feat(analyzer): analyze replaygain using ffmpeg

- remove pycairo pip install
- fix py36 compatibility
- reraise when executable was not found

BREAKING CHANGE: The analyzer requires 'ffmpeg'. The 'rgain3' python package and it's system dependencies can be removed.

analyzer/libretime_analyzer/steps/analyze_replaygain.py

@@ -1,42 +1,27 @@
-import re
-import subprocess
+from subprocess import CalledProcessError
 from typing import Any, Dict
 
-from loguru import logger
-
-REPLAYGAIN_EXECUTABLE = "replaygain"  # From the rgain3 python package
+from ..ffmpeg import compute_replaygain, probe_replaygain
 
 
-def analyze_replaygain(filename: str, metadata: Dict[str, Any]):
-    """Extracts the Replaygain loudness normalization factor of a track.
-    :param filename: The full path to the file to analyzer
-    :param metadata: A metadata dictionary where the results will be put
-    :return: The metadata dictionary
+def analyze_replaygain(filepath: str, metadata: Dict[str, Any]):
     """
-    """ The -d flag means do a dry-run, ie. don't modify the file directly.
+    Extracts the Replaygain loudness normalization factor of a track using ffmpeg.
     """
-    command = [REPLAYGAIN_EXECUTABLE, "-d", filename]
     try:
-        results = subprocess.check_output(
-            command,
-            stderr=subprocess.STDOUT,
-            close_fds=True,
-            universal_newlines=True,
-        )
-        gain_match = (
-            r"Calculating Replay Gain information \.\.\.(?:\n|.)*?:([\d.-]*) dB"
-        )
-        replaygain = re.search(gain_match, results).group(1)
-        metadata["replay_gain"] = float(replaygain)
+        # First probe for existing replaygain metadata.
+        track_gain = probe_replaygain(filepath)
+        if track_gain is not None:
+            metadata["replay_gain"] = track_gain
+            return metadata
+    except (CalledProcessError, OSError):
+        pass
 
-    except OSError as e:  # replaygain was not found
-        logger.warning(
-            "Failed to run: %s - %s. %s"
-            % (command[0], e.strerror, "Do you have python-rgain installed?")
-        )
-    except subprocess.CalledProcessError as e:  # replaygain returned an error code
-        logger.warning("%s %s %s", e.cmd, e.output, e.returncode)
-    except Exception as e:
-        logger.warning(e)
+    try:
+        track_gain = compute_replaygain(filepath)
+        if track_gain is not None:
+            metadata["replay_gain"] = track_gain
+    except (CalledProcessError, OSError):
+        pass
 
     return metadata