Fix capture output when running tests that call the ConanRunner in the conanfile

conans/client/runner.py

@@ -14,7 +14,7 @@
 
 class _UnbufferedWrite(object):
     def __init__(self, stream):
-        self._stream = stream
+        self._stream = stream._stream if hasattr(stream, "_stream") else stream
 
     def write(self, *args, **kwargs):
         self._stream.write(*args, **kwargs)
@@ -43,7 +43,7 @@ def __call__(self, command, output=True, log_filepath=None, cwd=None, subprocess
                   "use six.StringIO() instead ***")
 
         stream_output = output if output and hasattr(output, "write") else self._output or sys.stdout
-        if hasattr(output, "flush"):
+        if hasattr(stream_output, "flush"):
             # We do not want output from different streams to get mixed (sys.stdout, os.system)
             stream_output = _UnbufferedWrite(stream_output)
 

conans/test/functional/conanfile/runner_test.py

@@ -1,4 +1,5 @@
 import os
+import textwrap
 import unittest
 
 import six
@@ -180,3 +181,15 @@ def build(self):
         client.run("build .", assert_error=True)
         self.assertIn("Error while executing 'mkdir test_folder'", client.out)
         self.assertFalse(os.path.exists(test_folder))
+
+    def runner_capture_output_test(self):
+        conanfile = textwrap.dedent("""
+            from conans import ConanFile
+            class Pkg(ConanFile):
+                def source(self):
+                    self.run("echo 'hello Conan!'")
+        """)
+        client = TestClient()
+        client.save({"conanfile.py": conanfile})
+        client.run("source .")
+        self.assertIn("hello Conan!", client.out)