ccon: Add '--socket PATH' to support a create/start split

Along the lines of the create/start split we've been discussing for
the OCI [1,2].  Pull some common functionality into a libccon,
although I don't intend to make that a more than an internal helper.

Post-error cleanup in ccon-cli is pretty slim, since the process is
just about to die anyway.  I'll probably go back through and add
proper cleanup later.

serve_socket gets too deeply nested for my taste, but it's not quite
to the point where I'd pull out the request handling into a
handle_start_request helper or some such.  Mostly because I'd have to
either setup a new buf/iov/msg in the helper's stack or pass that all
through from serve_socket ;).

A few notes on the tests:

I don't have a stand-alone 'wait' on my system (it's built into most
shells [3]), but I've added an explicit check for it because POSIX
doesn't require it to be built in.

The waiting in the create/start tests is a bit awkward, but here's the
reasoning for the various steps:

* Put the socket in a 'sock' subdirectory so we don't have to mess
  with inotifywait's --exclude (when what we really want is an
  --include).  In most cases, the socket would be the first thing
  created in the test directory, but the process.host test will create
  a pivot-root.* before creating the socket.

* Clear a 'wait' file before launching the inotifywait/start subshell
  and append to it from that subshell so grep has something to look at
  (even if it racily starts looking before the subshell processes the
  inotifywait line).

* Block on a busy-wait grep until inotifywait sets up its watcher.
  This ensures we don't call ccon and create the socket before the
  watcher is looking.

  The zero-second sleep while we wait for inotifywait to setup its
  watcher is busy, but POSIX doesn't require 'sleep' to support
  non-integer times [4].

All of these issues could be abstracted out into an 'event' command
[5], but they're fine for a proof-of-concept create/start split.

[1]: https://groups.google.com/a/opencontainers.org/d/msg/dev/qWHoKs8Fsrk/k55FQrBzBgAJ
     Subject: Re: Splitting Start into create() and run()
     Date: Thu, 24 Mar 2016 15:04:14 -0700
     Message-ID: <[email protected]>
[2]: opencontainers/runtime-spec#384
     Subject: Split create and start
[3]: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap01.html#tag_17_06
[4]: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/sleep.html
[5]: opencontainers/runtime-spec#508
     Subject: runtime: Add an 'event' operation for subscribing to
       pushes

.gitignore

@@ -1,2 +1,3 @@
 ccon
+ccon-cli
 *.o

Makefile

@@ -6,16 +6,16 @@ LDLIBS := $(shell pkg-config --libs-only-l jansson libcap-ng)
 .PHONY: all clean fmt
 .PRECIOUS: %.o
 
-all: ccon
+all: ccon ccon-cli
 
 %.o: %.c
 	$(CC) $(CFLAGS) -c -o "$@" "$<"
 
-ccon: %: %.o
-	$(CC) $(LDFLAGS) -o "$@" "$<" $(LDLIBS) 
+ccon ccon-cli: %: %.o libccon.o
+	$(CC) $(LDFLAGS) -o "$@" $^ $(LDLIBS)
 
 clean:
-	rm -f *.o ccon
+	rm -f *.o ccon ccon-cli
 
 fmt:
-	indent --ignore-profile --linux-style *.c
+	indent --ignore-profile --linux-style *.h *.c

README.md

@@ -11,6 +11,10 @@ than [LXC][lxc.container.conf.5]).
 ## Table of contents
 
 * [Lifecycle](#lifecycle)
+* [Socket communication](#socket-communication)
+  * [Getting the container process's
+    PID](#getting-the-container-processs-pid)
+  * [Start request](#start-request)
 * [Configuration](#configuration)
   * [Version](#version)
   * [Namespaces](#namespaces)
@@ -58,8 +62,11 @@ synchronize the container setup.  Here's an outline of the lifecycle:
 |                                  | mounts filesystems            |
 |                                  | ← sends namespaces-complete   |
 | runs pre-start hooks             | blocks on exec-message        |
-| sends exec-message →             |                               |
-|                                  | opens the local ptmx          |
+| binds to socket path             |                               |
+| sends connection socket →        |                               |
+| blocks on exec-process message   | listens for process JSON      |
+|                                  | ← sends exec-process message  |
+| removes socket path              | opens the local ptmx          |
 |                                  | ← sends pseudoterminal master |
 | waits on child death             | executes user process         |
 |   splicing standard streams      | …                             |
@@ -83,6 +90,77 @@ use [`nsenter`][nsenter.1] or a wrapping ccon invocation to join those
 namespaces before the main ccon invocation creates the new mount
 namespace.
 
+## Socket communication
+
+With `--socket=PATH`, ccon will bind a [`SOCK_SEQPACKET` Unix
+socket][unix.7] to `PATH`.  This path is created after namespace-setup
+completes, so users can use its presence as a trigger for further
+configuration (e.g. network setup) before [starting](#start-request)
+the [user-specified code](#process).  The path is removed after a
+[start request](#start-request) is received or after the container
+process exits, whichever comes first.
+
+The [`ccon-cli`](ccon-cli.c) program distributed with this repository
+is one client for the ccon socket.
+
+### Getting the container process's PID
+
+An [`SO_PEERCRED`][socket.7] request will return the container
+process's PID [in the receiving process's PID
+namespace][pid_namespaces.7].  The client can use this to look up the
+container process in their local [`/proc`][proc.5].  This request may
+be performed as many times as you like.
+
+### Start request
+
+The request is a single [`struct iovec`][recv.2] containing either a
+leading null byte or process JSON.  Sending a single null-byte message
+will trigger the [**`process`**](#process) field present in the
+original configuration, while non-empty strings will completely
+override that field.
+
+The response is a single [`struct iovec`][recv.2] containing either a
+single null-byte message (for success) or an error message encoded in
+[ASCII][ascii.7].  In this context, “success” means “successfully
+received the start request”, because the container process sends the
+response before actually executing the
+[user-specified code](#process).
+
+If you set [**`host`**](#host) in your process JSON, `ccon-cli` will
+open the referenced path and pass the open file descriptor to the
+container over the Unix socket.
+
+### Example
+
+In one shell, launch ccon and have it listen on a socket at
+`/tmp/ccon-sock`:
+
+```
+$ ccon --socket /tmp/ccon-sock
+```
+
+In a second shell, get the container process's PID, but don't trigger
+the user-specified code:
+
+```
+$ PID=$(ccon-cli --socket /tmp/ccon-sock --pid)
+$ echo "${PID}"
+2186
+```
+
+You can then perform additional configuration using that PID:
+
+```
+$ ip link set ccon-ex-veth1 netns "${PID}"
+```
+
+And when you're finished setting up the environment, you can trigger
+the [user-specified code](#process):
+
+```
+$ ccon-cli  --socket /tmp/ccon-sock --config-string '{"args": ["busybox", "sh"]}'
+```
+
 ## Configuration
 
 Ccon is similar to an [Open Container Specification][ocs] runtime in
@@ -822,6 +900,7 @@ be distributed under the GPLv3+.
 [setgid.2]: http://man7.org/linux/man-pages/man2/setgid.2.html
 [setuid.2]: http://man7.org/linux/man-pages/man2/setuid.2.html
 [syscall.2]: http://man7.org/linux/man-pages/man2/syscall.2.html
+[recv.2]: http://man7.org/linux/man-pages/man2/recv.2.html
 [environ.3p]: https://www.kernel.org/pub/linux/docs/man-pages/man-pages-posix/
 [exec.3]: http://man7.org/linux/man-pages/man3/exec.3.html
 [getcwd.3]: http://man7.org/linux/man-pages/man3/getcwd.3.html
@@ -830,11 +909,14 @@ be distributed under the GPLv3+.
 [pts.4]: http://man7.org/linux/man-pages/man4/pty.4.html
 [filesystems.5]: http://man7.org/linux/man-pages/man5/filesystems.5.html
 [lxc.container.conf.5]: https://linuxcontainers.org/lxc/manpages/man5/lxc.container.conf.5.html
+[proc.5]: https://linuxcontainers.org/lxc/manpages/man5/proc.5.html
+[ascii.7]: http://man7.org/linux/man-pages/man7/ascii.7.html
 [capabilities.7]: http://man7.org/linux/man-pages/man7/capabilities.7.html
 [namespaces.7]: http://man7.org/linux/man-pages/man7/namespaces.7.html
 [pid_namespaces.7]: http://man7.org/linux/man-pages/man7/pid_namespaces.7.html
 [pty.7]: http://man7.org/linux/man-pages/man7/pty.7.html
 [signal.7]: http://man7.org/linux/man-pages/man7/signal.7.html
+[socket.7]: http://man7.org/linux/man-pages/man7/socket.7.html
 [unix.7]: http://man7.org/linux/man-pages/man7/unix.7.html
 [user_namespaces.7]: http://man7.org/linux/man-pages/man7/user_namespaces.7.html
 [mount.8]: http://man7.org/linux/man-pages/man8/pty.8.html
@@ -843,4 +925,5 @@ be distributed under the GPLv3+.
 [cgroups]: https://www.kernel.org/doc/Documentation/cgroup-v1/cgroups.txt
 [cgroups-unified]: https://www.kernel.org/doc/Documentation/cgroup-v2.txt
 [devpts]: https://www.kernel.org/doc/Documentation/filesystems/devpts.txt
+[rfc1345.s5]: https://tools.ietf.org/html/rfc1345#section-5
 [sd_listen_fds]: http://www.freedesktop.org/software/systemd/man/sd_listen_fds.html