handlers: add http-redirect example

bin/handlers/README.md

@@ -20,6 +20,7 @@ each plugin must define a `main()` which takes 3 arguments;
 
 ## on404
 
+* [redirect.py](redirect.py) sends an HTTP 301 or 302, redirecting the client to another page/file
 * [sorry.py](answer.py) replies with a custom message instead of the usual 404
 * [nooo.py](nooo.py) replies with an endless noooooooooooooo
 * [never404.py](never404.py) 100% guarantee that 404 will never be a thing again as it automatically creates dummy files whenever necessary

bin/handlers/redirect.py

@@ -0,0 +1,52 @@
+# if someone hits a 404, redirect them to another location
+
+
+def send_http_302_temporary_redirect(cli, new_path):
+    """
+    replies with an HTTP 302, which is a temporary redirect;
+    "new_path" can be any of the following:
+      - "http://a.com/" would redirect to another website,
+      - "/foo/bar" would redirect to /foo/bar on the same server;
+          note the leading '/' in the location which is important
+    """
+    cli.reply(b"redirecting...", 302, headers={"Location": new_path})
+
+
+def send_http_301_permanent_redirect(cli, new_path):
+    """
+    replies with an HTTP 301, which is a permanent redirect;
+    otherwise identical to send_http_302_temporary_redirect
+    """
+    cli.reply(b"redirecting...", 301, headers={"Location": new_path})
+
+
+def send_errorpage_with_redirect_link(cli, new_path):
+    """
+    replies with a website explaining that the page has moved;
+    "new_path" must be an absolute location on the same server
+    but without a leading '/', so for example "foo/bar"
+    would redirect to "/foo/bar"
+    """
+    cli.redirect(new_path, click=False, msg="this page has moved")
+
+
+def main(cli, vn, rem):
+    """
+    this is the function that gets called by copyparty;
+    note that vn.vpath and cli.vpath does not have a leading '/'
+    so we're adding the slash in the debug messages below
+    """
+    print(f"this client just hit a 404: {cli.ip}")
+    print(f"they were accessing this volume: /{vn.vpath}")
+    print(f"and the original request-path (straight from the URL) was /{cli.vpath}")
+    print(f"...which resolves to the following filesystem path: {vn.canonical(rem)}")
+
+    new_path = "/foo/bar/"
+    print(f"will now redirect the client to {new_path}")
+
+    # uncomment one of these:
+    send_http_302_temporary_redirect(cli, new_path)
+    #send_http_301_permanent_redirect(cli, new_path)
+    #send_errorpage_with_redirect_link(cli, new_path)
+
+    return "true"