runtime: Add 'exit' to state for collecting the container exit code

This gives us a more portable way to discover the container exit code
(vs. requiring callers to use subreapers [1] or other
platform-specific approaches which require knowledge of the runtime
implementation).

[1]: opencontainers/runc#827 (comment)

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

runtime.md

@@ -22,6 +22,7 @@ The value MAY be one of:
 
     Additional values MAY be defined by the runtime, however, they MUST be used to represent new runtime states not defined above.
 * **`pid`** (int, REQUIRED when `status` is `created` or `running`) is the ID of the container process, as seen by the host.
+* **`exit`** (uint, REQUIRED when `status` is `stopped`) is the exit code of the container process.
 * **`bundlePath`** (string, REQUIRED) is the absolute path to the container's bundle directory.
 This is provided so that consumers can find the container's configuration and root filesystem on the host.
 * **`annotations`** (map, OPTIONAL) contains the list of annotations associated with the container.

schema/state-schema.json

@@ -27,6 +27,11 @@
             "type": "integer",
             "minimum": 0
         },
+        "exit": {
+            "id": "https://opencontainers.org/schema/runtime/state/exit",
+            "type": "integer",
+            "minimum": 0
+        },
         "bundlePath": {
             "id": "https://opencontainers.org/schema/runtime/state/bundlePath",
             "type": "string"

specs-go/state.go

@@ -10,6 +10,8 @@ type State struct {
 	Status string `json:"status"`
 	// Pid is the process ID for the container process.
 	Pid int `json:"pid"`
+	// Exit is the exit code the container process.
+	Exit uint `json:"exit,omitempty"`
 	// BundlePath is the path to the container's bundle directory.
 	BundlePath string `json:"bundlePath"`
 	// Annotations are the annotations associated with the container.