Compatibility fixes for Blender 2.78c

Blender 2.78c is shipped with a version of the io_blend_utils module that
doesn't have a `pythonpath()` function yet, and that's bundled with an
older version of BAM. To work around this, we ship BAM as wheel, and detect
whether this version is needed to run.

As an added bonus, Blender 2.78c can now also use the file exclude filter
for Flamenco. The `bam_supports_exclude_option()` function is thus no
longer necessary.

blender_cloud/flamenco/bam_interface.py

@@ -1,6 +1,5 @@
 """BAM packing interface for Flamenco."""
 
-import functools
 import logging
 from pathlib import Path
 import typing
@@ -15,26 +14,26 @@ class CommandExecutionError(Exception):
     pass
 
 
-if 'bam_supports_exclude_option' in locals():
-    locals()['bam_supports_exclude_option'].cache_clear()
+def wheel_pythonpath_278() -> str:
+    """Returns the value of a PYTHONPATH environment variable needed to run BAM from its wheel file.
 
-
-@functools.lru_cache(maxsize=1)
-def bam_supports_exclude_option() -> bool:
-    """Returns True if the version of BAM bundled with Blender supports --exclude.
-
-    This feature was added to BAM 1.1.7, so we can do a simple version check.
+    Workaround for Blender 2.78c not having io_blend_utils.pythonpath()
     """
 
-    try:
-        import io_blend_utils
-    except ImportError:
-        # If this happens, BAM won't work at all. However, this function can be called from
-        # the GUI; by being a bit careful while importing, we avoid breaking Blender's GUI.
-        log.exception('Error importing io_blend_utils module.')
-        return False
+    import os
+    from ..wheels import wheel_filename
 
-    return io_blend_utils.bl_info['version'] >= (1, 1, 7)
+    # Find the wheel to run.
+    wheelpath = wheel_filename('blender_bam')
+
+    log.info('Using wheel %s to run BAM-Pack', wheelpath)
+
+    # Update the PYTHONPATH to include that wheel.
+    existing_pypath = os.environ.get('PYTHONPATH', '')
+    if existing_pypath:
+        return os.pathsep.join((existing_pypath, wheelpath))
+
+    return wheelpath
 
 
 async def bam_copy(base_blendfile: Path, target_blendfile: Path,
@@ -66,18 +65,20 @@ async def bam_copy(base_blendfile: Path, target_blendfile: Path,
     ]
 
     if exclusion_filter:
-        if bam_supports_exclude_option():
-            args.extend(['--exclude', exclusion_filter])
-        else:
-            log.warning('Your version of Blender does not support the exclusion filter, '
-                        'copying all files.')
+        args.extend(['--exclude', exclusion_filter])
 
     cmd_to_log = ' '.join(shlex.quote(s) for s in args)
     log.info('Executing %s', cmd_to_log)
 
+    # Workaround for Blender 2.78c not having io_blend_utils.pythonpath()
+    if hasattr(io_blend_utils, 'pythonpath'):
+        pythonpath = io_blend_utils.pythonpath()
+    else:
+        pythonpath = wheel_pythonpath_278()
+
     proc = await asyncio.create_subprocess_exec(
         *args,
-        env={'PYTHONPATH': io_blend_utils.pythonpath()},
+        env={'PYTHONPATH': pythonpath},
         stdin=subprocess.DEVNULL,
         stdout=subprocess.PIPE,
         stderr=subprocess.STDOUT,