S2: limit max repeat length (#188)

On single, extremely large single block encodes with high extremely long repeats, we could store the wrong length.

Does not affect streaming mode.

s2/encode_other.go

@@ -161,12 +161,21 @@ func emitRepeat(dst []byte, offset, length int) int {
 		dst[0] = 6<<2 | tagCopy1
 		return 4
 	}
+	const maxRepeat = (1 << 24) - 1
 	length -= 1 << 16
+	left := 0
+	if length > maxRepeat {
+		left = length - maxRepeat
+		length = maxRepeat
+	}
 	dst[4] = uint8(length >> 16)
 	dst[3] = uint8(length >> 8)
 	dst[2] = uint8(length >> 0)
 	dst[1] = 0
 	dst[0] = 7<<2 | tagCopy1
+	if left > 0 {
+		return 5 + emitRepeat(dst[5:], offset, left)
+	}
 	return 5
 }
 

s2/s2_test.go

@@ -1531,6 +1531,32 @@ func testFile(t *testing.T, i, repeat int) {
 	})
 }
 
+func TestDataRoundtrips(t *testing.T) {
+	test := func(t *testing.T, data []byte) {
+		t.Run("s2", func(t *testing.T) {
+			testWriterRoundtrip(t, data)
+		})
+		t.Run("s2-better", func(t *testing.T) {
+			testWriterRoundtrip(t, data, WriterBetterCompression())
+		})
+		t.Run("block", func(t *testing.T) {
+			d := data
+			testBlockRoundtrip(t, d)
+		})
+		t.Run("block-better", func(t *testing.T) {
+			d := data
+			testBetterBlockRoundtrip(t, d)
+		})
+		t.Run("snappy", func(t *testing.T) {
+			testSnappyDecode(t, data)
+		})
+	}
+	t.Run("longblock", func(t *testing.T) {
+		data := make([]byte, 1<<25)
+		test(t, data)
+	})
+}
+
 // Naming convention is kept similar to what snappy's C++ implementation uses.
 func Benchmark_UFlat0(b *testing.B)  { benchFile(b, 0, true) }
 func Benchmark_UFlat1(b *testing.B)  { benchFile(b, 1, true) }