Handle possible NodeList in source files when processing Middlewares //

Resolve #3531

env.Object() returns a list of objects that breaks the processing of
subsequent middlewares since we only expected File nodes.

HISTORY.rst

@@ -26,6 +26,7 @@ PlatformIO Core 4
 * Added support for "globstar/`**`" (recursive) pattern for the different commands and configuration options (`platformio ci <https://docs.platformio.org/page/core/userguide/cmd_ci.html>`__, `src_filter <https://docs.platformio.org/page/projectconf/section_env_build.html#src-filter>`__, `check_patterns <https://docs.platformio.org/page/projectconf/section_env_check.html#check-patterns>`__, `library.json > srcFilter <https://docs.platformio.org/page/librarymanager/config.html#srcfilter>`__). Python 3.5+ is required.
 * Added a new ``-e, --environment`` option to `platformio project init <https://docs.platformio.org/page/core/userguide/project/cmd_init.html#cmdoption-platformio-project-init-e>`__ command that helps to update a PlatformIO project using existing environment
 * Fixed an issue with PIO Unit Testing when running multiple environments (`issue #3523 <https://github.com/platformio/platformio-core/issues/3523>`_)
+* Fixed an issue with improper processing of source files added via multiple Build Middlewares (`issue #3531 <https://github.com/platformio/platformio-core/issues/3531>`_)
 
 4.3.4 (2020-05-23)
 ~~~~~~~~~~~~~~~~~~

platformio/builder/tools/platformio.py

@@ -20,6 +20,7 @@
 
 from SCons import Builder, Util  # pylint: disable=import-error
 from SCons.Node import FS  # pylint: disable=import-error
+from SCons.Node import NodeList  # pylint: disable=import-error
 from SCons.Script import COMMAND_LINE_TARGETS  # pylint: disable=import-error
 from SCons.Script import AlwaysBuild  # pylint: disable=import-error
 from SCons.Script import DefaultEnvironment  # pylint: disable=import-error
@@ -285,6 +286,8 @@ def CollectBuildFiles(
     for callback, pattern in env.get("__PIO_BUILD_MIDDLEWARES", []):
         tmp = []
         for node in sources:
+            if isinstance(node, NodeList):
+                node = node[0]
             if pattern and not fnmatch.fnmatch(node.srcnode().get_path(), pattern):
                 tmp.append(node)
                 continue