fix(tracing): fix msgpack import error (#7271)

## Description

- This change fixes an import error raised when `DD_TRACE_API` is set to
`v0.5`. This occurs because msgpack is not installed by ddtrace.
- This feature was reverted on 2.x due to performance reasons (#7263).
It only exists on 1.20

## Motivation
[Import
msgpack](https://github.com/DataDog/dd-trace-py/blob/v2.1.1/ddtrace/internal/_encoding.pyx#L425)
is problematic and breaks v0.5 encoding. To fix this issue we can ensure
the ddtrace library installs msgpack (or we can vendor it) but this
comes with it's own drawbacks. Instead of fixing the importerror this PR
only imports msgpack if `_DD_TRACE_WRITER_LOG_ERROR_PAYLOADS=True`.


- [x] checkbox

---------

Co-authored-by: Munir Abdinur <[email protected]>
Co-authored-by: Munir Abdinur <[email protected]>

Author: github-actions[bot]

ddtrace/internal/_encoding.pyi

@@ -17,7 +17,10 @@ class BufferItemTooLarge(Exception):
     pass
 
 class EncodingValidationError(Exception):
-    pass
+    def __init__(self, msg="", debug_message=""):
+        Exception.__init__(self, msg)
+        self.msg = msg
+        self.debug_message = debug_message
 
 class BufferedEncoder(object):
     max_size: int

ddtrace/internal/_encoding.pyx

@@ -68,7 +68,10 @@ class BufferItemTooLarge(Exception):
 
 
 class EncodingValidationError(Exception):
-    pass
+    def __init__(self, msg="", debug_message=""):
+        Exception.__init__(self, msg)
+        self.msg = msg
+        self.debug_message = debug_message
 
 
 cdef inline const char * string_to_buff(str s):
@@ -853,13 +856,16 @@ cdef class MsgpackEncoderV05(MsgpackEncoderBase):
         return 0
 
     cpdef _verify_encoding(self, encoded_bytes):
-        import msgpack
-
         from ddtrace import config
 
         if not config._trace_writer_log_err_payload:
             return
 
+        # msgpack is not installed by ddtrace. To decode ddtrace payloads ensure
+        # msgpack is installed
+        # TODO: vendor msgpack.unpackb()
+        import msgpack
+
         unpacked = msgpack.unpackb(encoded_bytes, raw=True, strict_map_key=False)
         if not unpacked or not unpacked[0]:
             return unpacked
@@ -916,11 +922,7 @@ cdef class MsgpackEncoderV05(MsgpackEncoderBase):
                     )
                 except Exception as e:
                     eve = EncodingValidationError(
-                        str(e) + "\nDecoded Span does not match encoded span: {}".format(og_span._pprint())
-                    )
-                    setattr(
-                        eve,
-                        "_debug_message",
+                        str(e) + "\nDecoded Span does not match encoded span: {}".format(og_span._pprint()),
                         "Malformed String table values\ntable:{}\ntraces:{}\nencoded_bytes:{}\nspans:{}".format(
                             table, packed_traces, encoded_bytes, self._encoded_spans
                         ),

ddtrace/internal/writer/writer.py

@@ -401,8 +401,7 @@ def _flush_queue_with_client(self, client, raise_exc=False):
                 return
         except EncodingValidationError as e:
             log.error("Encoding Error (or span was modified after finish): %s", str(e))
-            if hasattr(e, "_debug_message"):
-                log.debug(e._debug_message)
+            log.debug(e.debug_message)
             return
         except Exception:
             log.error("failed to encode trace with encoder %r", client.encoder, exc_info=True)

releasenotes/notes/fix-msgpack-errors-3fcf9f77a77c9786.yaml

@@ -0,0 +1,4 @@
+---
+fixes:
+  - |
+    tracing: Fixes a msgpack import error when ``DD_TRACE_API`` is set to ``v0.5``