[ruby-core:92456] [Ruby trunk Feature#15797] Use realpath(3) instead of custom realpath implementation if available

Issue #15797 has been updated by akr (Akira Tanaka).


PATH_MAX is dangerous.

Quotes from http://man7.org/linux/man-pages/man3/realpath.3.html

```
BUGS
       The POSIX.1-2001 standard version of this function is broken by
       design, since it is impossible to determine a suitable size for the
       output buffer, resolved_path.  According to POSIX.1-2001 a buffer of
       size PATH_MAX suffices, but PATH_MAX need not be a defined constant,
       and may have to be obtained using pathconf(3).  And asking
       pathconf(3) does not really help, since, on the one hand POSIX warns
       that the result of pathconf(3) may be huge and unsuitable for
       mallocing memory, and on the other hand pathconf(3) may return -1 to
       signify that PATH_MAX is not bounded.  The resolved_path == NULL
       feature, not standardized in POSIX.1-2001, but standardized in
       POSIX.1-2008, allows this design problem to be avoided.
```

----------------------------------------
Feature #15797: Use realpath(3) instead of custom realpath implementation if available
https://bugs.ruby-lang.org/issues/15797#change-77806

* Author: jeremyevans0 (Jeremy Evans)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
One reason to do this is simplicity, as this approach is ~30 lines of
code instead of ~200.

Performance wise, this performs 25%-115% better, using the following
benchmark on OpenBSD 6.5:

```ruby
require 'benchmark'

f = File
pwd = Dir.pwd
Dir.mkdir('b') unless f.directory?('b')
f.write('b/a', '') unless f.file?('b/a')

args = [
  ["b/a", nil],
  ["#{pwd}/b/a", nil],
  ['a', 'b'],
  ["#{pwd}/b/a", 'b'],
  ["b/a", pwd]
]

args.each do |path, base|
  print "File.realpath(#{path.inspect}, #{base.inspect}): ".ljust(50)
  puts Benchmark.measure{100000.times{f.realpath(path, base)}}
end
```

Before:

```
File.realpath("b/a", nil):                          4.330000   2.990000   7.320000 (  7.316244)
File.realpath("/home/testr/ruby/b/a", nil):         3.560000   2.680000   6.240000 (  6.240951)
File.realpath("a", "b"):                            4.370000   3.080000   7.450000 (  7.452511)
File.realpath("/home/testr/ruby/b/a", "b"):         3.730000   2.640000   6.370000 (  6.371979)
File.realpath("b/a", "/home/testr/ruby"):           3.590000   2.630000   6.220000 (  6.226824)
```

After:

```
File.realpath("b/a", nil):                          1.370000   2.030000   3.400000 (  3.400775)
File.realpath("/home/testr/ruby/b/a", nil):         1.260000   2.770000   4.030000 (  4.024957)
File.realpath("a", "b"):                            2.090000   1.990000   4.080000 (  4.080284)
File.realpath("/home/testr/ruby/b/a", "b"):         1.400000   2.620000   4.020000 (  4.015505)
File.realpath("b/a", "/home/testr/ruby"):           2.150000   2.760000   4.910000 (  4.910634)
```

If someone could benchmark before/after with this patch on Linux and/or MacOS X,
and post the results here, I would appreciate it.

My personal reason for wanting this is that the custom realpath
implementation does not work with OpenBSD's unveil(2) system call,
which limits access to the file system, allowing for security
similar to chroot(2), without most of the downsides.

This change passes all tests except for one assertion related to
taintedness.  Previously, if either argument to `File.realpath` is an
absolute path, then the returned value is considered not tainted.
However, I believe that behavior to be incorrect, because if there is
a symlink anywhere in the path, the returned value can contain a
section that was taken from the file system (unreliable source) that
was not marked as untainted. Example:

```ruby
Dir.mkdir('b') unless File.directory?('b')
File.write('b/a', '') unless File.file?('b/a')
File.symlink('b', 'c') unless File.symlink?('c')
path = File.realpath('c/a'.untaint, Dir.pwd.untaint)
path # "/home/testr/ruby/b/a"
path.tainted? # should be true, as 'b' comes from file system
```

I believe it is safer to always mark the output of realpath as tainted
to prevent this issue, which is what this commit does.

---Files--------------------------------
use-native-realpath.patch (6.31 KB)
use-native-realpath-v2.patch (4.64 KB)


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