Add simple support for redirects

[mogenson]

Follow 3XX status redirects with a new request. Parse the location
header for an absolute url, absolute path, or relative path. Replace or
amend the url from the original request and place a new request.

Expand the requests_simpletest_cpython.py example with new requests
utilizing the three supported types of redirects.

Note: We're using the httpbingo.org domain for redirects until
the following issue on httpbin.org is resolved:
postmanlabs/httpbin#617

This has been tested on CPython 3.8 and an ESP32-S2 based MagTag board
with CircuitPython 6.0.

[mogenson]

By the way, a fun request that requires redirect support is using the Wikipedia API to fetch a random article:

import socket
import ssl
import adafruit_requests

URL = "https://en.wikipedia.org/api/rest_v1/page/random/summary"
http = adafruit_requests.Session(socket, ssl.create_default_context())

response = http.get(URL)
article = response.json()

print(article["title"])
print(article["extract"])

[anecdata]

looks like it's failing on black, check out: https://learn.adafruit.com/creating-and-sharing-a-circuitpython-library/check-your-code

[mogenson]

Ah, black doesn't like that line 610 is folded but my editor doesn't like lines over 80 characters.

Good point about case sensitive headers.

Multiple redirects work, however since request() has been converted into a recursive method a very large number of consecutive redirects may crash the stack, especially on a microcontroller. We could set a flag to indicate we're handling a redirect, increment a counter, and compare to a static or user-defined limit. Or, assume most CircuitPython products are fetching from a pre-defined URL that will not redirect more than a few times.

I'll fixup this PR tonight.

[anecdata]

Tested with modified simpletest from the library examples using::
Adafruit CircuitPython 7.0.0-alpha.3 on 2021-06-03; Adafruit PyPortal with samd51j20

Got RuntimeError: pystack exhausted with redirects > 3 using URLs of this form:
https://httpbingo.org/absolute-redirect/3

PYSTACK is the same on virtually every CP device, but handling even 1 or 2 redirects is a huge improvement, and may help solve some previously-reported user issues.

[mogenson]

Ok so maybe allow 3 redirects, then throw an exception?

[anecdata]

I'm not sure what the Pythonic thing to do is. pystack may get further limited by other aspects of user code, so there's probably nothing foolproof. Users should try to minimize redirects by supplying a canonical URL. Worst case, users catch the RuntimeError, but it won't necessarily be clear what's causing it. Maybe @brentru or @tannewt would like to weigh in.

[tannewt]

I think for now we can just let it run out of stack. If folks have this issue a lot we can switch this code to using a loop instead of recursion.

[mogenson]

Left out a redirect/recursion limit check.

In order to accomplish a case-insensitive header search I'm converting all headers to lowercase. I'm unaware of other CircuitPython libraries that rely on mixed-case HTTP headers, but let me know! Also, this removes a previous optimization of doing an initial string length match for the "content-length" and "transfer-encoding" labels, since lower() will be used on all headers.

[mogenson]

@anecdata When you get a chance, can you let me know if this PR looks good now?

[anecdata]

Adjusted PR example for ESP32SPI (Adafruit CircuitPython 6.3.0 on 2021-06-01; Adafruit PyPortal with samd51j20) and re-tested latest commit:
Success with 3 redirects on each URL

Also tested against a single-redirect URL with Mixed Case HTTP/1.1 header keys:
Success

I'm not sure what the memory impact of using .lower() on all of the headers is, but functionally LGTM!

[mogenson]

Great! Whose approval is required to merge this PR?

[tannewt]

Looks good to me. This will generate brief intermediate string objects but they should be short since they are header keys. I'd worry more if it was the header values. (It'd be nice if .lower() was in-place but I don't think it is.)

[anecdata]

@mogenson Thanks for implementing this long-desired feature!

[ysiegel29]

Many thanks for implementing this!