[Hangfire] Fix NullReferenceException for unresolvable jobs

[reeloadead]

Changes

The metrics instrumentation registers HangfireMetricsStateFilter, a global IApplyStateFilter. It formats a job name on every state transition, and JobNameFormatter dereferences BackgroundJob.Job with no null check.

When a job's invocation data cannot be deserialized (missing assembly, renamed type/method), Job is null. The filter throws NullReferenceException.

Affects all versions incl. 1.16.0-beta.1.

Note

This typically happens in platforms where the Hangfire scheduler (enqueues jobs) and the worker (executes jobs) are separate applications.
The job's type/method can exist in the scheduler but be missing in the worker, so the worker can't deserialize the job and Job is null.

Observed in production (stack trimmed):

"10 state change attempt(s) failed... moving job to the FailedState"
System.NullReferenceException
   at ...JobNameFormatter.FormatJobName(Job job)
   at ...JobNameFormatter.FormatJobName(BackgroundJob backgroundJob)
   at ...HangfireTagBuilder.GetDefinitionName(BackgroundJob backgroundJob)

Preceded by System.IO.FileNotFoundException: Could not resolve assembly '...' at InvocationData.DeserializeJob().

Fix: null-guard JobNameFormatter and return a sentinel when the job definition cannot be resolved.

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable) — n/a, no public API change

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: reeloadead / name: roman hrishin (00e09f7, 5fb9a24)

[github-actions]

Welcome, contributor! Thank you for your contribution to opentelemetry-dotnet-contrib.

Important reminders:

• Read our Contributing Guidelines.
• Sign the CLA if you haven't already.
• Follow the OpenTelemetry Generative AI policy: disclose any AI use in your contribution, and communicate (PR descriptions, review replies) in your own words rather than AI-generated text.
• Give reviewers at least a few days before pinging them for feedback.
• If you need help with general setup, development process, or contributor etiquette, ask in #opentelemetry-new-contributors.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.32%. Comparing base (41e7326) to head (da7e278).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4731   +/-   ##
=======================================
  Coverage   77.32%   77.32%           
=======================================
  Files         466      466           
  Lines       19710    19712    +2     
=======================================
+ Hits        15240    15242    +2     
  Misses       4470     4470           

Flag	Coverage Δ
unittests-Instrumentation.Hangfire	88.88% <100.00%> (+0.05%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...tation.Hangfire/Implementation/JobNameFormatter.cs	100.00% <100.00%> (ø)

🚀 New features to boost your workflow:

• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.