logging: capture warnings emitted using warnings.warn()

By default, warnings emitted by warnings.warn() will show on stderr. For
a TUI application, this is a problem because they will be displayed over
the UI and create visual glitches.

For parts of the UI that are regularly redrawn, the text of the warning
will eventually go away (sometimes instantly, making the warning
impossible to read).

For parts of the screen that are *not* regularly redrawn, the text of
the warning will stay until the user sends a Ctrl-L sequence.

We now make use of logging.captureWarnings() to redirect the warnings to
the log files. This prevents the UI glitches and if any warning is
emitted, we can actually find it in the logs.

Signed-off-by: Olivier Gayot <olivier.gayot@canonical.com>

subiquity/cmd/server.py

@@ -193,6 +193,7 @@ def main():
 
     logfiles = setup_logger(dir=logdir, base="subiquity-server")
 
+    logging.captureWarnings(True)
     logger = logging.getLogger("subiquity")
     revision = os.environ.get("SNAP_REVISION", "unknown")
     version = os.environ.get("SNAP_VERSION", "unknown")

subiquity/cmd/tui.py

@@ -124,6 +124,7 @@ def main():
         logdir = opts.output_base
     logfiles = setup_logger(dir=logdir, base="subiquity-client")
 
+    logging.captureWarnings(True)
     logger = logging.getLogger("subiquity")
     version = os.environ.get("SNAP_REVISION", "unknown")
     snap = os.environ.get("SNAP", "unknown")