Race Condition in put when use_sudo=True

[arnimarj]

Abstract

There is a race in put, and the use_sudo=True, and writing a file with the same content from multiple connections concurrently.

Steps To Reproduce

Run this script concurrently:

import os, itertools, StringIO

from fabric.api import task, run, put, settings

@task
def megaput():
	content = '0' * 1024

	with settings(host_string = '127.0.0.1', user = os.environ['USER']):
		run('mkdir -p /tmp/megaput')

		for i in itertools.count():
			put(StringIO.StringIO(content), '/tmp/megaput/file', use_sudo = True)
			print 'copied', i, 'times'

It seems that the name of temporary file created is a hash of the host string and the file content. When puting files with the same content to the same host, there is a race for the mv operation, and with non-zero probability only the first operation will succeed.

Adding a random string to https://github.com/fabric/fabric/blob/master/fabric/sftp.py#L164 seems to make this script work without problems (e.g. hasher.update(uuid.uuid4().hex)).

[ploxiln]

The pseudo-random hash currently depends on the destination path, not on the file content. So this is not as bad as it sounds: this doesn't happen for putting identical files to different places simultaneously, this happens for putting multiple files (could be different content) to the same place simultaneously.

Using a random temp file makes it so that all of the mv will succeed. But also it's simpler and more effective. So I'll make a PR for it anyway ...