Name the cause of a failed toolchain or module download, not Go's progress line before it (issue 1560, review round 1)

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

@@ -128,7 +128,7 @@ func (m *Module) build(ctx context.Context, args map[string]any) (any, error) {
 	}
 	cmd := []string{"-C", dir, "build", "-ldflags", linker, "-o", filepath.Join(out, binary), "./" + strings.Trim(art.From, "./")}
 	if r := m.Run(ctx, m.buildEnv(dir), "go", cmd...); r.Status != 0 || r.Err != "" {
-		return nil, fmt.Errorf("go build of %s: %s", art.Name, firstLine(r.Err+"\n"+r.Stderr+"\n"+r.Stdout))
+		return nil, fmt.Errorf("go build of %s: %s", art.Name, causeLine(r.Err+"\n"+r.Stderr+"\n"+r.Stdout))
 	}
 	archive, digest, err := buildstep.Pack(out)
 	if err != nil {

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

@@ -148,7 +148,7 @@ func (m *Module) listPackages(ctx context.Context, tree, dir string, tags []stri
 	args = append(args, "./...")
 	r := m.Run(ctx, m.goEnvFor(abs), "go", args...)
 	if r.Status != 0 || r.Err != "" {
-		return nil, fmt.Errorf("go list in %s: %s", dir, firstLine(r.Err+"\n"+r.Stderr+"\n"+r.Stdout))
+		return nil, fmt.Errorf("go list in %s: %s", dir, causeLine(r.Err+"\n"+r.Stderr+"\n"+r.Stdout))
 	}
 	return parsePackages(r.Stdout)
 }

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

@@ -89,7 +89,7 @@ func (m *Module) fetch(ctx context.Context, args map[string]any) (any, error) {
 		for _, verb := range []string{"download", "verify"} {
 			r := m.Run(ctx, m.goEnv(), "go", "-C", filepath.Join(tree, d), "mod", verb)
 			if r.Status != 0 || r.Err != "" {
-				return nil, fmt.Errorf("go mod %s in %s: %s", verb, d, firstLine(r.Err+"\n"+r.Stdout+"\n"+r.Stderr))
+				return nil, fmt.Errorf("go mod %s in %s: %s", verb, d, causeLine(r.Err+"\n"+r.Stdout+"\n"+r.Stderr))
 			}
 		}
 	}
@@ -164,6 +164,18 @@ func (m *Module) clock() time.Time {
 }
 func (m *Module) now() time.Time { return m.clock() }
 
+// causeLine is the first line of a go command's output that says something other than Go's own progress: `go:
+// downloading ...` comes first when Go fetches a toolchain or a module, and the cause of a failure on a later line
+// (issue 1560: a refusal read only "go: downloading go1.27.1 (linux/amd64)").
+func causeLine(s string) string {
+	for _, l := range strings.Split(s, "\n") {
+		if l = strings.TrimSpace(l); l != "" && !strings.HasPrefix(l, "go: downloading ") {
+			return l
+		}
+	}
+	return firstLine(s)
+}
+
 func firstLine(s string) string {
 	for _, l := range strings.Split(s, "\n") {
 		if l = strings.TrimSpace(l); l != "" {

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

@@ -55,7 +55,7 @@ func probeGo(ctx context.Context, run forgekey.Runner, home, cache string) (stri
 	r := run(ctx, append(baseEnv(home), probeEnv(cache)...), "go", "version")
 	if r.Status != 0 || r.Err != "" {
 		return "", fmt.Errorf("`go version` with GOTOOLCHAIN=%s failed (the recipe builds with %s): %s", RecipeGo, RecipeGo,
-			firstLine(r.Err+"\n"+r.Stderr))
+			causeLine(r.Err+"\n"+r.Stderr))
 	}
 	return parseGoVersion(r.Stdout)
 }

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

@@ -77,7 +77,7 @@ func TestAFailedFetchOrAnotherVersionMakesHealthUnhealthyNamingBoth(t *testing.T
 		ran  forgekey.Ran
 		want []string
 	}{
-		{"fetch fails", forgekey.Ran{Status: 1, Stderr: "go: download go1.27.1 for linux/amd64: dial tcp: no route to host\n"},
+		{"fetch fails", forgekey.Ran{Status: 1, Stderr: "go: downloading go1.27.1 (linux/amd64)\ngo: download go1.27.1 for linux/amd64: dial tcp: no route to host\n"},
 			[]string{"go1.27.1", "no route to host"}},
 		{"other version", forgekey.Ran{Stdout: "go version go1.27.1-X:nodwarf5 linux/amd64\n"},
 			[]string{"go1.27.1", "go1.27.1-X:nodwarf5"}},
@@ -122,3 +122,14 @@ func TestTheProbeIsBoundLongOnlyWhileTheReleaseIsMissing(t *testing.T) {
 		t.Errorf("cached release: bound %v", got)
 	}
 }
+
+// 1560 (review): Go's progress line is not the cause: the refusal names the line after it.
+func TestCauseLineSkipsGosDownloadingLine(t *testing.T) {
+	got := causeLine("go: downloading go1.27.1 (linux/amd64)\ngo: download go1.27.1: checksum mismatch\n")
+	if got != "go: download go1.27.1: checksum mismatch" {
+		t.Errorf("causeLine = %q", got)
+	}
+	if got := causeLine("go: downloading go1.27.1 (linux/amd64)\n"); got != "go: downloading go1.27.1 (linux/amd64)" {
+		t.Errorf("only a progress line: %q", got)
+	}
+}