Stop flag interpretation at unknown flags

internal/cobra/flags_test.go

@@ -264,3 +264,31 @@ func TestParseFailuresRetainEnteredFlagSpellings(t *testing.T) {
 		t.Fatalf("earlier parser failure = %#v", result.Failure)
 	}
 }
+
+func TestFlagInterpretationStopsAtTheBackendFailurePrefix(t *testing.T) {
+	for _, test := range []struct {
+		name string
+		args []string
+		want []string
+	}{
+		{"long", []string{"--debug", "--unknown"}, []string{"--debug", "--unknown"}},
+		{"short cluster", []string{"-dqz"}, []string{"--debug", "--quiet", "-z"}},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			// The later value must not be canonicalized or retained in spellings.
+			args := append(append([]string(nil), test.args...), "--message="+strings.Repeat("value", 1024), "--quiet=false")
+			flags := append(commandmeta.ReportingFlags(), commandmeta.Flag{Name: "message"})
+			interpreted, spellings, occurrences, failure := interpretFlagTokens(args, flags, reportingFlagOptions().FlagAliases)
+			if failure != nil || !reflect.DeepEqual(interpreted, test.want) || len(spellings["message"]) != 0 {
+				t.Fatalf("interpreted=%q spellings=%v failure=%v", interpreted, spellings, failure)
+			}
+			if len(occurrences) != len(test.want)-1 || occurrences[0].Name != "debug" {
+				t.Fatalf("consumed occurrences=%#v", occurrences)
+			}
+			result := Parse(reportingFlagTree(), append([]string{"run"}, args...), reportingFlagOptions())
+			if result.Failure == nil || !strings.Contains(result.Failure.Message, "unknown") || !reflect.DeepEqual(result.FlagOccurrences, occurrences) {
+				t.Fatalf("backend result=%#v", result)
+			}
+		})
+	}
+}