feat: pin vantage-shared v0.5.0, add package-manager output tail buffer

internal/updates/tailbuf.go

@@ -0,0 +1,34 @@
+package updates
+
+import "sync"
+
+// outputTailMax bounds the package-manager output carried back in a
+// PatchResult. The end of the output is where apt and dnf say what failed, so
+// the newest bytes are the ones kept.
+const outputTailMax = 64 << 10
+
+// tailBuffer is an io.Writer that retains only the last max bytes written.
+// Stdout and stderr are both pointed at one, so it is safe for concurrent use.
+type tailBuffer struct {
+	mu  sync.Mutex
+	max int
+	buf []byte
+}
+
+func newTailBuffer(max int) *tailBuffer { return &tailBuffer{max: max} }
+
+func (t *tailBuffer) Write(p []byte) (int, error) {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	t.buf = append(t.buf, p...)
+	if over := len(t.buf) - t.max; over > 0 {
+		t.buf = append([]byte(nil), t.buf[over:]...)
+	}
+	return len(p), nil
+}
+
+func (t *tailBuffer) String() string {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	return string(t.buf)
+}

internal/updates/tailbuf_test.go

@@ -0,0 +1,34 @@
+package updates
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestTailBufferKeepsNewestBytes(t *testing.T) {
+	b := newTailBuffer(10)
+	_, _ = b.Write([]byte("0123456789"))
+	_, _ = b.Write([]byte("abcde"))
+	if got := b.String(); got != "56789abcde" {
+		t.Fatalf("got %q, want %q", got, "56789abcde")
+	}
+}
+
+func TestTailBufferSingleWriteLargerThanMax(t *testing.T) {
+	b := newTailBuffer(4)
+	n, err := b.Write([]byte("abcdefgh"))
+	if err != nil || n != 8 {
+		t.Fatalf("Write must report the full length consumed, got %d, %v", n, err)
+	}
+	if got := b.String(); got != "efgh" {
+		t.Fatalf("got %q", got)
+	}
+}
+
+func TestTailBufferDefaultCap(t *testing.T) {
+	b := newTailBuffer(outputTailMax)
+	_, _ = b.Write([]byte(strings.Repeat("x", outputTailMax+100)))
+	if len(b.String()) != outputTailMax {
+		t.Fatalf("len %d, want %d", len(b.String()), outputTailMax)
+	}
+}