From dcae699323c8bda7d648a2e1303afaa4f6e19913 Mon Sep 17 00:00:00 2001
From: jochen <jschoubben@gmail.com>
Date: Fri, 23 Oct 2026 22:26:29 +0200
Subject: [PATCH] mesh-build-go: a clipped failure starts with its cause, not
 the marker, for an older checker (issue 1901/AC3)

---
 .../cmd/mesh-build-go/failure_words_test.go   | 38 ++++++++++++++++++-
 .../mesh-build-go/cmd/mesh-build-go/levels.go | 17 +++++++--
 2 files changed, 49 insertions(+), 6 deletions(-)

diff --git a/modules/mesh-build-go/cmd/mesh-build-go/failure_words_test.go b/modules/mesh-build-go/cmd/mesh-build-go/failure_words_test.go
index 7a061993..9f82aa72 100644
--- a/modules/mesh-build-go/cmd/mesh-build-go/failure_words_test.go
+++ b/modules/mesh-build-go/cmd/mesh-build-go/failure_words_test.go
@@ -39,11 +39,11 @@ func TestAFailureLongerThanTheBoundKeepsItsLastLines(t *testing.T) {
 		lines = append(lines, "    x_test.go:1: log line "+strings.Repeat("y", 40))
 	}
 	lines = append(lines, "    x_test.go:9: the real cause", "--- FAIL: TestX (0.00s)")
-	out := clip(lines)
+	out := clip(lines, "")
 	if len(out) > outputBytes+100 {
 		t.Errorf("1901/AC1: kept %d bytes, bound is %d", len(out), outputBytes)
 	}
-	if !strings.HasSuffix(out, "the real cause\n--- FAIL: TestX (0.00s)") || !strings.HasPrefix(out, "... (") {
+	if !strings.HasSuffix(out, "the real cause\n--- FAIL: TestX (0.00s)") || !strings.Contains(out, "\n... (") {
 		t.Errorf("1901/AC1: out = %q...%q", out[:40], out[len(out)-60:])
 	}
 }
@@ -76,3 +76,37 @@ func TestAnOlderCheckerReadsTheNewFailureAndGetsAtLeastWhatItKept(t *testing.T)
 		t.Errorf("1901/AC3: older checker's output = %q", old[0].Output)
 	}
 }
+
+// A long failure, whose earlier lines are left out, still starts with a real line for an older checker, which keeps only the
+// first non-empty line of the output cut to 120 characters, and not with the marker.
+func TestALongFailureStillStartsWithARealLineForAnOlderChecker(t *testing.T) { // 1901/AC3
+	var lines []string
+	for i := 0; i < 400; i++ {
+		lines = append(lines, "    x_test.go:1: log line "+strings.Repeat("y", 40))
+	}
+	lines = append(lines, "    x_test.go:9: the real cause", "--- FAIL: TestX (0.00s)")
+	oldFirst := func(out string) string { // the older checker's rule (judge.go firstLine)
+		for _, l := range strings.Split(out, "\n") {
+			if l = strings.TrimSpace(l); l != "" {
+				if len(l) > 120 {
+					l = l[:120]
+				}
+				return l
+			}
+		}
+		return ""
+	}
+	for _, cause := range []string{"the real cause", ""} {
+		out := clip(lines, cause)
+		got := oldFirst(out)
+		if strings.HasPrefix(got, "... (") || got == "" {
+			t.Errorf("1901/AC3: cause %q: an older checker would read %q", cause, got)
+		}
+		if cause != "" && got != cause {
+			t.Errorf("1901/AC3: older checker reads %q, want the cause", got)
+		}
+		if !strings.HasSuffix(out, "--- FAIL: TestX (0.00s)") {
+			t.Errorf("1901/AC3: the last lines are lost: %q", out[len(out)-40:])
+		}
+	}
+}
diff --git a/modules/mesh-build-go/cmd/mesh-build-go/levels.go b/modules/mesh-build-go/cmd/mesh-build-go/levels.go
index 3d921649..d247f1a9 100644
--- a/modules/mesh-build-go/cmd/mesh-build-go/levels.go
+++ b/modules/mesh-build-go/cmd/mesh-build-go/levels.go
@@ -207,15 +207,17 @@ func (r *reader) event(ev event) {
 			r.times = append(r.times, TestTime{Package: ev.Package, Test: ev.Test, Elapsed: ev.Elapsed})
 			if ev.Action == "fail" {
 				r.failedIn[ev.Package] = true
-				r.res.Failed = append(r.res.Failed, Failure{Package: ev.Package, Test: ev.Test, Output: clip(r.testOut[key]), Cause: r.causes[key]})
+				r.res.Failed = append(r.res.Failed, Failure{Package: ev.Package, Test: ev.Test, Output: clip(r.testOut[key], r.causes[key]), Cause: r.causes[key]})
 			}
 		}
 	}
 }
 
 // clip joins the last lines of a failure's output that fit outputBytes, each cut to outputLineBytes, and says how many
-// earlier lines it left out.
-func clip(lines []string) string {
+// earlier lines it left out. **When it leaves lines out, its first line is still a real one** (issue 1901/AC3): the failure's
+// cause, or failing that the first non-empty line of the output, then the marker. An older checker takes only the first
+// non-empty line of the output, and a marker there would be less than the older pair kept.
+func clip(lines []string, cause string) string {
 	size, from := 0, len(lines)
 	for from > 0 {
 		n := len(cutLine(lines[from-1])) + 1
@@ -227,6 +229,13 @@ func clip(lines []string) string {
 	}
 	kept := make([]string, 0, len(lines)-from+1)
 	if from > 0 {
+		head := strings.TrimSpace(cause)
+		for i := 0; head == "" && i < from; i++ {
+			head = strings.TrimSpace(lines[i])
+		}
+		if head != "" {
+			kept = append(kept, cutLine(head))
+		}
 		kept = append(kept, fmt.Sprintf("... (%d earlier lines)", from))
 	}
 	for _, l := range lines[from:] {
@@ -256,7 +265,7 @@ func (r *reader) finish() {
 	for _, name := range r.order {
 		p := r.pkgs[name]
 		if !p.Passed && !r.failedIn[name] {
-			r.res.Failed = append(r.res.Failed, Failure{Package: name, Output: clip(r.pkgOut[name])})
+			r.res.Failed = append(r.res.Failed, Failure{Package: name, Output: clip(r.pkgOut[name], "")})
 		}
 		if !p.NoTests && p.Elapsed > r.res.Elapsed {
 			r.res.Elapsed = p.Elapsed