issue #275: parent/ssh: centralize EC0_MARKER and change it for ssh.py.

Must maintain a minimum buffer length prior to deciding whether we have
an interesting token, and 'EC0' is too short for that.

docs/howitworks.rst

@@ -51,7 +51,7 @@ can be recovered by the bootstrapped process later. It then forks into a new
 process.
 
 After fork, the parent half overwrites its ``stdin`` with the read end of the
-pipe, and the child half writes the string ``EC0\n``, then begins reading the
+pipe, and the child half writes the string ``MITOGEN0\n``, then begins reading the
 :py:mod:`zlib`-compressed payload supplied on ``stdin`` by the master, and
 writing the decompressed result to the write-end of the UNIX pipe.
 
@@ -112,12 +112,17 @@ fetched from the master a second time.
 Signalling Success
 ##################
 
-Once the first stage has signalled ``EC0\n``, the master knows it is ready to
-receive the compressed bootstrap. After decompressing and writing the bootstrap
-source to its parent Python interpreter, the first stage writes the string
-``EC1\n`` to ``stdout`` before exiting. The master process waits for this
-string before considering bootstrap successful and the child's ``stdio`` ready
-to receive messages.
+Once the first stage has signalled ``MITO000\n``, the master knows it is ready
+to receive the compressed bootstrap. After decompressing and writing the
+bootstrap source to its parent Python interpreter, the first stage writes the
+string ``MITO001\n`` to ``stdout`` before exiting. The master process waits for
+this string before considering bootstrap successful and the child's ``stdio``
+ready to receive messages.
+
+The signal value is 8 bytes to match the minimum chunk size required to
+disambiguate between lines containing an interesting token during SSH password
+authentication, a debug message from the SSH client itself, or a message from
+the first stage.
 
 
 ExternalContext.main()

mitogen/parent.py

@@ -790,11 +790,11 @@ class Stream(mitogen.core.Stream):
                 sys.executable += sys.version[:3]
             os.environ['ARGV0']=sys.executable
             os.execl(sys.executable,sys.executable+'(mitogen:CONTEXT_NAME)')
-        os.write(1,'EC0\n')
+        os.write(1,'MITO000\n')
         C=_(os.fdopen(0,'rb').read(PREAMBLE_COMPRESSED_LEN),'zip')
         os.fdopen(W,'w',0).write(C)
         os.fdopen(w,'w',0).write('PREAMBLE_LEN\n'+C)
-        os.write(1,'EC1\n')
+        os.write(1,'MITO001\n')
 
     def get_boot_command(self):
         source = inspect.getsource(self._first_stage)
@@ -870,13 +870,17 @@ class Stream(mitogen.core.Stream):
             self._reap_child()
             raise
 
+    #: For ssh.py, this must be at least max(len('password'), len('debug1:'))
+    EC0_MARKER = 'MITO000\n'
+    EC1_MARKER = 'MITO001\n'
+
     def _ec0_received(self):
         LOG.debug('%r._ec0_received()', self)
         write_all(self.transmit_side.fd, self.get_preamble())
-        discard_until(self.receive_side.fd, 'EC1\n', self.connect_deadline)
+        discard_until(self.receive_side.fd, 'MITO001\n', self.connect_deadline)
 
     def _connect_bootstrap(self, extra_fd):
-        discard_until(self.receive_side.fd, 'EC0\n', self.connect_deadline)
+        discard_until(self.receive_side.fd, 'MITO000\n', self.connect_deadline)
         self._ec0_received()
 
 

mitogen/ssh.py

@@ -68,10 +68,10 @@ def filter_debug(stream, it):
         while buf:
             if state == 'start_of_line':
                 if len(buf) < 8:
-                    # short read near the buffer limit, block waiting for at
-                    # least 8 bytes so we can discern either a debug line, or
-                    # the minimum desired interesting token from above
-                    # ('password').
+                    # short read near buffer limit, block awaiting at least 8
+                    # bytes so we can discern a debug line, or the minimum
+                    # interesting token from above or the bootstrap
+                    # ('password', 'MITO000\n').
                     break
                 elif buf.startswith(DEBUG_PREFIXES):
                     state = 'in_debug'
@@ -229,7 +229,7 @@ class Stream(mitogen.parent.Stream):
 
         for buf in filter_debug(self, it):
             LOG.debug('%r: received %r', self, buf)
-            if buf.endswith('EC0\n'):
+            if buf.endswith(self.EC0_MARKER):
                 self._router.broker.start_receive(self.tty_stream)
                 self._ec0_received()
                 return

mitogen/su.py

@@ -97,7 +97,7 @@ class Stream(mitogen.parent.Stream):
 
         for buf in it:
             LOG.debug('%r: received %r', self, buf)
-            if buf.endswith('EC0\n'):
+            if buf.endswith(self.EC0_MARKER):
                 self._ec0_received()
                 return
             if any(s in buf.lower() for s in self.incorrect_prompts):

mitogen/sudo.py

@@ -168,7 +168,7 @@ class Stream(mitogen.parent.Stream):
 
         for buf in it:
             LOG.debug('%r: received %r', self, buf)
-            if buf.endswith('EC0\n'):
+            if buf.endswith(self.EC0_MARKER):
                 self._ec0_received()
                 return
             elif PASSWORD_PROMPT in buf.lower():