fuzzing on date: date -I @ triggers a panic

[sylvestre]

Found with the fuzzer introduced in #4389

$ ../target/debug/coreutils date -I@
thread 'main' panicked at 'Invalid format: @', src/uu/date/src/date.rs:118:18
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[shanmukhateja]

I ran the same on GNU date and I see this.

$ date -I@
date: invalid argument ‘@’ for ‘--iso-8601’
Valid arguments are:
  - ‘hours’
  - ‘minutes’
  - ‘date’
  - ‘seconds’
  - ‘ns’
Try 'date --help' for more information.

@sylvestre What should the correct response be in this case?

[sylvestre]

at least

date:` invalid argument ‘@’ for ‘--iso-8601’

would be great

[shanmukhateja]

So do I just update the String in panic! or is there a something more you'd like me to add?

[sylvestre]

I think you need to handle the error properly

[shanmukhateja]

@sylvestre I noticed the panic is placed inside the impl for Iso8601Format enum. I went ahead and updated the error message to look like GNU's date.

After the change, the output is as follows:

Failed case:

$ target/debug/date -I@    
thread 'main' panicked at 'date:` invalid argument `@` for '--iso-8601'', src/uu/date/src/date.rs:118:18
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Success case:

target/debug/date -Ihours
2023-03-12T11+05:30

[tertsdiepraam]

Note that the comment on this panic says that this case should have been handled by clap, but it wasn't. So probably we need to add a value parser to the argument in clap.