ls: reduce write syscalls & cleanup

[tertsdiepraam]

Instead of writing directly to stdout, with print! and println!, we can write to a BufWriter to reduce write syscalls. Doing so leads to the following improvements:

• GNU is now 4.06 times faster instead of 4.31.
• Our time spent on syscalls is now on par with GNU instead of 60% longer.
• Number of write calls goes from 344064 to 1090 when running ls -R on the Firefox source code.

The full benchmarks are in the details tag below. Because print! doesn't return a Result and write! does, I simply discarded the return values of write!.

Because this was a simple change I figured I could sneak in a few other changes to clean up the code some more. I hope that's okay. These changes are:

• Merging some imports
• Use Entry API with the cache HashMaps.
• Remove some redundant code from the indicator_style argument parsing.
• Make display_file_type return a char instead of String.

Full Benchmarks

Before

❯ hyperfine --warmup 2 'ls -R' '../coreutils/target/release/coreutils ls -R'
Benchmark #1: ls -R
  Time (mean ± σ):     537.6 ms ±   6.9 ms    [User: 379.8 ms, System: 156.0 ms]
  Range (min … max):   527.6 ms … 553.5 ms    10 runs
Benchmark #2: ../coreutils/target/release/coreutils ls -R

Time (mean ± σ):      2.315 s ±  0.026 s    [User: 2.089 s, System: 0.224 s]

Range (min … max):    2.287 s …  2.372 s    10 runs
Summary

'ls -R' ran

4.31 ± 0.07 times faster than '../coreutils/target/release/coreutils ls -R'

❯ strace -c ../coreutils/target/release/coreutils ls -R > /dev/null 
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ------------------
 72,26    6,736737          19    344064           write
 11,74    1,094781          29     37436           getdents64
  5,48    0,510923          27     18697           openat
  5,31    0,494837          26     18697           close
  5,14    0,479311          25     18697           newfstatat

After

❯ hyperfine --warmup 2 'ls -R' '../coreutils/target/release/coreutils ls -R'
Benchmark #1: ls -R
  Time (mean ± σ):     533.0 ms ±   3.2 ms    [User: 381.2 ms, System: 150.0 ms]
  Range (min … max):   528.2 ms … 540.5 ms    10 runs
Benchmark #2: ../coreutils/target/release/coreutils ls -R

Time (mean ± σ):      2.165 s ±  0.015 s    [User: 1.997 s, System: 0.165 s]

Range (min … max):    2.144 s …  2.195 s    10 runs
Summary

'ls -R' ran

4.06 ± 0.04 times faster than '../coreutils/target/release/coreutils ls -R'

❯ strace -c ../coreutils/target/release/coreutils ls -R > /dev/null
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ------------------
 41,39    0,510284          13     37436           getdents64
 19,68    0,242657          12     18697           openat
 19,38    0,238876          12     18697           close
 18,60    0,229333          12     18697           newfstatat
  0,51    0,006314           5      1090           write

[sylvestre]

well done :)

[siebenHeaven]

Please see if we can use something like in #2111 where we only buffer if stdout is not interactive (instead of buffering always).
Always buffering will slightly impact user experience when looking at output on a terminal as nothing will be printed until buffer is flushed.

Might I also suggest keeping this as a global because it might unnecessarily complicate function signatures (although globals are generally frowned upon, this seems like valid usecase to me - please feel free to ignore if you disagree :) )

[tertsdiepraam]

I figured we could just always buffer because the output of ls should always be generated fairly quickly. At least when I tried there seemed to be no real visible difference. The only time this might be an issue is with an --ignore flag that hides a lot of files? One thing I didn't like about that PR was that it needed Box<dyn Write>. Maybe it'd be nice to create a StdoutWriter type in uucore that automatically buffers if's not interactive?

I chose not to make this a global, because write! requires a mutable reference, so it would needs in a Mutex if we use lazy_static, which also complicates the code. Anyway, if you can find a nicer way to implement this, feel free to do so!

[siebenHeaven]

Agreed, I also noticed rust-lang/rust#60673 - seems like stdout buffering is already in the works.
As for the global related comment - yeah seems like the complicated lazy_static + mutex would be the way to implement it, I'll check if I can find a better way.

[tertsdiepraam]

That issue is very interesting, I hope some solution lands soon! Thanks for checking out the global variable!

[siebenHeaven]

looks good :)
Also thanks for cleaning up some things related to the gid'/uid caching. (I was seeing if we needed to get this into uucore - I do not see any usecases elsewhere that require caching at the moment).