feat(chord): ErrorCallback to handle Group failures

[ggodik]

A chord is a Group and a task that executes when the Group is done with the results of the group.
If a task fails in that group - nothing gets called at the end of the group execution.

Proposal: NewChordWithError which would make a Chord(Group, SuccessTask, FailureTask) using the following logic:

if all tasks in group succeed:
     SuccessTask(results of group)
else:
     FailureTask(errors of failed tasks)

The FailureTask would take an aggregate of errors of all the failed tasks e.g. func FailureTask(errs []string) error
Any successful completions of the tasks in the group would be 'dropped'

Changes in this PR

• tasks.NewChordWithError() function that takes an additional Signature
• tasks.NewChord maintains backwards compatiblity
• results.AsyncChordResult behavior modified to fetch ErrorCallback results if one of the tasks fails
• worker.go modifications for processing ErrorChord
• Consolidated Chord Callback logic handling in single function: processChord
• Interfaces and Mocks for Server and Worker
• Unit tests
• Integration tests

Note: Unit tests made this a larger change as they required making interfaces out of Server and Worker and injecting those into processing functions. I'd love to take this effort further - but I wanted to keep this change as small as possible. It's already 16 files so I somewhat failed at that.

Let me know if you have thoughts on the utility of the change, etc

[ggodik]

cc @zeroshade

[codecov]

Codecov Report

Merging #605 (c0f13fa) into master (831c54f) will decrease coverage by 1.80%.
The diff coverage is 72.50%.

❗ Current head c0f13fa differs from pull request most recent head 26f15fe. Consider uploading reports for the commit 26f15fe to get more accurate results

@@            Coverage Diff             @@
##           master     #605      +/-   ##
==========================================
- Coverage   40.86%   39.05%   -1.81%     
==========================================
  Files          34       32       -2     
  Lines        2763     2929     +166     
==========================================
+ Hits         1129     1144      +15     
- Misses       1490     1641     +151     
  Partials      144      144              

Impacted Files	Coverage Δ
v1/server.go	27.55% <0.00%> (+5.03%)	⬆️
v1/tasks/signature.go	46.15% <ø> (+14.57%)	⬆️
v2/server.go	25.42% <0.00%> (ø)
v1/worker.go	16.58% <70.96%> (+14.35%)	⬆️
v1/tasks/workflow.go	90.32% <100.00%> (+63.39%)	⬆️
v1/common/broker.go	73.33% <0.00%> (-5.05%)	⬇️
v1/common/redis.go	0.00% <0.00%> (ø)
v1/config/config.go	75.00% <0.00%> (ø)
v1/backends/redis/redis.go	67.08% <0.00%> (ø)
v1/backends/redis/goredis.go	0.00% <0.00%> (ø)
... and 11 more

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