Implement error/warning for the bad dynamic_context being set in config.

Author: ionelmc

Diff for: src/pytest_cov/engine.py

@@ -7,6 +7,7 @@
 import random
 import socket
 import sys
+import warnings
 from io import StringIO
 from pathlib import Path
 
@@ -15,6 +16,11 @@
 from coverage.sqldata import filename_suffix
 
 from .embed import cleanup
+from .plugin import PytestCovWarning
+
+
+class BrokenCovConfigError(Exception):
+    pass
 
 
 class _NullFile:
@@ -225,6 +231,10 @@ def summary(self, stream):
         return total
 
 
+class CentralCovContextWarning(PytestCovWarning):
+    pass
+
+
 class Central(CovController):
     """Implementation for centralised operation."""
 
@@ -238,6 +248,13 @@ def start(self):
             data_suffix=_data_suffix('c'),
             config_file=self.cov_config,
         )
+        if self.cov.config.dynamic_context == 'test_function':
+            message = (
+                'Detected dynamic_context=test_function in coverage configuration. '
+                'This is unnecessary as this plugin provides the more complete --cov-context option.'
+            )
+            warnings.warn(CentralCovContextWarning(message), stacklevel=1)
+
         self.combining_cov = coverage.Coverage(
             source=self.cov_source,
             branch=self.cov_branch,
@@ -269,6 +286,10 @@ def finish(self):
         self.node_descs.add(node_desc)
 
 
+class DistCovError(Exception):
+    pass
+
+
 class DistMaster(CovController):
     """Implementation for distributed master."""
 
@@ -282,6 +303,12 @@ def start(self):
             data_suffix=_data_suffix('m'),
             config_file=self.cov_config,
         )
+        if self.cov.config.dynamic_context == 'test_function':
+            raise DistCovError(
+                'Detected dynamic_context=test_function in coverage configuration. '
+                'This is known to cause issues when using xdist, see: https://github.com/pytest-dev/pytest-cov/issues/604\n'
+                'It is recommended to use --cov-context instead.'
+            )
         self.cov._warn_no_data = False
         self.cov._warn_unimported_source = False
         self.cov._warn_preimported_source = False

Diff for: tests/test_pytest_cov.py

@@ -1673,11 +1673,15 @@ def test_dynamic_context(pytester, testdir, opts, prop):
 {prop.conf}
 """)
     result = testdir.runpytest('-v', f'--cov={script.dirpath()}', script, *opts.split() + prop.args)
-    result.stdout.fnmatch_lines(
-        [
-            f'test_1* {prop.result}*',
-        ]
-    )
+    if opts:
+        result.stderr.fnmatch_lines(['pytest_cov.engine.DistCovError: Detected dynamic_context=test_function*'])
+    else:
+        result.stdout.fnmatch_lines(
+            [
+                '* CentralCovContextWarning: Detected dynamic_context=test_function*',
+                f'test_1* {prop.result}*',
+            ]
+        )
 
 
 @xdist_params