Avoid listing code path directories

[josevalim]

If a code path directory has too many .beam files,
then list_dir/1 becomes a quite expensive operation.
With this commit, code:where_is_file/2 and
code:which/1 attempt to directly find a file,
instead of listing the directory.

[github-actions]

CT Test Results

       2 files       65 suites   1h 3m 39s ⏱️
1 508 tests 1 256 ✔️ 252 💤 0 ❌
1 700 runs  1 399 ✔️ 301 💤 0 ❌

Results for commit 20f3c11.

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// Erlang/OTP Github Action Bot

[josevalim]

It is unclear why the previous code was written with list_dir/1 operations. I have blamed the code and it goes back to R13, so I was not able to learn much.

The proposed code should be faster in the huge majority of cases. That's because read_file_info/1 will only succeed when it finds a file (returning error most of the time) while list_dir will consistently list several entries per path. Here is some benchmark on top of kernel's ebin:

~/OSS/otp/lib/kernel/ebin[jv-which-read-file-info %]$ cat foo.exs
Mix.install [:benchee]
{:ok, p} = :file.get_cwd()

Benchee.run(
  %{
    "list_dir" => fn -> :erl_prim_loader.list_dir(p) end,
    "read_file_info" => fn -> :erl_prim_loader.read_file_info(p ++ ~c"unknown") end
  },
  time: 10,
  memory_time: 2
)
~/OSS/otp/lib/kernel/ebin[jv-which-read-file-info %]$ elixir foo.exs
Operating System: macOS
CPU Information: Apple M1 Max
Number of Available Cores: 10
Available memory: 32 GB
Elixir 1.16.0-dev
Erlang 26.0.2

Benchmark suite executing with the following configuration:
warmup: 2 s
time: 10 s
memory time: 2 s
reduction time: 0 ns
parallel: 1
inputs: none specified
Estimated total run time: 28 s

Benchmarking list_dir ...
Benchmarking read_file_info ...

Name                     ips        average  deviation         median         99th %
read_file_info      116.50 K        8.58 μs   ±243.74%        7.38 μs       37.96 μs
list_dir              7.75 K      128.96 μs    ±47.73%      106.17 μs      265.74 μs

Comparison:
read_file_info      116.50 K
list_dir              7.75 K - 15.02x slower +120.38 μs

**All measurements for memory usage were the same**