -test.run=TestAgentCommand_runForElection -test.timeout=1m0s (packet block)
  • Triggering command: /tmp/go-build1868015620/b001/dkron.test -test.testlogfile=/tmp/go-build1868015620/b001/testlog.txt -test.paniconexit0 -test.v=true -test.run=TestAgent|TestIsLeader -test.timeout=1m0s (packet block)
• 127.0.0.12
  • Triggering command: /tmp/go-build3610477801/b001/dkron.test -test.testlogfile=/tmp/go-build3610477801/b001/testlog.txt -test.paniconexit0 -test.v=true -test.run=API -test.timeout=2m0s (packet block)
• 127.0.0.14
  • Triggering command: /tmp/go-build3610477801/b001/dkron.test -test.testlogfile=/tmp/go-build3610477801/b001/testlog.txt -test.paniconexit0 -test.v=true -test.run=API -test.timeout=2m0s (packet block)
• 127.0.0.15
  • Triggering command: /tmp/go-build3610477801/b001/dkron.test -test.testlogfile=/tmp/go-build3610477801/b001/testlog.txt -test.paniconexit0 -test.v=true -test.run=API -test.timeout=2m0s (packet block)
• 127.0.0.21
  • Triggering command: /tmp/go-build3610477801/b001/dkron.test -test.testlogfile=/tmp/go-build3610477801/b001/testlog.txt -test.paniconexit0 -test.v=true -test.run=API -test.timeout=2m0s (packet block)
• 127.0.0.22
  • Triggering command: /tmp/go-build3610477801/b001/dkron.test -test.testlogfile=/tmp/go-build3610477801/b001/testlog.txt -test.paniconexit0 -test.v=true -test.run=API -test.timeout=2m0s (packet block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

Investigate the possible cause #1702

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 31.62%. Comparing base (8b55676) to head (15ce925).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
dkron/agent.go	66.66%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1831      +/-   ##
==========================================
+ Coverage   31.39%   31.62%   +0.22%     
==========================================
  Files          76       76              
  Lines        6806     6817      +11     
==========================================
+ Hits         2137     2156      +19     
+ Misses       4375     4364      -11     
- Partials      294      297       +3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.