fix(cwhisper): Fixing buffer merging logic

[deniszh]

Cwhisper buffer is circular, so, fetchCompressed return unsorted data and then it should be sorted - we're creating new array named values, calculate index and copy data from results there. But I found out that sometimes if select interval (end - start) is smaller then buffer size than logic breaks.
E.g. if buffer has 120 points and we're selecting last 1 hour - it works:

if 1h minus 5min - it breaks:

So, in fix below I create values with same size as fetchCompressed data and then cut rest before return.

[bom-d-van]

Hi @deniszh , just in case, this seems to have failed one of the test case. (ci isn't enabled in go-whisper, so it wasn't reported here).

--- FAIL: TestCompressedWhisperReadWrite1 (0.00s)
    --- FAIL: TestCompressedWhisperReadWrite1/buffer_overflow (0.00s)
        compress_test.go:295:   []whisper.TimeSeriesPoint{
              	{Time: 1660219749, Value: NaN},
              	{Time: 1660219750, Value: NaN},
              	{Time: 1660219751, Value: NaN},
            + 	{Time: 1660219752, Value: NaN},
            + 	{Time: 1660219753, Value: 10},
            + 	{Time: 1660219754, Value: NaN},
            + 	{Time: 1660219755, Value: NaN},
            + 	{Time: 1660219756, Value: NaN},
            + 	{Time: 1660219757, Value: NaN},
            + 	{Time: 1660219758, Value: 14},
            + 	{Time: 1660219759, Value: NaN},
            + 	{Time: 1660219760, Value: NaN},
            + 	{Time: 1660219761, Value: NaN},
            + 	{Time: 1660219762, Value: NaN},
            + 	{Time: 1660219763, Value: 15},
              }

--- FAIL: TestCompressedWhisperReadWrite2 (0.00s)
    compress_test.go:368:   &whisper.TimeSeries{
          	fromTime:  1544477925,
          	untilTime: 1544477945,
          	step:      5,
          	values: []float64{
        - 		NaN,
        + 		NaN, 666, NaN, NaN,
          	},
          }

    compress_test.go:389:   &whisper.TimeSeries{
          	fromTime:  1544478201,
          	untilTime: 1544478231,
          	step:      1,
          	values: []float64{
          		... // 10 identical elements
          		24,
          		15,
        + 		1,
        + 		2,
        + 		NaN,
        + 		NaN,
        + 		NaN,
        + 		NaN,
        + 		NaN,
        + 		NaN,
        + 		NaN,
        + 		NaN,
        + 		NaN,
        + 		3,
        + 		4,
        + 		15.5,
        + 		14.0625,
        + 		3.25,
        + 		8.625,
        + 		13.1,
          	},
          }

--- FAIL: TestCompressedWhisperReadWrite3 (0.54s)
    --- FAIL: TestCompressedWhisperReadWrite3/random_time (0.54s)
        compress_test.go:495: case: random_time
        compress_test.go:622: statTotalUpdates: 420 extended: 2 totalPoints: 30774
        compress_test.go:647: go run cmd/compare.go -v -now 1589720099 tmp/test3_random_time.wsp tmp/test3_random_time.wsp.cwsp
panic: runtime error: index out of range [86] with length 86 [recovered]
	panic: runtime error: index out of range [86] with length 86

goroutine 37 [running]:
testing.tRunner.func1.2({0x11d4560, 0xc00001e198})
	/usr/local/go/src/testing/testing.go:1209 +0x24e
testing.tRunner.func1()
	/usr/local/go/src/testing/testing.go:1212 +0x218
panic({0x11d4560, 0xc00001e198})
	/usr/local/go/src/runtime/panic.go:1038 +0x215
github.com/go-graphite/go-whisper.Compare({0xc000018d80, 0x19}, {0xc000500940, 0x1e}, 0x5ec13423, 0x0, {0x0, 0x1052300}, 0x0, 0x0, ...)
	/Users/bom_d_van/Code/go/workspace/src/github.com/go-graphite/go-whisper/debug.go:400 +0x1494
github.com/go-graphite/go-whisper.TestCompressedWhisperReadWrite3.func18(0xc000165ba0)
	/Users/bom_d_van/Code/go/workspace/src/github.com/go-graphite/go-whisper/compress_test.go:649 +0xf8d
testing.tRunner(0xc000165ba0, 0xc0001b6780)
	/usr/local/go/src/testing/testing.go:1259 +0x102
created by testing.(*T).Run
	/usr/local/go/src/testing/testing.go:1306 +0x35a
exit status 2
FAIL	github.com/go-graphite/go-whisper	0.877s

[deniszh]

Oh, you're right, forgot about tests :D
Doesn't looks good, let me check.