decoder: Return EOF, not panic on a proper end of stream

io.EOF handling was already there in instructions decoding loop, but it
was simply breaking out of the loop and in the end decode
unconditionally was trying to pop top element from stack, and since
stack is empty panicked. E.g. for "" input this way:

--- FAIL: TestDecode (0.00s)
panic: runtime error: index out of range [recovered]
        panic: runtime error: index out of range

goroutine 20 [running]:
testing.tRunner.func1(0xc4200765b0)
        /home/kirr/src/tools/go/go/src/testing/testing.go:624 +0x267
panic(0x5368c0, 0x603010)
        /home/kirr/src/tools/go/go/src/runtime/panic.go:489 +0x26e
github.com/kisielk/og-rek.Decoder.Decode(0xc4200ea5a0, 0x624908, 0x0, 0x0, 0xc4200e6b10, 0x52a180, 0xc4200ee308, 0x0, 0x0)
        /home/kirr/src/wendelin/neo/g.neo/src/github.com/kisielk/og-rek/ogorek.go:250 +0x4bb
github.com/kisielk/og-rek.TestDecode(0xc4200765b0)
        /home/kirr/src/wendelin/neo/g.neo/src/github.com/kisielk/og-rek/ogorek_test.go:80 +0x232f
testing.tRunner(0xc4200765b0, 0x567648)
        /home/kirr/src/tools/go/go/src/testing/testing.go:659 +0x98
created by testing.(*T).Run
        /home/kirr/src/tools/go/go/src/testing/testing.go:701 +0x2df
exit status 2

Fix it via reworking EOF handling:

- if we see EOF and no instructions was decoded so far - just return it
  to the caller

- if we see EOF in the middle of the stream - as valid pickle streams
  end with Stop opcode - it is unexpected EOF - return it as such.

A couple of tests had to be adjusted in TestDecode - as input there was
not a valid pickle - not ending with "." (which python pickle decoder
also does not decode at all).

ogorek.go

@@ -121,14 +121,16 @@ func (d Decoder) Decode() (interface{}, error) {
 	insn := 0
 loop:
 	for {
-		insn++
 		key, err := d.r.ReadByte()
-		if err == io.EOF {
-			break
-		} else if err != nil {
+		if err != nil {
+			if err == io.EOF && insn != 0 {
+				err = io.ErrUnexpectedEOF
+			}
 			return nil, err
 		}
 
+		insn++
+
 		switch key {
 		case opMark:
 			d.mark()

ogorek_test.go

@@ -3,6 +3,7 @@ package ogórek
 import (
 	"bytes"
 	"encoding/hex"
+	"io"
 	"math/big"
 	"reflect"
 	"testing"
@@ -52,7 +53,7 @@ func TestDecode(t *testing.T) {
 		{"list of numbers", "(lp0\nI1\naI2\naI3\naI4\na.", []interface{}{int64(1), int64(2), int64(3), int64(4)}},
 		{"string", "S'abc'\np0\n.", string("abc")},
 		{"unicode", "V\\u65e5\\u672c\\u8a9e\np0\n.", string("日本語")},
-		{"unicode", "V' \\u77e5\\u4e8b\\u5c11\\u65f6\\u70e6\\u607c\\u5c11\\u3001\\u8bc6\\u4eba\\u591a\\u5904\\u662f\\u975e\\u591a\\u3002", string("' 知事少时烦恼少、识人多处是非多。")},
+		{"unicode2", "V' \\u77e5\\u4e8b\\u5c11\\u65f6\\u70e6\\u607c\\u5c11\\u3001\\u8bc6\\u4eba\\u591a\\u5904\\u662f\\u975e\\u591a\\u3002\n.", string("' 知事少时烦恼少、识人多处是非多。")},
 		{"empty dict", "(dp0\n.", make(map[interface{}]interface{})},
 		{"dict with strings", "(dp0\nS'a'\np1\nS'1'\np2\nsS'b'\np3\nS'2'\np4\ns.", map[interface{}]interface{}{"a": "1", "b": "2"}},
 		{"GLOBAL and REDUCE opcodes", "cfoo\nbar\nS'bing'\n\x85R.", Call{Callable: Class{Module: "foo", Name: "bar"}, Args: []interface{}{"bing"}}},
@@ -60,7 +61,7 @@ func TestDecode(t *testing.T) {
 		{"graphite message1", string(graphitePickle1), []interface{}{map[interface{}]interface{}{"values": []interface{}{float64(473), float64(497), float64(540), float64(1497), float64(1808), float64(1890), float64(2013), float64(1821), float64(1847), float64(2176), float64(2156), float64(1250), float64(2055), float64(1570), None{}, None{}}, "start": int64(1383782400), "step": int64(86400), "end": int64(1385164800), "name": "ZZZZ.UUUUUUUU.CCCCCCCC.MMMMMMMM.XXXXXXXXX.TTT"}}},
 		{"graphite message2", string(graphitePickle2), []interface{}{map[interface{}]interface{}{"values": []interface{}{float64(473), float64(497), float64(540), float64(1497), float64(1808), float64(1890), float64(2013), float64(1821), float64(1847), float64(2176), float64(2156), float64(1250), float64(2055), float64(1570), None{}, None{}}, "start": int64(1383782400), "step": int64(86400), "end": int64(1385164800), "name": "user.login.area.machine.metric.minute"}}},
 		{"graphite message3", string(graphitePickel3), []interface{}{map[interface{}]interface{}{"intervals": []interface{}{}, "metric_path": "carbon.agents", "isLeaf": false}, map[interface{}]interface{}{"intervals": []interface{}{}, "metric_path": "carbon.aggregator", "isLeaf": false}, map[interface{}]interface{}{"intervals": []interface{}{}, "metric_path": "carbon.relays", "isLeaf": false}}},
-		{"too long line", "V28,34,30,55,100,130,87,169,194,202,232,252,267,274,286,315,308,221,358,368,401,406,434,452,475,422,497,530,517,559,400,418,571,578,599,600,625,630,635,647,220,715,736,760,705,785,794,495,808,852,861,863,869,875,890,893,896,922,812,980,1074,1087,1145,1153,1163,1171,445,1195,1203,1242,1255,1274,52,1287,1319,636,1160,1339,1345,1353,1369,1391,1396,1405,1221,1410,1431,1451,1460,1470,1472,1492,1517,1528,419,1530,1532,1535,1573,1547,1574,1437,1594,1595,847,1551,983,1637,1647,1666,1672,1691,1726,1515,1731,1739,1741,1723,1776,1685,505,1624,1436,1890,728,1910,1931,1544,2013,2025,2030,2043,2069,1162,2129,2160,2199,2210,1911,2246,804,2276,1673,2299,2315,2322,2328,2355,2376,2405,1159,2425,2430,2452,1804,2442,2567,2577,1167,2611,2534,1879,2623,2682,2699,2652,2742,2754,2774,2782,2795,2431,2821,2751,2850,2090,513,2898,592,2932,2933,1555,2969,3003,3007,3010,2595,3064,3087,3105,3106,3110,151,3129,3132,304,3173,3205,3233,3245,3279,3302,3307,714,316,3331,3347,3360,3375,3380,3442,2620,3482,3493,3504,3516,3517,3518,3533,3511,2681,3530,3601,3606,3615,1210,3633,3651,3688,3690,3781,1907,3839,3840,3847,3867,3816,3899,3924,2345,3912,3966,982,4040,4056,4076,4084,4105,2649,4171,3873,1415,3567,4188,4221,4227,4231,2279,4250,4253,770,894,4343,4356,4289,4404,4438,2572,3124,4334,2114,3953,4522,4537,4561,4571,641,4629,4640,4664,4687,4702,4709,4740,4605,4746,4768,3856,3980,4814,2984,4895,4908,1249,4944,4947,4979,4988,4995,32,4066,5043,4956,5069,5072,5076,5084,5085,5137,4262,5152,479,5156,3114,1277,5183,5186,1825,5106,5216,963,5239,5252,5218,5284,1980,1972,5352,5364,5294,5379,5387,5391,5397,5419,5434,5468,5471,3350,5510,5522,5525,5538,5554,5573,5597,5610,5615,5624,842,2851,5641,5655,5656,5658,5678,5682,5696,5699,5709,5728,5753,851,5805,3528,5822,801,5855,2929,5871,5899,5918,5925,5927,5931,5935,5939,5958,778,5971,5980,5300,6009,6023,6030,6032,6016,6110,5009,6155,6197,1760,6253,6267,4886,5608,6289,6308,6311,6321,6316,6333,6244,6070,6349,6353,6186,6357,6366,6386,6387,6389,6399,6411,6421,6432,6437,6465,6302,6493,5602,6511,6529,6536,6170,6557,6561,6577,6581,6590,5290,5649,6231,6275,6635,6651,6652,5929,6692,6693,6695,6705,6711,6723,6738,6752,6753,3629,2975,6790,5845,338,6814,6826,6478,6860,6872,6882,880,356,6897,4102,6910,6611,1030,6934,6936,6987,6984,6999,827,6902,7027,7049,7051,4628,7084,7083,7071,7102,7137,5867,7152,6048,2410,3896,7168,7177,7224,6606,7233,1793,7261,7284,7290,7292,5212,7315,6964,3238,355,1969,4256,448,7325,908,2824,2981,3193,3363,3613,5325,6388,2247,1348,72,131,5414,7285,7343,7349,7362,7372,7381,7410,7418,7443,5512,7470,7487,7497,7516,7277,2622,2863,945,4344,3774,1024,2272,7523,4476,256,5643,3164,7539,7540,7489,1932,7559,7575,7602,7605,7609,7608,7619,7204,7652,7663,6907,7672,7654,7674,7687,7718,7745,1202,4030,7797,7801,7799,2924,7871,7873,7900,7907,7911,7912,7917,7923,7935,8007,8017,7636,8084,8087,3686,8114,8153,8158,8171,8175,8182,8205,8222,8225,8229,8232,8234,8244,8247,7256,8279,6929,8285,7040,8328,707,6773,7949,8468,5759,6344,8509,1635\n", "28,34,30,55,100,130,87,169,194,202,232,252,267,274,286,315,308,221,358,368,401,406,434,452,475,422,497,530,517,559,400,418,571,578,599,600,625,630,635,647,220,715,736,760,705,785,794,495,808,852,861,863,869,875,890,893,896,922,812,980,1074,1087,1145,1153,1163,1171,445,1195,1203,1242,1255,1274,52,1287,1319,636,1160,1339,1345,1353,1369,1391,1396,1405,1221,1410,1431,1451,1460,1470,1472,1492,1517,1528,419,1530,1532,1535,1573,1547,1574,1437,1594,1595,847,1551,983,1637,1647,1666,1672,1691,1726,1515,1731,1739,1741,1723,1776,1685,505,1624,1436,1890,728,1910,1931,1544,2013,2025,2030,2043,2069,1162,2129,2160,2199,2210,1911,2246,804,2276,1673,2299,2315,2322,2328,2355,2376,2405,1159,2425,2430,2452,1804,2442,2567,2577,1167,2611,2534,1879,2623,2682,2699,2652,2742,2754,2774,2782,2795,2431,2821,2751,2850,2090,513,2898,592,2932,2933,1555,2969,3003,3007,3010,2595,3064,3087,3105,3106,3110,151,3129,3132,304,3173,3205,3233,3245,3279,3302,3307,714,316,3331,3347,3360,3375,3380,3442,2620,3482,3493,3504,3516,3517,3518,3533,3511,2681,3530,3601,3606,3615,1210,3633,3651,3688,3690,3781,1907,3839,3840,3847,3867,3816,3899,3924,2345,3912,3966,982,4040,4056,4076,4084,4105,2649,4171,3873,1415,3567,4188,4221,4227,4231,2279,4250,4253,770,894,4343,4356,4289,4404,4438,2572,3124,4334,2114,3953,4522,4537,4561,4571,641,4629,4640,4664,4687,4702,4709,4740,4605,4746,4768,3856,3980,4814,2984,4895,4908,1249,4944,4947,4979,4988,4995,32,4066,5043,4956,5069,5072,5076,5084,5085,5137,4262,5152,479,5156,3114,1277,5183,5186,1825,5106,5216,963,5239,5252,5218,5284,1980,1972,5352,5364,5294,5379,5387,5391,5397,5419,5434,5468,5471,3350,5510,5522,5525,5538,5554,5573,5597,5610,5615,5624,842,2851,5641,5655,5656,5658,5678,5682,5696,5699,5709,5728,5753,851,5805,3528,5822,801,5855,2929,5871,5899,5918,5925,5927,5931,5935,5939,5958,778,5971,5980,5300,6009,6023,6030,6032,6016,6110,5009,6155,6197,1760,6253,6267,4886,5608,6289,6308,6311,6321,6316,6333,6244,6070,6349,6353,6186,6357,6366,6386,6387,6389,6399,6411,6421,6432,6437,6465,6302,6493,5602,6511,6529,6536,6170,6557,6561,6577,6581,6590,5290,5649,6231,6275,6635,6651,6652,5929,6692,6693,6695,6705,6711,6723,6738,6752,6753,3629,2975,6790,5845,338,6814,6826,6478,6860,6872,6882,880,356,6897,4102,6910,6611,1030,6934,6936,6987,6984,6999,827,6902,7027,7049,7051,4628,7084,7083,7071,7102,7137,5867,7152,6048,2410,3896,7168,7177,7224,6606,7233,1793,7261,7284,7290,7292,5212,7315,6964,3238,355,1969,4256,448,7325,908,2824,2981,3193,3363,3613,5325,6388,2247,1348,72,131,5414,7285,7343,7349,7362,7372,7381,7410,7418,7443,5512,7470,7487,7497,7516,7277,2622,2863,945,4344,3774,1024,2272,7523,4476,256,5643,3164,7539,7540,7489,1932,7559,7575,7602,7605,7609,7608,7619,7204,7652,7663,6907,7672,7654,7674,7687,7718,7745,1202,4030,7797,7801,7799,2924,7871,7873,7900,7907,7911,7912,7917,7923,7935,8007,8017,7636,8084,8087,3686,8114,8153,8158,8171,8175,8182,8205,8222,8225,8229,8232,8234,8244,8247,7256,8279,6929,8285,7040,8328,707,6773,7949,8468,5759,6344,8509,1635"},
+		{"too long line", "V28,34,30,55,100,130,87,169,194,202,232,252,267,274,286,315,308,221,358,368,401,406,434,452,475,422,497,530,517,559,400,418,571,578,599,600,625,630,635,647,220,715,736,760,705,785,794,495,808,852,861,863,869,875,890,893,896,922,812,980,1074,1087,1145,1153,1163,1171,445,1195,1203,1242,1255,1274,52,1287,1319,636,1160,1339,1345,1353,1369,1391,1396,1405,1221,1410,1431,1451,1460,1470,1472,1492,1517,1528,419,1530,1532,1535,1573,1547,1574,1437,1594,1595,847,1551,983,1637,1647,1666,1672,1691,1726,1515,1731,1739,1741,1723,1776,1685,505,1624,1436,1890,728,1910,1931,1544,2013,2025,2030,2043,2069,1162,2129,2160,2199,2210,1911,2246,804,2276,1673,2299,2315,2322,2328,2355,2376,2405,1159,2425,2430,2452,1804,2442,2567,2577,1167,2611,2534,1879,2623,2682,2699,2652,2742,2754,2774,2782,2795,2431,2821,2751,2850,2090,513,2898,592,2932,2933,1555,2969,3003,3007,3010,2595,3064,3087,3105,3106,3110,151,3129,3132,304,3173,3205,3233,3245,3279,3302,3307,714,316,3331,3347,3360,3375,3380,3442,2620,3482,3493,3504,3516,3517,3518,3533,3511,2681,3530,3601,3606,3615,1210,3633,3651,3688,3690,3781,1907,3839,3840,3847,3867,3816,3899,3924,2345,3912,3966,982,4040,4056,4076,4084,4105,2649,4171,3873,1415,3567,4188,4221,4227,4231,2279,4250,4253,770,894,4343,4356,4289,4404,4438,2572,3124,4334,2114,3953,4522,4537,4561,4571,641,4629,4640,4664,4687,4702,4709,4740,4605,4746,4768,3856,3980,4814,2984,4895,4908,1249,4944,4947,4979,4988,4995,32,4066,5043,4956,5069,5072,5076,5084,5085,5137,4262,5152,479,5156,3114,1277,5183,5186,1825,5106,5216,963,5239,5252,5218,5284,1980,1972,5352,5364,5294,5379,5387,5391,5397,5419,5434,5468,5471,3350,5510,5522,5525,5538,5554,5573,5597,5610,5615,5624,842,2851,5641,5655,5656,5658,5678,5682,5696,5699,5709,5728,5753,851,5805,3528,5822,801,5855,2929,5871,5899,5918,5925,5927,5931,5935,5939,5958,778,5971,5980,5300,6009,6023,6030,6032,6016,6110,5009,6155,6197,1760,6253,6267,4886,5608,6289,6308,6311,6321,6316,6333,6244,6070,6349,6353,6186,6357,6366,6386,6387,6389,6399,6411,6421,6432,6437,6465,6302,6493,5602,6511,6529,6536,6170,6557,6561,6577,6581,6590,5290,5649,6231,6275,6635,6651,6652,5929,6692,6693,6695,6705,6711,6723,6738,6752,6753,3629,2975,6790,5845,338,6814,6826,6478,6860,6872,6882,880,356,6897,4102,6910,6611,1030,6934,6936,6987,6984,6999,827,6902,7027,7049,7051,4628,7084,7083,7071,7102,7137,5867,7152,6048,2410,3896,7168,7177,7224,6606,7233,1793,7261,7284,7290,7292,5212,7315,6964,3238,355,1969,4256,448,7325,908,2824,2981,3193,3363,3613,5325,6388,2247,1348,72,131,5414,7285,7343,7349,7362,7372,7381,7410,7418,7443,5512,7470,7487,7497,7516,7277,2622,2863,945,4344,3774,1024,2272,7523,4476,256,5643,3164,7539,7540,7489,1932,7559,7575,7602,7605,7609,7608,7619,7204,7652,7663,6907,7672,7654,7674,7687,7718,7745,1202,4030,7797,7801,7799,2924,7871,7873,7900,7907,7911,7912,7917,7923,7935,8007,8017,7636,8084,8087,3686,8114,8153,8158,8171,8175,8182,8205,8222,8225,8229,8232,8234,8244,8247,7256,8279,6929,8285,7040,8328,707,6773,7949,8468,5759,6344,8509,1635\n.", "28,34,30,55,100,130,87,169,194,202,232,252,267,274,286,315,308,221,358,368,401,406,434,452,475,422,497,530,517,559,400,418,571,578,599,600,625,630,635,647,220,715,736,760,705,785,794,495,808,852,861,863,869,875,890,893,896,922,812,980,1074,1087,1145,1153,1163,1171,445,1195,1203,1242,1255,1274,52,1287,1319,636,1160,1339,1345,1353,1369,1391,1396,1405,1221,1410,1431,1451,1460,1470,1472,1492,1517,1528,419,1530,1532,1535,1573,1547,1574,1437,1594,1595,847,1551,983,1637,1647,1666,1672,1691,1726,1515,1731,1739,1741,1723,1776,1685,505,1624,1436,1890,728,1910,1931,1544,2013,2025,2030,2043,2069,1162,2129,2160,2199,2210,1911,2246,804,2276,1673,2299,2315,2322,2328,2355,2376,2405,1159,2425,2430,2452,1804,2442,2567,2577,1167,2611,2534,1879,2623,2682,2699,2652,2742,2754,2774,2782,2795,2431,2821,2751,2850,2090,513,2898,592,2932,2933,1555,2969,3003,3007,3010,2595,3064,3087,3105,3106,3110,151,3129,3132,304,3173,3205,3233,3245,3279,3302,3307,714,316,3331,3347,3360,3375,3380,3442,2620,3482,3493,3504,3516,3517,3518,3533,3511,2681,3530,3601,3606,3615,1210,3633,3651,3688,3690,3781,1907,3839,3840,3847,3867,3816,3899,3924,2345,3912,3966,982,4040,4056,4076,4084,4105,2649,4171,3873,1415,3567,4188,4221,4227,4231,2279,4250,4253,770,894,4343,4356,4289,4404,4438,2572,3124,4334,2114,3953,4522,4537,4561,4571,641,4629,4640,4664,4687,4702,4709,4740,4605,4746,4768,3856,3980,4814,2984,4895,4908,1249,4944,4947,4979,4988,4995,32,4066,5043,4956,5069,5072,5076,5084,5085,5137,4262,5152,479,5156,3114,1277,5183,5186,1825,5106,5216,963,5239,5252,5218,5284,1980,1972,5352,5364,5294,5379,5387,5391,5397,5419,5434,5468,5471,3350,5510,5522,5525,5538,5554,5573,5597,5610,5615,5624,842,2851,5641,5655,5656,5658,5678,5682,5696,5699,5709,5728,5753,851,5805,3528,5822,801,5855,2929,5871,5899,5918,5925,5927,5931,5935,5939,5958,778,5971,5980,5300,6009,6023,6030,6032,6016,6110,5009,6155,6197,1760,6253,6267,4886,5608,6289,6308,6311,6321,6316,6333,6244,6070,6349,6353,6186,6357,6366,6386,6387,6389,6399,6411,6421,6432,6437,6465,6302,6493,5602,6511,6529,6536,6170,6557,6561,6577,6581,6590,5290,5649,6231,6275,6635,6651,6652,5929,6692,6693,6695,6705,6711,6723,6738,6752,6753,3629,2975,6790,5845,338,6814,6826,6478,6860,6872,6882,880,356,6897,4102,6910,6611,1030,6934,6936,6987,6984,6999,827,6902,7027,7049,7051,4628,7084,7083,7071,7102,7137,5867,7152,6048,2410,3896,7168,7177,7224,6606,7233,1793,7261,7284,7290,7292,5212,7315,6964,3238,355,1969,4256,448,7325,908,2824,2981,3193,3363,3613,5325,6388,2247,1348,72,131,5414,7285,7343,7349,7362,7372,7381,7410,7418,7443,5512,7470,7487,7497,7516,7277,2622,2863,945,4344,3774,1024,2272,7523,4476,256,5643,3164,7539,7540,7489,1932,7559,7575,7602,7605,7609,7608,7619,7204,7652,7663,6907,7672,7654,7674,7687,7718,7745,1202,4030,7797,7801,7799,2924,7871,7873,7900,7907,7911,7912,7917,7923,7935,8007,8017,7636,8084,8087,3686,8114,8153,8158,8171,8175,8182,8205,8222,8225,8229,8232,8234,8244,8247,7256,8279,6929,8285,7040,8328,707,6773,7949,8468,5759,6344,8509,1635"},
 		{"FRAME Opcode and int", "\x95\x00\x00\x00\x00\x00\x00\x00\x00I5\n.", int64(5)},
 		{"SHORTBINUNICODE opcode", "\x8c\t\xe6\x97\xa5\xe6\x9c\xac\xe8\xaa\x9e\x94.", "日本語"},
 	}
@@ -75,6 +76,11 @@ func TestDecode(t *testing.T) {
 		if !reflect.DeepEqual(v, test.expected) {
 			t.Errorf("%s: got\n%q\n expected\n%q", test.name, v, test.expected)
 		}
+
+		v, err = dec.Decode()
+		if !(v == nil && err == io.EOF) {
+			t.Errorf("decode: no EOF at end: v = %#v  err = %#v", v, err)
+		}
 	}
 }
 
@@ -97,8 +103,23 @@ func TestDecodeMultiple(t *testing.T) {
 			t.Errorf("step #%v: %q  ; want %q", i, obj, objOk)
 		}
 	}
+
+	obj, err := dec.Decode()
+	if !(obj == nil && err == io.EOF) {
+		t.Errorf("decode: no EOF at end: obj = %#v  err = %#v", obj, err)
+	}
 }
 
+func TestDecodeUnexpectedEOF(t *testing.T) {
+	input := "I5\n"
+	buf := bytes.NewBufferString(input)
+	dec := NewDecoder(buf)
+
+	obj, err := dec.Decode()
+	if !(obj == nil && err == io.ErrUnexpectedEOF) {
+		t.Errorf("decode: no ErrUnexpectedEOF on truncated stream: obj = %#v  err = %#v", obj, err)
+	}
+}
 
 func TestZeroLengthData(t *testing.T) {
 	data := ""