Notes on adding low-bandwidth mode detection to Homebrew

brew-lowbandwidthmode.md

Low-bandwidth mode

Goal: Find a way to make Homebrew conscious of when a user is on a low-bandwidth mode WiFi when running brew commands that may download large files.

Inspiration: I burned ~25% of my monthly LTE plan bandwidth in one day in May 2024 on installing a package that unbeknownst to me had multiple gigabytes of dependencies.

Testing

With detect_low_bandwidth_mode.swift (attached to this gist)…

When low data mode is disabled for the WiFi SSID:

[11:40:50 colin@workmac ~/Source/scratch ]
$ swift detect_low_bandwidth_mode.swift
[11:41:30 colin@workmac ~/Source/scratch ]
$ echo $?
0

When low data mode is enabled for the WiFi SSID:

[11:41:35 colin@workmac ~/Source/scratch ]
swift detect_low_data_mode.swift 
nok
libc++abi: terminating due to uncaught exception of type std::__1::system_error: mutex lock failed: Invalid argument
libc++abi: Pure virtual function called!
Abort trap: 6

This error is assuredly fixable and purely a side effect of this being the first Swift code I've ever written ;-)

Future work

• Convince Homebrew maintainers to let Swift code into Homebrew
  • Precedent: It's already in Cask
• Fix Swift code
• Choose hook
  • Dumb: halt and prompt to continue if low data mode detected?
  • Smarter: ☝️ but show bottle/package download size after asking permission to brew update?

See also

• https://developer.apple.com/documentation/network/nwpathmonitor
• https://developer.apple.com/documentation/network/nwinterface
• https://developer.apple.com/documentation/dispatch/dispatchqueue

detect_low_bandwidth_mode.swift

#!/usr/bin/swift

import Foundation
import Network

func pathIsOk(path: NWPath) -> Bool {
	!path.isExpensive && !path.isConstrained
}

let pathMon = NWPathMonitor.init()
let queue = DispatchQueue.global()

pathMon.start(queue: queue)

Task {
    let result = await pathMon.allSatisfy(pathIsOk)
    if result {
      print("ok")
      exit(0)
    } else {
      print("nok")
      exit(1)
    }
}