parent: allow Python to determine its install prefix from argv[0]

Fixes support for virtualenv. Closes #152.

docs/api.rst

@@ -525,6 +525,11 @@ Router Class
 
             If unspecified, defaults to ``<username>@<hostname>:<pid>``.
 
+            This variable cannot contain slash characters, as the resulting
+            ``argv[0]`` must be presented in such a way as to allow Python to
+            determine its installation prefix. This is required to support
+            virtualenv.
+
         :param str python_path:
             Path to the Python interpreter to use for bootstrap. Defaults to
             ``python2.7``. In future this may default to ``sys.executable``.

docs/index.rst

@@ -68,7 +68,7 @@ using the UNIX **ps** command:
 
     20051 ?        Ss     0:00  \_ sshd: dmw [priv]
     20053 ?        S      0:00  |   \_ sshd: dmw@notty
-    20054 ?        Ssl    0:00  |       \_ mitogen:dmw@Eldil.home:22476
+    20054 ?        Ssl    0:00  |       \_ /usr/bin/python(mitogen:dmw@Eldil.home:22476)
     20103 ?        S      0:00  |           \_ tar zxvf myapp.tar.gz
 
 The example context was started by UID ``dmw`` on host ``Eldil.home``, process

mitogen/parent.py

@@ -313,6 +313,8 @@ class Stream(mitogen.core.Stream):
         if remote_name is None:
             remote_name = '%s@%s:%d'
             remote_name %= (getpass.getuser(), socket.gethostname(), os.getpid())
+        if '/' in remote_name or '\\' in remote_name:
+            raise ValueError('remote_name= cannot contain slashes')
         self.remote_name = remote_name
         self.debug = debug
         self.profiling = profiling
@@ -332,9 +334,11 @@ class Stream(mitogen.core.Stream):
     # Minimised, gzipped, base64'd and passed to 'python -c'. It forks, dups
     # file descriptor 0 as 100, creates a pipe, then execs a new interpreter
     # with a custom argv.
-    # 'CONTEXT_NAME', 'PREAMBLE_COMPRESSED_LEN', and 'PREAMBLE_LEN' are
+    #   * Optimized for minimum byte count after minification & compression.
+    #   * 'CONTEXT_NAME', 'PREAMBLE_COMPRESSED_LEN', and 'PREAMBLE_LEN' are
     #     substituted with their respective values.
-    # Optimized for minimum byte count after minification & compression.
+    #   * CONTEXT_NAME must be prefixed with the name of the Python binary in
+    #     order to allow virtualenvs to detect their install prefix.
     @staticmethod
     def _first_stage():
         R,W=os.pipe()
@@ -348,7 +352,7 @@ class Stream(mitogen.core.Stream):
             os.close(W)
             os.close(w)
             os.environ['ARGV0']=sys.executable
-            os.execl(sys.executable,'mitogen:CONTEXT_NAME')
+            os.execl(sys.executable,sys.executable+'(mitogen:CONTEXT_NAME)')
         os.write(1,'EC0\n')
         C=_(os.fdopen(0,'rb').read(PREAMBLE_COMPRESSED_LEN),'zip')
         os.fdopen(W,'w',0).write(C)