diff options
Diffstat (limited to 'lib')
| -rw-r--r-- | lib/external_command.rb | 131 |
1 files changed, 131 insertions, 0 deletions
diff --git a/lib/external_command.rb b/lib/external_command.rb new file mode 100644 index 000000000..de8bfc11e --- /dev/null +++ b/lib/external_command.rb @@ -0,0 +1,131 @@ +# Run an external command, capturing its stdout and stderr +# streams into variables. +# +# So it’s rather like the `backtick` built-in, except that: +# - The command is run as-is, rather than being parsed by the shell; +# - Standard error is also captured. +# +# After the run() method has been called, the instance variables +# out, err and status contain the contents of the process’s stdout, +# the contents of its stderr, and the exit status. +# +# Example usage: +# require 'external_command' +# xc = ExternalCommand("ls", "-l").run() +# puts "Ran ls -l with exit status #{xc.status}" +# puts "===STDOUT===\n#{xc.out}" +# puts "===STDERR===\n#{xc.err}" +# +# The out and err attributes are writeable. If you assign +# a string, after calling the constructor and before calling +# run(), then the subprocess output/error will be appended +# to this string. + +# <rant author="robin"> +# In any sane language, this would be implemented with a +# single child process. The parent process would block on +# select(), and when the child process terminated, the +# select call would be interrupted by a CHLD signal +# and return EINTR. Unfortunately Ruby goes out of its +# way to prevent this from working, automatically restarting +# the select call if EINTR is returned. Therefore we +# use a parent-child-grandchild arrangement, where the +# parent blocks on select() and the child blocks on +# waitpid(). When the child detects that the grandchild +# has finished, it writes to a pipe that’s included in +# the parent’s select() for this purpose. +# </rant> + +class ExternalCommand + attr_accessor :out, :err + attr_reader :status + + def initialize(cmd, *args) + @cmd = cmd + @args = args + + # Strings to collect stdout and stderr from the child process + # These may be replaced by the caller, to append to existing strings. + @out = "" + @err = "" + @fin = "" + end + + def run() + # Pipes for parent-child communication + @out_read, @out_write = IO::pipe + @err_read, @err_write = IO::pipe + @fin_read, @fin_write = IO::pipe + + @pid = fork do + # Here we’re in the child process. + child_process + end + + # Here we’re in the parent process. + parent_process + + return self + end + + private + + def child_process() + # Reopen stdout and stderr to point at the pipes + STDOUT.reopen(@out_write) + STDERR.reopen(@err_write) + + # Close all the filehandles other than the ones we intend to use. + ObjectSpace.each_object(IO) do |fh| + fh.close unless ( + [STDOUT, STDERR, @fin_write].include?(fh) || fh.closed?) + end + + Process::waitpid(fork { grandchild_process }) + @fin_write.puts($?.exitstatus.to_s) + + exit! 0 + end + + def grandchild_process() + exec(@cmd, *@args) + + # This is only reached if the exec fails + @err_write.print("Failed to exec: #{[@cmd, *@args].join(' ')}") + exit! 99 + end + + def parent_process() + # Close the writing ends of the pipes + @out_write.close + @err_write.close + @fin_write.close + + @fhs = {@out_read => @out, @err_read => @err, @fin_read => @fin} + + while @fin.empty? + r = read_data + if r.nil? + raise "select() timed out even with a nil (infinite) timeout" + end + end + + Process::waitpid(@pid) + @status = @fin.to_i + @out_read.close + @err_read.close + end + + def read_data() + ready_array = IO.select(@fhs.keys, [], [], nil) + return false if ready_array.nil? + ready_array[0].each do |fh| + begin + @fhs[fh] << fh.readpartial(8192) + rescue EOFError + @fhs.delete fh + end + end + return true + end +end |