msgpack: support decimals with negative scale

Current decimal external type parser do not expect negative scale in
decimal payload. Negative scale is a positive exponent. It seems that
the only way to obtain positive exponent is to use E-notation since
Tarantool library do not truncate trailing zeroes before decimal point:

```
tarantool> msgpack.encode(decimal.new('1e33')):hex()
---
- c70301d0df1c
...

tarantool> msgpack.encode(decimal.new('1000000000000000000000000000000000')):hex()
---
- c713010001000000000000000000000000000000000c
...
```

There are two different bugs in current implementation:
- we support only `positive fixint` scale and do not expect `int 8` [2],
- we do not expect negative scale so positive exponent will be ignored.

This patch fixes both of them. See also [3].

1. https://github.com/tarantool/tarantool/blob/ba749e820bf0638aa3f79f266848590f9713c1cf/src/lib/core/decimal.c#L432-L450
2. https://github.com/msgpack/msgpack/blob/master/spec.md
3. tarantool/go-tarantool#314

Author: DifferentialOrange

Diff for: CHANGELOG.md

@@ -9,6 +9,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Added
 - Allow to require specific server protocol version and features (#267).
 
+### Fixed
+- Parsing of E-notation Tarantool decimals with positive exponent (PR #298).
+
 ## 1.0.0 - 2023-04-17
 
 ### Changed

Diff for: tarantool/msgpack_ext/decimal.py

@@ -56,6 +56,8 @@
 
 from decimal import Decimal
 
+import msgpack
+
 from tarantool.error import MsgpackError, MsgpackWarning, warn
 
 EXT_ID = 1
@@ -353,7 +355,12 @@ def decode(data, _):
     :raise: :exc:`~tarantool.error.MsgpackError`
     """
 
-    scale = data[0]
+    # A decimal starts with mp_int or mp_uint followed by raw bytes.
+    unpacker = msgpack.Unpacker()
+    unpacker.feed(data)
+
+    scale = unpacker.unpack()
+    scale_size = unpacker.tell()
 
     sign = get_str_sign(data[-1] & 0x0f)
 
@@ -362,7 +369,7 @@ def decode(data, _):
 
     add_str_digit(data[-1] >> 4, digits_reverted, scale)
 
-    for i in range(len(data) - 2, 0, -1):
+    for i in range(len(data) - 2, scale_size - 1, -1):
         add_str_digit(data[i] & 0x0f, digits_reverted, scale)
         add_str_digit(data[i] >> 4, digits_reverted, scale)
 
@@ -372,6 +379,12 @@ def decode(data, _):
 
     digits_reverted.append(sign)
 
-    str_repr = ''.join(digits_reverted[::-1])
+    digits = digits_reverted[::-1]
+
+    # Add trailing zeroes in case of a negative scale
+    for i in range(0, -1 * scale):
+        add_str_digit(0, digits, scale)
+
+    str_repr = ''.join(digits)
 
     return Decimal(str_repr)

Diff for: test/suites/test_decimal.py

@@ -226,6 +226,39 @@ def setUp(self):
                         b'\x09\x87\x65\x43\x21\x98\x76\x54\x32\x1d'),
             'tarantool': "decimal.new('-1234567891234567890.0987654321987654321')",
         },
+        'decimal_exponent_1': {
+            'python': decimal.Decimal('1e33'),
+            'msgpack': (b'\x00\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00'
+                        b'\x00\x00\x00\x00\x00\x00\x00\x0c'),
+            'tarantool': "decimal.new('1e33')",
+        },
+        'decimal_exponent_2': {
+            'python': decimal.Decimal('1.2345e33'),
+            'msgpack': (b'\x00\x01\x23\x45\x00\x00\x00\x00\x00\x00\x00'
+                        b'\x00\x00\x00\x00\x00\x00\x00\x0c'),
+            'tarantool': "decimal.new('1.2345e33')",
+        },
+        'decimal_exponent_3': {
+            'python': decimal.Decimal('1.2345e2'),
+            'msgpack': (b'\x02\x12\x34\x5c'),
+            'tarantool': "decimal.new('1.2345e2')",
+        },
+        'decimal_exponent_4': {
+            'python': decimal.Decimal('1.2345e4'),
+            'msgpack': (b'\x00\x12\x34\x5c'),
+            'tarantool': "decimal.new('1.2345e4')",
+        },
+        'decimal_exponent_5': {
+            'python': decimal.Decimal('-1e33'),
+            'msgpack': (b'\x00\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00'
+                        b'\x00\x00\x00\x00\x00\x00\x00\x0d'),
+            'tarantool': "decimal.new('-1e33')",
+        },
+        'decimal_exponent_6': {
+            'python': decimal.Decimal('1e-33'),
+            'msgpack': (b'\x21\x1c'),
+            'tarantool': "decimal.new('1e-33')",
+        },
     }
 
     def test_msgpack_decode(self):