Use only one worker if debug mode is activated

CHANGELOG.rst

@@ -18,6 +18,7 @@ Version 0.5.0
 - Command line should be a lot faster when no work is done, e.g. for help
   output.
 - Fix compatibility with iCloud again.
+- Use only one worker if debug mode is activated.
 
 Version 0.4.4
 =============

tests/cli/test_main.py

@@ -38,6 +38,33 @@ def test_simple_run(tmpdir, runner):
     assert tmpdir.join('path_b/haha.txt').read() == 'UID:haha'
 
 
+def test_debug_connections(tmpdir, runner):
+    runner.write_with_general(dedent('''
+    [pair my_pair]
+    a = my_a
+    b = my_b
+
+    [storage my_a]
+    type = filesystem
+    path = {0}/path_a/
+    fileext = .txt
+
+    [storage my_b]
+    type = filesystem
+    path = {0}/path_b/
+    fileext = .txt
+    ''').format(str(tmpdir)))
+
+    tmpdir.mkdir('path_a')
+    tmpdir.mkdir('path_b')
+
+    result = runner.invoke(['-vdebug', 'sync', '--max-workers=3'])
+    assert 'using 3 maximal workers' in result.output.lower()
+
+    result = runner.invoke(['-vdebug', 'sync'])
+    assert 'using 1 maximal workers' in result.output.lower()
+
+
 def test_empty_storage(tmpdir, runner):
     runner.write_with_general(dedent('''
     [pair my_pair]

vdirsyncer/cli/__init__.py

@@ -48,14 +48,25 @@ def app(ctx, verbosity):
     if ctx.obj is None:
         ctx.obj = {}
 
+    ctx.obj['verbosity'] = verbosity
+
     if 'config' not in ctx.obj:
         ctx.obj['config'] = load_config()
 
 main = app
 
+def max_workers_callback(ctx, param, value):
+    if value == 0 and ctx.obj['verbosity'] == log.logging.DEBUG:
+        return 1
+    return value
+
+
 max_workers_option = click.option(
     '--max-workers', default=0, type=click.IntRange(min=0, max=None),
-    help=('Use at most this many connections, 0 means unlimited.')
+    callback=max_workers_callback,
+    help=('Use at most this many connections. With debug messages enabled, '
+          'the default is 1, otherwise an unlimited amount of connections is '
+          'used.')
 )