Clamp the overwrite discard to what the buffer holds

ring_buffer.go

@@ -664,6 +664,10 @@ func (r *RingBuffer) write(p []byte) (n int, err error) {
 		if len(p) > avail {
 			// Advance read pointer to make room
 			needed := len(p) - avail
+			// Never discard more than the buffer actually holds.
+			if used := r.size - avail; needed > used {
+				needed = used
+			}
 			r.r = (r.r + needed) % r.size
 			// If buffer was full, it's no longer full after advancing read pointer
 			r.isFull = false

ring_buffer_test.go

@@ -2193,3 +2193,33 @@ type result struct {
 	data string
 	err  error
 }
+
+func TestRingBuffer_OverwriteMode_WriteLargerThanCapacity(t *testing.T) {
+	for _, tt := range []struct {
+		name    string
+		prefill string
+		write   string
+		want    string
+	}{
+		{"empty", "", "abcdef", "abcd"},
+		{"empty_odd", "", "abcde", "abcd"},
+		{"partial", "ab", "cdefgh", "cdef"},
+		{"full", "abcd", "efghij", "efgh"},
+	} {
+		t.Run(tt.name, func(t *testing.T) {
+			rb := New(4).SetOverwrite(true)
+			if tt.prefill != "" {
+				if _, err := rb.Write([]byte(tt.prefill)); err != nil {
+					t.Fatalf("prefill: %v", err)
+				}
+			}
+			n, err := rb.Write([]byte(tt.write))
+			if got := string(rb.Bytes(nil)); got != tt.want {
+				t.Errorf("Bytes = %q, want %q (n=%d err=%v)", got, tt.want, n, err)
+			}
+			if rb.Length() != 4 {
+				t.Errorf("Length = %d, want 4", rb.Length())
+			}
+		})
+	}
+}