Twisted Documentation: Using Processes

# Using Processes

## Overview

Along with connection to servers across the internet, Twisted also connects to local processes with much the same API. The API is described in more detail in the documentation of:

- `twisted.internet.interfaces.IReactorProcess`
- `twisted.internet.interfaces.IProcessTransport`
- `twisted.internet.protocol.ProcessProtocol`

## Running Another Process

Processes are run through the reactor, using `reactor.spawnProcess()`. Pipes are created to the child process, and added to the reactor core so that the application will not block while sending data into or pulling data out of the new process. `reactor.spawnProcess()` requires two arguments, processProtocol and executable, and optionally takes six more: arguments, environment, path, userID, groupID, and usePTY.

    from twisted.internet import reactor

    mypp = MyProcessProtocol()
    reactor.spawnProcess(processProtocol, executable, args=[program, arg1, arg2],
                         env={'HOME': os.environ['HOME']}, path,
                         uid, gid, usePTY, childFDs)

- `processProtocol` should be an instance of a subclass of `twisted.internet.protocol.ProcessProtocol`. The interface is described below.
- `executable` is the full path of the program to run. It will be connected to processProtocol.
- `args` is a list of command line arguments to be passed to the process. `args[0]` should be the name of the process.
- `env` is a dictionary containing the environment to pass through to the process.
- `path` is the directory to run the process in. The child will switch to the given directory just before starting the new program. The default is to stay in the current directory.
- `uid` and `gid` are the user ID and group ID to run the subprocess as. Of course, changing identities will be more likely to succeed if you start as root.
- `usePTY` specifies whether the child process should be run with a pty, or if it should just get a pair of pipes. Interactive programs (where you don't know when it may read or write) need to be run with ptys.
- `childFDs` lets you specify how the child's file descriptors should be set up. Each key is a file descriptor number (an integer) as seen by the child. 0, 1, and 2 are usually stdin, stdout, and stderr, but some programs may be instructed to use additional fds through command-line arguments or environment variables. Each value is either an integer specifying one of the parent's current file descriptors, the string r which creates a pipe that the parent can read from, or the string w which creates a pipe that the parent can write to. If `childFDs` is not provided, a default is used which creates the usual stdin-writer, stdout-reader, and stderr-reader pipes.

`args` and `env` have empty default values, but many programs depend upon them to be set correctly. At the very least, `args[0]` should probably be the same as `executable`. If you just provide `os.environ` for `env`, the child program will inherit the environment from the current process, which is usually the civilized thing to do (unless you want to explicitly clean the environment as a security precaution). The default is to give an empty `env` to the child.

`reactor.spawnProcess()` returns an instance that implements the `twisted.internet.interfaces.IProcessTransport`.

## Writing a ProcessProtocol

The ProcessProtocol you pass to spawnProcess is your interaction with the process. It has a very similar signature to a regular Protocol, but it has several extra methods to deal with events specific to a process. In our example, we will interface with 'wc' to create a word count of user-given text. First, we'll start by importing the required modules, and writing the initialization for our ProcessProtocol.

    from twisted.internet import protocol
    class WCProcessProtocol(protocol.ProcessProtocol):

        def __init__(self, text):
            self.text = text

When the ProcessProtocol is connected to the protocol, it has the connectionMade method called. In our protocol, we will write our text to the standard input of our process and then close standard input, to the let the process know we are done writing to it.

    def connectionMade(self):
            self.transport.write(self.text)
            self.transport.closeStdin()

At this point, the process has receieved the data, and it's time for us to read the results. Instead of being receieved in dataReceived, data from standard output is receieve in outReceived. This is to distinguish it from data on standard error.

    def outReceived(self, data):
            fieldLength = len(data) / 3
            lines = int(data[:fieldLength])
            words = int(data[fieldLength:fieldLength*2])
            chars = int(data[fieldLength*2:])
            self.transport.loseConnection()
            self.receiveCounts(lines, words, chars)

Now, the process has parsed the output, and ended the connection to the process. Then it sends the results on to the final method, receiveCounts. This is for users of the class to override, so as to do other things with the data. For our demonstration, we will just print the results.

    def receiveCounts(self, lines, words, chars):
            print 'Received counts from wc.'
            print 'Lines:', lines
            print 'Words:', words
            print 'Characters:', chars

We're done! To use our WCProcessProtocol, we create an instance, and pass it to spawnProcess.

    from twisted.internet import reactor
    wcProcess = WCProcessProtocol("accessing protocols through Twisted is fun!\n")
    reactor.spawnProcess(wcProcess, 'wc', ['wc'])
    reactor.run()

## Things that can happen to your ProcessProtocol

These are the methods that you can usefully override in your subclass of `ProcessProtocol`:

- `.connectionMade`: This is called when the program is started, and makes a good place to write data into the stdin pipe (using `self.transport.write()`).
- `.outReceived(data)`: This is called with data that was received from the process' stdout pipe. Pipes tend to provide data in larger chunks than sockets (one kilobyte is a common buffer size), so you may not experience the random dribs and drabs behavior typical of network sockets, but regardless you should be prepared to deal if you don't get all your data in a single call. To do it properly, `outReceived` ought to simply accumulate the data and put off doing anything with it until the process has finished.
- `.errReceived(data)`: This is called with data from the process' stderr pipe. It behaves just like `outReceived`.
- `.inConnectionLost`: This is called when the reactor notices that the process' stdin pipe has closed. Programs don't typically close their own stdin, so this will probably get called when your ProcessProtocol has shut down the write side with `self.transport.loseConnection()`.
- `.outConnectionLost`: This is called when the program closes its stdout pipe. This usually happens when the program terminates.
- `.errConnectionLost`: Same as `outConnectionLost`, but for stderr instead of stdout.

- `.processEnded(status)`: This is called when the child process has been reaped, and receives information about the process' exit status. The status is passed in the form of a `Failure` instance, created with a `.value` that either holds a `ProcessDone` object if the process terminated normally (it died of natural causes instead of receiving a signal, and if the exit code was 0), or a `ProcessTerminated` object (with an `.exitCode` attribute) if something went wrong. This scheme may seem a bit weird, but I trust that it proves useful when dealing with exceptions that occur in asynchronous code.

  This will always be called after`inConnectionLost`, `outConnectionLost`, and `errConnectionLost` are called.

The base-class definitions of these functions are all no-ops. This will result in all stdout and stderr being thrown away. Note that it is important for data you don't care about to be thrown away: if the pipe were not read, the child process would eventually block as it tried to write to a full pipe.

## Things you can do from your ProcessProtocol

The following are the basic ways to control the child process:

- `self.transport.write(data)`: Stuff some data in the stdin pipe. Note that this `write` method will queue any data that can't be written immediately. Writing will resume in the future when the pipe becomes writable again.
- `self.transport.closeStdin`: Close the stdin pipe. Programs which act as filters (reading from stdin, modifying the data, writing to stdout) usually take this as a sign that they should finish their job and terminate. For these programs, it is important to close stdin when you're done with it, otherwise the child process will never quit.
- `self.transport.closeStdout`: Not usually called, since you're putting the process into a state where any attempt to write to stdout will cause a SIGPIPE error. This isn't a nice thing to do to the poor process.
- `self.transport.closeStderr`: Not usually called, same reason as `closeStdout`.
- `self.transport.loseConnection`: Close all three pipes.
- `os.kill(self.transport.pid, signal.SIGKILL)`: Kill the child process. This will eventually result in `processEnded` being called.

## Verbose Example

Here is an example that is rather verbose about exactly when all the methods are called. It writes a number of lines into the `wc` program and then parses the output.

    #! /usr/bin/python

    from twisted.internet import protocol
    from twisted.internet import reactor
    import re

    class MyPP(protocol.ProcessProtocol):
        def __init__(self, verses):
            self.verses = verses
            self.data = ""
        def connectionMade(self):
            print "connectionMade!"
            for i in range(self.verses):
                self.transport.write("Aleph-null bottles of beer on the wall,\n" +
                                     "Aleph-null bottles of beer,\n" +
                                     "Take one down and pass it around,\n" +
                                     "Aleph-null bottles of beer on the wall.\n")
                self.transport.closeStdin() # tell them we're done
        def outReceived(self, data):
            print "outReceived! with %d bytes!" % len(data)
            self.data = self.data + data
        def errReceived(self, data):
            print "errReceived! with %d bytes!" % len(data)
        def inConnectionLost(self):
            print "inConnectionLost! stdin is closed! (we probably did it)"
        def outConnectionLost(self):
            print "outConnectionLost! The child closed their stdout!"
            # now is the time to examine what they wrote
            #print "I saw them write:", self.data
            (dummy, lines, words, chars, file) = re.split(r'\s+', self.data)
            print "I saw %s lines" % lines
        def errConnectionLost(self):
            print "errConnectionLost! The child closed their stderr."
        def processEnded(self, status_object):
            print "processEnded, status %d" % status_object.value.exitCode
            print "quitting"
            reactor.stop()

    pp = MyPP(10)
    reactor.spawnProcess(pp, "wc", ["wc"], {})
    reactor.run()

Source listing - listings/process/process.py

The exact output of this program depends upon the relative timing of some un-synchronized events. In particular, the program may observe the child process close its stderr pipe before or after it reads data from the stdout pipe. One possible transcript would look like this:

    % ./process.py
    connectionMade!
    inConnectionLost! stdin is closed! (we probably did it)
    errConnectionLost! The child closed their stderr.
    outReceived! with 24 bytes!
    outConnectionLost! The child closed their stdout!
    I saw 40 lines
    processEnded, status 0
    quitting
    Main loop terminated.
    %

## Doing it the Easy Way

Frequently, one just needs a simple way to get all the output from a program. In the blocking world, you might use `commands.getoutput` from the standard library, but using that in an event-driven program will cause everything else to stall until the command finishes. (in addition, the SIGCHLD handler used by that function does not play well with Twisted's own signal handling). For these cases, the `twisted.internet.utils.getProcessOutput` function can be used. Here is a simple example:

    from twisted.internet import protocol, utils, reactor
    from twisted.python import failure
    from cStringIO import StringIO

    class FortuneQuoter(protocol.Protocol):

        fortune = '/usr/games/fortune'

        def connectionMade(self):
            output = utils.getProcessOutput(self.fortune)
            output.addCallbacks(self.writeResponse, self.noResponse)

        def writeResponse(self, resp):
            self.transport.write(resp)
            self.transport.loseConnection()

        def noResponse(self, err):
            self.transport.loseConnection()

    if __name__ == '__main__':
        f = protocol.Factory()
        f.protocol = FortuneQuoter
        reactor.listenTCP(10999, f)
        reactor.run()

Source listing - listings/process/quotes.py

If you only need the final exit code (like `commands.getstatusoutput(cmd)[0]`), the `twisted.internet.utils.getProcessValue` function is useful. Here is an example:

    from twisted.internet import utils, reactor

    def printTrueValue(val):
        print "/bin/true exits with rc=%d" % val
        output = utils.getProcessValue('/bin/false')
        output.addCallback(printFalseValue)

    def printFalseValue(val):
        print "/bin/false exits with rc=%d" % val
        reactor.stop()

    output = utils.getProcessValue('/bin/true')
    output.addCallback(printTrueValue)
    reactor.run()

Source listing - listings/process/trueandfalse.py

## Mapping File Descriptors

stdin, stdout, and stderr are just conventions. Programs which operate as filters generally accept input on fd0, write their output on fd1, and emit error messages on fd2. This is common enough that the standard C library provides macros like stdin to mean fd0, and shells interpret the pipe character | to mean redirect fd1 from one command into fd0 of the next command.

But these are just conventions, and programs are free to use additional file descriptors or even ignore the standard three entirely. The childFDs argument allows you to specify exactly what kind of files descriptors the child process should be given.

Each child FD can be put into one of three states:

- Mapped to a parent FD: this causes the child's reads and writes to come from or go to the same source/destination as the parent.
- Feeding into a pipe which can be read by the parent.
- Feeding from a pipe which the parent writes into.

Mapping the child FDs to the parent's is very commonly used to send the child's stderr output to the same place as the parent's. When you run a program from the shell, it will typically leave fds 0, 1, and 2 mapped to the shell's 0, 1, and 2, allowing you to see the child program's output on the same terminal you used to launch the child. Likewise, inetd will typically map both stdin and stdout to the network socket, and may map stderr to the same socket or to some kind of logging mechanism. This allows the child program to be implemented with no knowledge of the network: it merely speaks its protocol by doing reads on fd0 and writes on fd1.

Feeding into a parent's read pipe is used to gather output from the child, and is by far the most common way of interacting with child processes.

Feeding from a parent's write pipe allows the parent to control the child. Programs like bc or ftp can be controlled this way, by writing commands into their stdin stream.

The childFDs dictionary maps file descriptor numbers (as will be seen by the child process) to one of these three states. To map the fd to one of the parent's fds, simply provide the fd number as the value. To map it to a read pipe, use the string r as the value. To map it to a write pipe, use the string w.

For example, the default mapping sets up the standard stdin/stdout/stderr pipes. It is implemented with the following dictionary:

    childFDs = { 0: "w", 1: "r", 2: "r" }

To launch a process which reads and writes to the same places that the parent python program does, use this:

    childFDs = { 0: 0, 1: 1, 2: 2}

To write into an additional fd (say it is fd number 4), use this:

    childFDs = { 0: "w", 1: "r", 2: "r" , 4: "w"}

### ProcessProtocols with extra file descriptors

When you provide a childFDs dictionary with more than the normal three fds, you need addtional methods to access those pipes. These methods are more generalized than the `.outReceived` ones described above. In fact, those methods (`outReceived` and `errReceived`) are actually just wrappers left in for compatibility with older code, written before this generalized fd mapping was implemented. The new list of things that can happen to your ProcessProtocol is as follows:

- `.connectionMade`: This is called when the program is started.
- `.childDataReceived(childFD, data)`: This is called with data that was received from one of the process' output pipes (i.e. where the childFDs value was r. The actual file number (from the point of view of the child process) is in childFD. For compatibility, the default implementation of `.dataReceived` dispatches to `.outReceived` or `.errReceived` when childFD is 1 or 2.
- `.childConnectionLost(childFD)`: This is called when the reactor notices that one of the process' pipes has been closed. This either means you have just closed down the parent's end of the pipe (with `.transport.closeChildFD`), the child closed the pipe explicitly (sometimes to indicate EOF), or the child process has terminated and the kernel has closed all of its pipes. The childFD argument tells you which pipe was closed. Note that you can only find out about file descriptors which were mapped to pipes: when they are mapped to existing fds the parent has no way to notice when they've been closed. For compatibility, the default implementation dispatches to `.inConnectionLost`, `.outConnectionLost`, or `.errConnectionLost`.
- `.processEnded(status)`: This is called when the child process has been reaped, and all pipes have been closed. This insures that all data written by the child prior to its death will be received before `.processEnded` is invoked.

In addition to those methods, there are other methods available to influence the child process:

- `self.transport.writeToChild(childFD, data)`: Stuff some data into an input pipe. `.write` simply writes to childFD=0.
- `self.transport.closeChildFD(childFD)`: Close one of the child's pipes. Closing an input pipe is a common way to indicate EOF to the child process. Closing an output pipe is neither very friendly nor very useful.
- `os.kill(self.transport.pid, signal.SIGKILL)`: Kill the child process. This will eventually result in `processEnded` being called.

### Examples

GnuPG, the encryption program, can use additional file descriptors to accept a passphrase and emit status output. These are distinct from stdin (used to accept the crypttext), stdout (used to emit the plaintext), and stderr (used to emit human-readable status/warning messages). The passphrase FD reads until the pipe is closed and uses the resulting string to unlock the secret key that performs the actual decryption. The status FD emits machine-parseable status messages to indicate the validity of the signature, which key the message was encrypted to, etc.

gpg accepts command-line arguments to specify what these fds are, and then assumes that they have been opened by the parent before the gpg process is started. It simply performs reads and writes to these fd numbers.

To invoke gpg in decryption/verification mode, you would do something like the following:

    class GPGProtocol(ProcessProtocol):
        def __init__(self, crypttext):
            self.crypttext = crypttext
            self.plaintext = ""
            self.status = ""
        def connectionMade(self):
            self.transport.writeToChild(3, self.passphrase)
            self.transport.closeChildFD(3)
            self.transport.writeToChild(0, self.crypttext)
            self.transport.closeChildFD(0)
        def childDataReceived(self, childFD, data):
            if childFD == 1: self.plaintext += data
            if childFD == 4: self.status += data
        def processEnded(self, status):
            rc = status.value.exitCode
            if rc == 0:
                self.deferred.callback(self)
            else:
                self.deferred.errback(rc)

    def decrypt(crypttext):
        gp = GPGProtocol(crypttext)
        gp.deferred = Deferred()
        cmd = ["gpg", "--decrypt", "--passphrase-fd", "3", "--status-fd", "4",
               "--batch"]
        p = reactor.spawnProcess(gp, cmd[0], cmd, env=None,
                                 childFDs={0:"w", 1:"r", 2:2, 3:"w", 4:"r"})
        return gp.deferred

In this example, the status output could be parsed after the fact. It could, of course, be parsed on the fly, as it is a simple line-oriented protocol. Methods from LineReceiver could be mixed in to make this parsing more convenient.

The stderr mapping (2:2) used will cause any GPG errors to be emitted by the parent program, just as if those errors had caused in the parent itself. This is sometimes desireable (it roughly corresponds to letting exceptions propagate upwards), especially if you do not expect to encounter errors in the child process and want them to be more visible to the end user. The alternative is to map stderr to a read-pipe and handle any such output from within the ProcessProtocol (roughly corresponding to catching the exception locally).

Index

Version: 2.4.0