pillar/qmp: return a proper error if response on "query-status" can't be parsed

The 'query-status' QMP request is a special one: if status can't
be parsed - QEMU is stopped by EVE ('quit' is sent over QMP).

There are many customer complains on sudden QEMU stops with only
a few lines in the log, e.g.:

  "domain c54579e8-f67e-43fa-ae44-8e03584606c2.1.2 reported to be in unexpected state "
  "one of the c54579e8-f67e-43fa-ae44-8e03584606c2 tasks has crashed (domain c54579e8-f67e-43fa-ae44-8e03584606c2.1.2 reported to be in unexpected state )"
  "qmpEventHandler: Received event: SHUTDOWN event details: map[guest:false reason:host-qmp-quit]. Calling quit on socket: /run/hypervisor/kvm/c54579e8-f67e-43fa-ae44-8e03584606c2.1.2/qmp"

Which indicates that `getQemuStatus()` routine returns a "" (empty) string.
(according to the code the actual returned state is printed:
 "domain %s reported to be in unexpected state %s", domainName, res)

The `getQemuStatus()` can return a "" (empty string) and no error only
in one case: when received JSON does not match the expected structure.

This commit disallows unknown JSON fields, so that a returned string
should match exactly the structure.

This does not fix the error or sudden stop, but rather makes an error
explicit, so instead of:

   "domain c54579e8-f67e-43fa-ae44-8e03584606c2.1.2 reported to be in unexpected state "

the following error may appear:

   "couldn't retrieve status for domain c54579e8-f67e-43fa-ae44-8e03584606c2.1.2: json: unknown field "error"; (the bytes received: '{"error": {"class": "CommandNotFound", "desc": "Expecting capabilities negotiation with 'qmp_capabilities'"}}"

which explicitly indicates the expecting capabilities negotiations.
(the error is made up and does not reflect the real problem!)

Again. This does not fix a problem, rather exposes it with a clear
error message, so QEMU will be stopped by the domainmgr, but I hope
clear error help to fix a real problem.

Signed-off-by: Roman Penyaev <[email protected]>

Author: rouming

pkg/pillar/hypervisor/qmp.go

@@ -1,6 +1,7 @@
 package hypervisor
 
 import (
+	"bytes"
 	"context"
 	"encoding/json"
 	"fmt"
@@ -77,7 +78,14 @@ func getQemuStatus(socket string) (string, error) {
 				Status     string `json:"status"`
 			} `json:"return"`
 		}
-		err = json.Unmarshal(raw, &result)
+		dec := json.NewDecoder(bytes.NewReader(raw))
+		dec.DisallowUnknownFields()
+		err = dec.Decode(&result)
+		if err != nil {
+			err = fmt.Errorf("%v; (JSON received: '%s')", err, raw)
+		} else if result.Return.Status == "" {
+			err = fmt.Errorf("'status' is empty; (JSON received: '%s')", raw)
+		}
 		return result.Return.Status, err
 	} else {
 		return "", err