Add portfile support (#7724)

Convenient when manipulating mitmproxy programmatically, as it's often
useful to specify port 0 (for all sorts of servers) but then the user
needs a way to retrieve the actual port being used.

mitmproxy logs the bound ports out by default, however because it logs
to the fully buffered stdout *and* can generate quite a bit of log,
this tends to be inconvenient. Writing out a file at a user-specified
location makes it much easier for them to poll, and using json even if
there is a minor race the user can just keep reading until they see a
valid JSON file.

examples/contrib/portfile.py

@@ -0,0 +1,40 @@
+import json
+import pathlib
+from typing import Optional
+
+from mitmproxy import ctx
+
+
+class PortFile:
+    def load(self, loader):
+        loader.add_option(
+            name="datadir",
+            typespec=Optional[str],
+            default=None,
+            help="Creates `portfile` mapping proxies (by mode spec) to the port "
+            "they use in the provided directory.",
+        )
+
+    def running(self):
+        if not ctx.options.datadir:
+            return
+
+        datadir = pathlib.Path(ctx.options.datadir)
+        if not datadir.is_dir():
+            ctx.log.warning("%s is not a directory", datadir)
+            return
+
+        proxies = ctx.master.addons.get("proxyserver")
+        modemap = {
+            instance.mode.full_spec: addr[1]
+            for instance in proxies.servers
+            # assumes all listen_addrs of a given instance are bound
+            # to the same port, but as far as I can tell mitmproxy
+            # works very hard to try and make it so
+            if (addr := next(iter(instance.listen_addrs), None))
+        }
+        with datadir.joinpath("portfile").open("w", encoding="utf-8") as fp:
+            json.dump(modemap, fp)
+
+
+addons = [PortFile()]