Feat/issue 605 debug logging via env variable (#608)

* feat: Debug Logging via Environment Variable

* refactor: deleted libp2p/utils.py

* fix: double messages logging fix

* doc: add logging info to getting_started.rst

libp2p/utils/varint.py

@@ -0,0 +1,87 @@
+import itertools
+import logging
+import math
+
+from libp2p.exceptions import (
+    ParseError,
+)
+from libp2p.io.abc import (
+    Reader,
+)
+from libp2p.io.utils import (
+    read_exactly,
+)
+
+logger = logging.getLogger("libp2p.utils.varint")
+
+# Unsigned LEB128(varint codec)
+# Reference: https://github.com/ethereum/py-wasm/blob/master/wasm/parsers/leb128.py
+
+LOW_MASK = 2**7 - 1
+HIGH_MASK = 2**7
+
+# The maximum shift width for a 64 bit integer.  We shouldn't have to decode
+# integers larger than this.
+SHIFT_64_BIT_MAX = int(math.ceil(64 / 7)) * 7
+
+
+def encode_uvarint(number: int) -> bytes:
+    """Pack `number` into varint bytes."""
+    buf = b""
+    while True:
+        towrite = number & 0x7F
+        number >>= 7
+        if number:
+            buf += bytes((towrite | 0x80,))
+        else:
+            buf += bytes((towrite,))
+            break
+    return buf
+
+
+async def decode_uvarint_from_stream(reader: Reader) -> int:
+    """https://en.wikipedia.org/wiki/LEB128."""
+    res = 0
+    for shift in itertools.count(0, 7):
+        if shift > SHIFT_64_BIT_MAX:
+            raise ParseError("TODO: better exception msg: Integer is too large...")
+
+        byte = await read_exactly(reader, 1)
+        value = byte[0]
+
+        res += (value & LOW_MASK) << shift
+
+        if not value & HIGH_MASK:
+            break
+    return res
+
+
+def encode_varint_prefixed(msg_bytes: bytes) -> bytes:
+    varint_len = encode_uvarint(len(msg_bytes))
+    return varint_len + msg_bytes
+
+
+async def read_varint_prefixed_bytes(reader: Reader) -> bytes:
+    len_msg = await decode_uvarint_from_stream(reader)
+    data = await read_exactly(reader, len_msg)
+    return data
+
+
+# Delimited read/write, used by multistream-select.
+# Reference: https://github.com/gogo/protobuf/blob/07eab6a8298cf32fac45cceaac59424f98421bbc/io/varint.go#L109-L126  # noqa: E501
+
+
+def encode_delim(msg: bytes) -> bytes:
+    delimited_msg = msg + b"\n"
+    return encode_varint_prefixed(delimited_msg)
+
+
+async def read_delim(reader: Reader) -> bytes:
+    msg_bytes = await read_varint_prefixed_bytes(reader)
+    if len(msg_bytes) == 0:
+        raise ParseError("`len(msg_bytes)` should not be 0")
+    if msg_bytes[-1:] != b"\n":
+        raise ParseError(
+            f'`msg_bytes` is not delimited by b"\\n": `msg_bytes`={msg_bytes!r}'
+        )
+    return msg_bytes[:-1]