Test 1901/AC3: an older pair of build module and checker still agree (issue 1901)

modules/mesh-build-go/cmd/mesh-build-go/failure_words_test.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"encoding/json"
 	"os"
 	"strings"
 	"testing"
@@ -27,7 +28,7 @@ func TestAFailedTestKeepsItsFatalMessageAfterItsLogLines(t *testing.T) {
 		}
 	}
 	if want := "3 subscription(s) derive to a subject no emitter owns:"; f.Cause != want && !strings.HasSuffix(f.Cause, want) {
-		t.Errorf("1901/AC2: cause = %q, want the t.Fatalf line, not the t.Logf line", f.Cause)
+		t.Errorf("1901/AC1: cause = %q, want the t.Fatalf line, not the t.Logf line", f.Cause)
 	}
 }
 
@@ -46,3 +47,32 @@ func TestAFailureLongerThanTheBoundKeepsItsLastLines(t *testing.T) {
 		t.Errorf("1901/AC1: out = %q...%q", out[:40], out[len(out)-60:])
 	}
 }
+
+// An older checker, which knows no "cause" field, still reads the new build module's answer, and the output it gets
+// holds at least what the older build module kept (its first lines, for an output shorter than the bound).
+func TestAnOlderCheckerReadsTheNewFailureAndGetsAtLeastWhatItKept(t *testing.T) { // 1901/AC3
+	raw, err := os.ReadFile("testdata/captured/go-test-json-log-then-fatal.jsonl")
+	if err != nil {
+		t.Fatal(err)
+	}
+	res := &TestResult{}
+	rd := newReader(res, ".")
+	rd.read(string(raw))
+	rd.finish()
+	b, err := json.Marshal(res.Failed)
+	if err != nil {
+		t.Fatal(err)
+	}
+	var old []struct {
+		Package string `json:"package"`
+		Test    string `json:"test,omitempty"`
+		Output  string `json:"output,omitempty"`
+	}
+	if err := json.Unmarshal(b, &old); err != nil || len(old) != 1 {
+		t.Fatalf("1901/AC3: an older checker refused the answer: %v, %d failures", err, len(old))
+	}
+	first := strings.SplitN(old[0].Output, "\n", 2)[0]
+	if first == "" || !strings.Contains(old[0].Output, "judged against") || !strings.Contains(old[0].Output, "3 subscription(s)") {
+		t.Errorf("1901/AC3: older checker's output = %q", old[0].Output)
+	}
+}