bledev.filetransfer: CircuitPython's BLE file-transfer protocol on MicroPython

[bdbarnett]

A MicroPython board can now serve files over Bluetooth using CircuitPython's own BLE file-transfer protocol, so one client reaches files on either interpreter. Stacked on #85 (bledev-midi); the diff is the three commits on top.

import bledev.filetransfer
bledev.filetransfer.start(password="...", name="rack")   # files and the REPL, from main.py

What's in it

• bledev.filetransfer: read (with an offset), write (at an offset), delete (recursive), mkdir (with parents), listdir, move, and the free-space pacing on every write. FileServer is the protocol with no radio. The board serves it as a second service on bledev.repl's server (repl.start(files=True)). There's also a client built on the bledev contract, so it runs on bleak or on a board.
• Locked like the REPL. It's opt-in and needs the REPL's password, written to one extra characteristic (ADAF0300-…, which CircuitPython doesn't have). One attempt per connection, and a wrong password gets hung up on. Until the password arrives, every command is answered with status 0x80. Logging in to the REPL on the same connection unlocks files too.
• File work runs on the main thread. The esp32 BLE IRQ runs in NimBLE's task, which has only a few KB of stack, and a recursive delete there crashed the T-Embed. Now the IRQ only queues the bytes, and micropython.schedule plus the REPL's 5 ms timer do the file work.

Checks

• tests/bledev_filetransfer.py passes 104 of 104 on CPython and on unix MicroPython. It runs at MTUs 23, 185 and 247 with windows from 512 to 4096 bytes. test_bledev_filetransfer.py also plants a flipped bit and a dropped packet, and both fail the run.
• On the T-Embed from the laptop (bleak), tests/bledev_board/files_client.py passed: no password and a wrong password were refused, 20 KB went up and down byte for byte, and read-at-offset, listdir, move and recursive delete all worked. A bit flip planted on the client side and one planted on the board side both failed the run.
• Adafruit's own FileTransferClient also worked against FileServer in-process. That was a one-off check; the details are in the internals doc.

Measured. This is what decides whether mpftp needs this. A 20 KB file, laptop to T-Embed, with throughput connection parameters:

	Up	Down
file transfer	0.83 s	0.30 s
raw REPL (mpremote)	15.6 s	10.6 s

mpftp's ble:// transport uses file transfer, and falls back to the REPL when a board doesn't serve it: PyDevices/mpftp#47.

Not done

• No CircuitPython board has been tried with bledev's client, because CircuitPython's service needs pairing and bledev doesn't pair yet.
• A board acting as the client hasn't been run on hardware. Only one S3 was free.