Regression: Fix assigning to nil map in DeepMerge (#18143)

CHANGELOG.next.asciidoc

@@ -81,6 +81,7 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD d
 - Fix goroutine leak and Elasticsearch output file descriptor leak when output reloading is in use. {issue}10491[10491] {pull}17381[17381]
 - Fix `setup.dashboards.index` setting not working. {pull}17749[17749]
 - Fix Elasticsearch license endpoint URL referenced in error message. {issue}17880[17880] {pull}18030[18030]
+- Fix panic when assigning a key to a `nil` value in an event. {pull}18143[18143]
 
 *Auditbeat*
 

libbeat/common/mapstr.go

@@ -96,20 +96,26 @@ func (m MapStr) deepUpdateMap(d MapStr, overwrite bool) {
 }
 
 func deepUpdateValue(old interface{}, val MapStr, overwrite bool) interface{} {
-	if old == nil {
-		return val
-	}
-
 	switch sub := old.(type) {
 	case MapStr:
+		if sub == nil {
+			return val
+		}
+
 		sub.deepUpdateMap(val, overwrite)
 		return sub
 	case map[string]interface{}:
+		if sub == nil {
+			return val
+		}
+
 		tmp := MapStr(sub)
 		tmp.deepUpdateMap(val, overwrite)
 		return tmp
 	default:
-		// This should never happen
+		// We reach the default branch if old is no map or if old == nil.
+		// In either case we return `val`, such that the old value is completely
+		// replaced when merging.
 		return val
 	}
 }

libbeat/common/mapstr_test.go

@@ -87,6 +87,11 @@ func TestMapStrDeepUpdate(t *testing.T) {
 			MapStr{"a.b": 1},
 			MapStr{"a": 1, "a.b": 1},
 		},
+		{
+			MapStr{"a": (MapStr)(nil)},
+			MapStr{"a": MapStr{"b": 1}},
+			MapStr{"a": MapStr{"b": 1}},
+		},
 	}
 
 	for i, test := range tests {