add exece

[aabacchus]

A wrapper of execve to be able to specify the environment of the new process. I'm not convinced this is good code: should the Lua strings, used to fill the C array envp, be removed from the stack before (or after) execve? Surely when execv returns (in case of error) there will be more values on the stack than reported by the return value of the function?

Also, valgrind reports errors with my function but not with unistd.exec, I'm not sure why (both leave extra Lua strings on the stack).

Also, would it be better to change unistd.exec to have an optional env arg rather than a separate unistd.exece function?

[aabacchus]

I suppose the same features could be implemented using stdlib.setenv and unistd.exec but that would be a) much more verbose, b) probably slower, and c) execve is in POSIX, after all.

[aabacchus]

Indeed, I am currently using this in place of an exece:

-- path is a string, cmd is an array, env is a table
-- Despite the variable name, path doesn't have to be absolute because execp is used.
function run(path, cmd, env)
    io.stderr:write(path .. ' ' .. table.concat(cmd, ' ', 1) .. '\n')

    local pid = unistd.fork()

    if not pid then
        die("fork failed")
    elseif pid == 0 then
        if env then
            for k,v in pairs(env) do
                stdlib.setenv(k, v)
            end
        end

        unistd.execp(path, cmd)
    else
        local _, msg, code = sys_wait.wait(pid)
        if msg ~= "exited" then
            die("run failed: " .. msg)
        end
        return code == 0
    end
end