ffmpeg: Detect and fail on software decoder fallback

That goal of fluster is to test one specific implementation, allowing
software fallback hides possible integration issues.

fluster/decoders/ffmpeg.py

@@ -119,16 +119,26 @@ def decode(
         # MD5 muxer
         if self.use_md5_muxer and not keep_files:
             command.extend(["-f", "md5", "-"])
-            output = run_command_with_output(command, timeout=timeout, verbose=verbose)
+        # Output file
+        else:
+            command.extend(["-f", "rawvideo", output_filepath])
+
+        output = run_command_with_output(command, timeout=timeout,
+                verbose=verbose, keep_stderr=True)
+
+        # Detect software fallback and turn this into an error
+        if self.hw_acceleration:
+            hw_error = re.search(r"Failed setup for format", output)
+            if hw_error:
+               raise Exception("Failed to use HW accelerator.");
+
+        if self.use_md5_muxer and not keep_files:
             md5sum = re.search(r"MD5=([0-9a-fA-F]+)\s*", output)
             if not md5sum:
                 raise Exception("No MD5 found in the program trace.")
             return md5sum.group(1).lower()
-
-        # Output file
-        command.extend(["-f", "rawvideo", output_filepath])
-        run_command(command, timeout=timeout, verbose=verbose)
-        return file_checksum(output_filepath)
+        else:
+            return file_checksum(output_filepath)
 
     @lru_cache(maxsize=128)
     def check(self, verbose: bool) -> bool:

fluster/utils.py

@@ -88,9 +88,12 @@ def run_command_with_output(
     verbose: bool = False,
     check: bool = True,
     timeout: Optional[int] = None,
+    keep_stderr: bool = False,
 ) -> str:
     """Runs a command and returns std output trace"""
-    serr = subprocess.DEVNULL if not verbose else subprocess.STDOUT
+    serr = subprocess.DEVNULL
+    if verbose or keep_stderr:
+        serr = subprocess.STDOUT
     if verbose:
         print(f'\nRunning command "{" ".join(command)}"')