lib/protocol: Invalid files should always lose (#4747)

lib/protocol/bep_extensions.go

@@ -95,6 +95,11 @@ func (f FileInfo) SequenceNo() int64 {
 // WinsConflict returns true if "f" is the one to choose when it is in
 // conflict with "other".
 func (f FileInfo) WinsConflict(other FileInfo) bool {
+	// If only one of the files is invalid, that one loses
+	if f.IsInvalid() != other.IsInvalid() {
+		return !f.IsInvalid()
+	}
+
 	// If a modification is in conflict with a delete, we pick the
 	// modification.
 	if !f.IsDeleted() && other.IsDeleted() {

lib/protocol/conflict_test.go

@@ -9,6 +9,7 @@ func TestWinsConflict(t *testing.T) {
 		// The first should always win over the second
 		{{ModifiedS: 42}, {ModifiedS: 41}},
 		{{ModifiedS: 41}, {ModifiedS: 42, Deleted: true}},
+		{{Deleted: true}, {ModifiedS: 10, Invalid: true}},
 		{{ModifiedS: 41, Version: Vector{[]Counter{{42, 2}, {43, 1}}}}, {ModifiedS: 41, Version: Vector{[]Counter{{42, 1}, {43, 2}}}}},
 	}