Fix unfailing CheckExternalCommands

[skylt]

On Unix, with shell=True, the shell default to /bin/sh.
Using Popen(['type', command], shell=True) is equivalent to calling
Popen(['/bin/sh', '-c', 'type', command]).
In this case 'command' becomes a positional parameter to the shell,
and not an argument to the command 'type'.

One solution would be to pass a single string as parameter.
The problem is that with shell=True, we wouldn't be safe from a shell injection,
so it is wiser to use a python only solution.

The package distutils is part of the standard distribution, so it doesn't add
extra dependencies.
The method find_executable has the same behaviour as 'which' on bash, it
will return the path to the executable or None if it was not found.

Logging needs to be added as it was previously handled by log_check_call().

This fixes #380 which was broken here.

[andsens]

I checked out the implementation of find_executable, and it honestly leaves a little to be desired:

def find_executable(executable, path=None):
    """Tries to find 'executable' in the directories listed in 'path'.

    A string listing directories separated by 'os.pathsep'; defaults to
    os.environ['PATH'].  Returns the complete filename or None if not found.
    """
    if path is None:
        path = os.environ['PATH']
    paths = path.split(os.pathsep)
    base, ext = os.path.splitext(executable)

    if (sys.platform == 'win32' or os.name == 'os2') and (ext != '.exe'):
        executable = executable + '.exe'

    if not os.path.isfile(executable):
        for p in paths:
            f = os.path.join(p, executable)
            if os.path.isfile(f):
                # the file exists, we have a shot at spawn working
                return f
        return None
    else:
        return executable

Note that there is no check for executability. Also, we will not be able to check the availability of shell functions or shell builtins with this.

I think your suggestion of making the whole checking command a string might be a better solution to this problem, we needn't worry about shell injection too much, since all of the input is trusted.

[skylt]

I have updated with an executability check.
Shell builtins are not used in the project, and I see no valid use for them.
The same goes for shell functions, wouldn't it better to implement them in python directly ?

[andsens]

eh, true. Alright, I'm fine with merging this. Nice touch with adding debug logging, since there is no log_check_call any longer :-)