pio: simplify single-source builder

Generate resulting .cpp in the $BUILD_DIR, no need for atexit
Register middleware only once, there's no need to manually walk the
project directory since fnmatch.fnmatch(...) supports globs

Provide a SCons Action instead of simple function, so there's a way to
generate things on demand via command line
```
$ pio run -e $env -t .pio/build/$env/espurna_single_source/src/main.cpp
```

code/scripts/espurna_utils/__init__.py

@@ -19,7 +19,12 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
-from .build import firmware_destination, app_add_target_build_and_copy, app_add_target_build_re2c
+from .build import (
+    firmware_destination,
+    app_add_builder_single_source,
+    app_add_target_build_and_copy,
+    app_add_target_build_re2c,
+)
 from .checks import check_env, check_cppcheck, check_printsize
 from .flags import app_inject_flags
 from .float_support import remove_float_support
@@ -28,14 +33,15 @@ from .postmortem import dummy_ets_printf
 from .version import app_inject_version, app_full_version_for_env
 
 __all__ = [
+    "app_add_builder_single_source",
     "app_add_target_build_and_copy",
     "app_add_target_build_re2c",
     "app_full_version_for_env",
     "app_inject_flags",
     "app_inject_version",
     "app_version",
-    "check_env",
     "check_cppcheck",
+    "check_env",
     "check_printsize",
     "dummy_ets_printf",
     "firmware_destination",

code/scripts/espurna_utils/build.py

@@ -1,33 +1,43 @@
-import atexit
 import os
 import shutil
 import tempfile
 
-from .display import print_warning
 from .version import app_full_version_for_env
 
 
-def try_remove(path):
-    try:
-        os.remove(path)
-    except:  # pylint: disable=bare-except
-        print_warning("Please manually remove the file `{}`".format(path))
-
-
 # emulate .ino concatenation to speed up compilation times
-def merge_cpp(sources, output):
+def merge_cpp(target, source, env, encoding="utf-8"):
     with tempfile.TemporaryFile() as tmp:
         tmp.write(b"// !!! Automatically generated file; DO NOT EDIT !!! \n")
-        tmp.write(b'#include "espurna.h"\n')
-        for source in sources:
-            src_include = '#include "{}"\n'.format(source)
-            tmp.write(src_include.encode("utf-8"))
+        tmp.write(
+            '#include "{}"\n'.format(
+                env.File("${PROJECT_DIR}/espurna/espurna.h").get_abspath()
+            ).encode(encoding)
+        )
+        for src in source:
+            src_include = '#include "{}"\n'.format(src.get_abspath())
+            tmp.write(src_include.encode(encoding))
 
         tmp.seek(0)
 
-        with open(output, "wb") as fobj:
+        with open(target[0].get_abspath(), "wb") as fobj:
             shutil.copyfileobj(tmp, fobj)
-        atexit.register(try_remove, output)
+
+
+def app_add_builder_single_source(env):
+    # generate things in the $BUILD_DIR, so there's no need for any extra clean-up code
+    source = os.path.join("${BUILD_DIR}", "espurna_single_source", "src", "main.cpp")
+
+    # substitute a single node instead of building it somewhere else as a lib or extra source dir
+    # (...and since we can't seem to modify src_filter specifically for the project dir, only middleware works :/)
+    def ignore_node(node):
+        if node.name.endswith("main.cpp"):
+            return env.File(source)
+        return None
+
+    project = env.Dir("${PROJECT_DIR}/espurna")
+    env.AddBuildMiddleware(ignore_node, os.path.join(project.get_abspath(), "*.cpp"))
+    env.Command(source, env.Glob("${PROJECT_DIR}/espurna/*.cpp"), env.Action(merge_cpp))
 
 
 def firmware_prefix(env):

code/scripts/pio_pre.py

@@ -15,7 +15,7 @@ import sys
 from SCons.Script import Import, ARGUMENTS
 
 from espurna_utils import check_env
-from espurna_utils.build import merge_cpp, app_add_target_build_re2c
+from espurna_utils.build import app_add_builder_single_source, app_add_target_build_re2c
 
 Import("env")
 env = globals()["env"]
@@ -127,18 +127,7 @@ if len(ino) == 1 and ino[0].name == "espurna.ino":
 
 # merge every .cpp into a single file and **only** build that single file
 if check_env("ESPURNA_BUILD_SINGLE_SOURCE", "0"):
-    cpp_files = []
-    for root, dirs, filenames in os.walk("espurna"):
-        for name in filenames:
-            if not name.endswith(".cpp"):
-                continue
-
-            abspath = os.path.join(os.path.abspath(root), name)
-            env.AddBuildMiddleware(lambda node: None, abspath)
-
-            relpath = os.path.relpath(abspath, "espurna")
-            cpp_files.append(relpath)
-    merge_cpp(cpp_files, "espurna/espurna_single_source.cpp")
+    app_add_builder_single_source(env)
 
 # handle explicit targets that have .re.cpp.inc, build them before falling into the next sconsfile
 app_add_target_build_re2c(env)