config: Make 'process.args' optional

Since be59415 (Split create and start, 2016-04-01, opencontainers#384), it's
possible for a container process to never execute user-specified code
(e.g. you can call 'create', 'kill', 'delete' without calling
'start').  For folks who expect to do that, there's no reason to
define process.args.

The only other process property required for all platforms is 'cwd',
but the runtime's idler code isn't specified in sufficient detail for
the configuration author to have an opinion about what its working
directory should be.

On Linux and Solaris, 'user' is also required for 'uid' and 'gid'.  My
preferred approach here is to make those optional and define defaults
[1,2]:

  If unset, the runtime will not attempt to manipulate the user ID
  (e.g. not calling setuid(2) or similar).

But the maintainer consensus is that they want those to be explicitly
required properties [3,4,5].  With the current spec, one option could
be to make process optional (with the idler's working directory
unspecified) for OSes besides Linux and Solaris.  On Windows, username
is optional, but it's not clear how intentional that was [6].

[1]: opencontainers#417 (comment)
[2]: https://groups.google.com/a/opencontainers.org/forum/#!topic/dev/DWdystx5X3A
     Subject: Exposing platform defaults
     Date: Thu, 14 Jan 2016 15:36:26 -0800
     Message-ID: <20160114233625.GN6362@odin.tremily.us>
[3]: http://ircbot.wl.linuxfoundation.org/meetings/opencontainers/2016/opencontainers.2016-05-04-17.00.log.html#l-44
[4]: opencontainers#417 (comment)
[5]: opencontainers#417 (comment)
[6]: opencontainers#618

Signed-off-by: W. Trevor King <wking@tremily.us>

config.md

@@ -128,7 +128,7 @@ See links for details about [mountvol](http://ss64.com/nt/mountvol.html) and [Se
 * **`env`** (array of strings, OPTIONAL) contains a list of variables that will be set in the process's environment prior to execution.
   Elements in the array are specified as Strings in the form "KEY=value".
   The left hand side MUST consist solely of letters, digits, and underscores `_` as outlined in [IEEE Std 1003.1-2001](http://pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap08.html).
-* **`args`** (array of strings, REQUIRED) executable to launch and any flags as an array.
+* **`args`** (array of strings, OPTIONAL) executable to launch and any flags as an array.
   The executable is the first element and MUST be available at the given path inside of the rootfs.
   If the executable path is not an absolute path then the search $PATH is interpreted to find the executable.
 

runtime.md

@@ -103,6 +103,7 @@ This operation MUST generate an error if it is not provided the container ID.
 Attempting to start a container that does not exist MUST generate an error.
 Attempting to start an already started container MUST have no effect on the container and MUST generate an error.
 This operation MUST run the user-specified code as specified by [`process`](config.md#process-configuration).
+If `process.args` was not configured, the runtime MUST generate an error.
 
 Upon successful completion of this operation the `status` property of this container MUST be `running`.
 

schema/config-schema.json

@@ -74,8 +74,7 @@
             "id": "https://opencontainers.org/schema/bundle/process",
             "type": "object",
             "required": [
-                "cwd",
-                "args"
+                "cwd"
             ],
             "properties": {
                 "args": {

specs-go/config.go

@@ -38,7 +38,7 @@ type Process struct {
 	// User specifies user information for the process.
 	User User `json:"user"`
 	// Args specifies the binary and arguments for the application to execute.
-	Args []string `json:"args"`
+	Args []string `json:"args,omitempty"`
 	// Env populates the process environment for the process.
 	Env []string `json:"env,omitempty"`
 	// Cwd is the current working directory for the process and must be