iotests: add support for capturing and matching QMP events

When using the _launch_qemu and _send_qemu_cmd functions from
common.qemu, any QMP events get mixed in with the output from
the commands and responses.

This makes it difficult to write a test case as the ordering
of events in the output is not stable.

This introduces a variable 'capture_events' which can be set
to a list of event names. Any events listed in this variable
will not be printed, instead collected in the $QEMU_EVENTS
environment variable.

A new '_wait_event' function can be invoked to collect events
at a fixed point in time. The function will first pull events
cached in $QEMU_EVENTS variable, and if none are found, will
then read more from QMP.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20210204124834.774401-11-berrange@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

tests/qemu-iotests/common.qemu

@@ -53,6 +53,15 @@ _in_fd=4
 # If $mismatch_only is set, only non-matching responses will
 # be echoed.
 #
+# If $capture_events is non-empty, then any QMP event names it lists
+# will not be echoed out, but instead collected in the $QEMU_EVENTS
+# variable. The _wait_event function can later be used to receive
+# the cached events.
+#
+# If $only_capture_events is set to anything but an empty string,
+# then an error will be raised if a QMP message is seen which is
+# not an event listed in $capture_events.
+#
 # If $success_or_failure is set, the meaning of the arguments is
 # changed as follows:
 # $2: A string to search for in the response; if found, this indicates
@@ -78,6 +87,31 @@ _timed_wait_for()
     QEMU_STATUS[$h]=0
     while IFS= read -t ${QEMU_COMM_TIMEOUT} resp <&${QEMU_OUT[$h]}
     do
+        if [ -n "$capture_events" ]; then
+            capture=0
+            local evname
+            for evname in $capture_events
+            do
+                case ${resp} in
+                    *\"event\":\ \"${evname}\"* ) capture=1 ;;
+                esac
+            done
+            if [ $capture = 1 ];
+            then
+                ev=$(echo "${resp}" | tr -d '\r' | tr % .)
+                QEMU_EVENTS="${QEMU_EVENTS:+${QEMU_EVENTS}%}${ev}"
+                if [ -n "$only_capture_events" ]; then
+                    return
+                else
+                    continue
+                fi
+            fi
+        fi
+        if [ -n "$only_capture_events" ]; then
+            echo "Only expected $capture_events but got ${resp}"
+            exit 1
+        fi
+
         if [ -z "${silent}" ] && [ -z "${mismatch_only}" ]; then
             echo "${resp}" | _filter_testdir | _filter_qemu \
                            | _filter_qemu_io | _filter_qmp | _filter_hmp
@@ -172,12 +206,82 @@ _send_qemu_cmd()
         let count--;
     done
     if [ ${QEMU_STATUS[$h]} -ne 0 ] && [ -z "${qemu_error_no_exit}" ]; then
-        echo "Timeout waiting for ${1} on handle ${h}"
+        echo "Timeout waiting for command ${1} response on handle ${h}"
         exit 1 #Timeout means the test failed
     fi
 }
 
 
+# Check event cache for a named QMP event
+#
+# Input parameters:
+# $1:       Name of the QMP event to check for
+#
+# Checks if the named QMP event that was previously captured
+# into $QEMU_EVENTS. When matched, the QMP event will be echoed
+# and the $matched variable set to 1.
+#
+# _wait_event is more suitable for test usage in most cases
+_check_cached_events()
+{
+    local evname=${1}
+
+    local match="\"event\": \"$evname\""
+
+    matched=0
+    if [ -n "$QEMU_EVENTS" ]; then
+        CURRENT_QEMU_EVENTS=$QEMU_EVENTS
+        QEMU_EVENTS=
+        old_IFS=$IFS
+        IFS="%"
+        for ev in $CURRENT_QEMU_EVENTS
+        do
+            grep -q "$match" < <(echo "${ev}")
+            if [ $? -eq 0 ] && [ $matched = 0 ]; then
+                echo "${ev}" | _filter_testdir | _filter_qemu \
+                           | _filter_qemu_io | _filter_qmp | _filter_hmp
+                matched=1
+            else
+                QEMU_EVENTS="${QEMU_EVENTS:+${QEMU_EVENTS}%}${ev}"
+            fi
+        done
+        IFS=$old_IFS
+    fi
+}
+
+# Wait for a named QMP event
+#
+# Input parameters:
+# $1:       QEMU handle to use
+# $2:       Name of the QMP event to wait for
+#
+# Checks if the named QMP even was previously captured
+# into $QEMU_EVENTS. If none are present, then waits for the
+# event to arrive on the QMP channel. When matched, the QMP
+# event will be echoed
+_wait_event()
+{
+    local h=${1}
+    local evname=${2}
+
+    while true
+    do
+        _check_cached_events $evname
+
+        if [ $matched = 1 ];
+        then
+            return
+        fi
+
+        only_capture_events=1 qemu_error_no_exit=1 _timed_wait_for ${h}
+
+        if [ ${QEMU_STATUS[$h]} -ne 0 ] ; then
+            echo "Timeout waiting for event ${evname} on handle ${h}"
+            exit 1 #Timeout means the test failed
+        fi
+    done
+}
+
 # Launch a QEMU process.
 #
 # Input parameters: