Introduce cuts: a flow dissector

This PR introduces the cuts addon, a flow dissector that  allows you to select
and operate on specific components of flows. It also adds the first consumer
for cuts - the cuts.save command.

Save the content of the focus to /tmp/foo:

    cuts.save s.content|@focus /tmp/foo

Save the URL and response content-type headers for all flows currently shown to
file, comma-separated, one flow per line:

    cuts.save s.url,q.header[content-type]|@focus /tmp/foo

We also use this to replace the body save shortcut in the console flowlist.

2 files changed, 14 insertions, 3 deletions

diff --git a/mitmproxy/utils/strutils.py b/mitmproxy/utils/strutils.py
index 1b90c2e5..db0cfd2e 100644
--- a/mitmproxy/utils/strutils.py
+++ b/mitmproxy/utils/strutils.py
@@ -25,9 +25,10 @@ def always_str(str_or_bytes: Optional[AnyStr], *decode_args) -> Optional[str]:
         raise TypeError("Expected str or bytes, but got {}.".format(type(str_or_bytes).__name__))
 
 
-# Translate control characters to "safe" characters. This implementation initially
-# replaced them with the matching control pictures (http://unicode.org/charts/PDF/U2400.pdf),
-# but that turned out to render badly with monospace fonts. We are back to "." therefore.
+# Translate control characters to "safe" characters. This implementation
+# initially replaced them with the matching control pictures
+# (http://unicode.org/charts/PDF/U2400.pdf), but that turned out to render badly
+# with monospace fonts. We are back to "." therefore.
 _control_char_trans = {
     x: ord(".")  # x + 0x2400 for unicode control group pictures
     for x in range(32)
diff --git a/mitmproxy/utils/typecheck.py b/mitmproxy/utils/typecheck.py
index 20791e17..c97ff529 100644
--- a/mitmproxy/utils/typecheck.py
+++ b/mitmproxy/utils/typecheck.py
@@ -19,6 +19,16 @@ def check_command_return_type(value: typing.Any, typeinfo: typing.Any) -> bool:
         for v in value:
             if not check_command_return_type(v, T):
                 return False
+    elif typename.startswith("typing.Union"):
+        try:
+            types = typeinfo.__args__  # type: ignore
+        except AttributeError:
+            # Python 3.5.x
+            types = typeinfo.__union_params__  # type: ignore
+        for T in types:
+            checks = [check_command_return_type(value, T) for T in types]
+            if not any(checks):
+                return False
     elif value is None and typeinfo is None:
         return True
     elif not isinstance(value, typeinfo):