Fix 1.21.0 pass through regression

[coilysiren]

Changes

• adds an explicit "regression test" in its own file, distinct from the other tests
• changes the reorderArgs logic to ignore args that aren't defined for the command you are running
• refactors reorderArgs for understandability
• changes a TestCommandFlagParsing test case that was expecting an error, to no longer expect an error

Motivation

Fixes #850

#850 caused a stop everything and fix this now P0 bug that since then taken a few dozen hours of work to fully root-out and stabilize (eg. at my dayjob). Because of that, I've been working to get a change into this package to avoid that occurring in the future.

What I'm looking for here

• I feel like it's valuable to keep the new regression test I added distinct from the other tests, but do let me know if that doesn't make sense. I'm very ok with moving that test into command_test.go
• I put a lot of effort into making the logic in reorderArgs as clear as possible, please let me know if I can add more context / docs / update variable names / etc.
• The change to TestCommandFlagParsing is conceptually dangerous, but in a practical sense I think it's actually safe. Please let me know if you agree or disagree? I think it's safe in a practical sense because user's shouldn't be strictly relying on this package to raise "flag provided but not defined" errors in all kinds of nuanced places.

Future Work

The whole idea of argument re-ordering feels dangerous to me. Immediately after this PR I plan on investigating how I could potentially remove that functionality, without impacting the API.

[codecov]

Codecov Report

Merging #872 into master will increase coverage by 0.16%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #872      +/-   ##
==========================================
+ Coverage   71.48%   71.64%   +0.16%     
==========================================
  Files          30       30              
  Lines        2395     2409      +14     
==========================================
+ Hits         1712     1726      +14     
  Misses        577      577              
  Partials      106      106

Impacted Files	Coverage Δ
command.go	84.02% <100%> (+1.44%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 49ad3df...0480001. Read the comment docs.

[coilysiren]

#691 <= I looks like this PR is what caused the bug, but it doesn't revert cleanly. Which is to be expected, since it's 2 years old 😅 I'll figure this out

[coilysiren]

I believe what happened was that the reorder args functionality here https://github.com/urfave/cli/pull/691/files#diff-9c42b524cb14ca001c61267826cbefb1R206 started accidentally pulling in args meant for pass-through commands. So like the --no-trunc in my regression test.

[coilysiren]

There's a line in the flag re-ordering function that checks for --,

cli/command.go

Line 223 in 6cc7e98

if arg == "--" {

. This hints to me that we already kind of knew that flag re-ordering could cause issues with pass-through commands.

[coilysiren]

I'm putting this on hold until the v2 released plan is figured out => #826

[coilysiren]

👋 I have returned to this PR

[saschagrunert]

Just one non-blocking comment, the rest LGTM

[coilysiren]

review ping @AudriusButkevicius / @asahasrabuddhe

[asahasrabuddhe]

Suggested change

	nextIndexMayContainValue := false
	var nextIndexMayContainValue bool

This is also a possibility as it would be false by default! It's fine if you don't change it too :)

[coilysiren]

I'm gonna opt against adding this into the PR - but only because I wanna merge this ASAP so that we can fix the regression 🔧

[asahasrabuddhe]

This looks good 👍 I have added a very optional change. You may or may not choose to do it 😄