Clean Flash Magisk on Pixel 3a (Android 9)

magisk_pixel3a.txt

@author LongCat (Pichaya Morimoto)

1. Enable ADB
Settings > About Phone > Tap on the "Build Number" entry 7 times
Settings > System > Advanced > Developer options > Enable "USB debugging"
Settings > System > Advanced > Developer options > Enable "OEM unlocking"

Note: If you cannot enable "OEM unlocking", then you are out of luck - Buy the new one :)

Connect Pixel 3a to MBP > allow access in the device's prompt.

2. Get Android 9 ROM
Go to https://developers.google.com/android/images#sargo
Use 9.0.0 (PQ3B.190801.002, Aug 2019)	- The latest version for Android 9
$ wget https://dl.google.com/dl/android/aosp/bonito-pq3b.190801.002-factory-61a2836d.zip
$ unzip bonito-pq3b.190801.002-factory-61a2836d.zip

3. Unlock Boot Loader
$ brew cask install android-platform-tools
$ adb reboot bootloader
$ fastboot devices
$ fastboot flashing unlock

4. Flash Android 9 ROM
$ cd bonito-pq3b.190801.002-factory-61a2836d && ./flash-all.sh

4. Install Magisk 
Download Magisk Manager (APK) at:
https://magiskmanager.com/downloading-magisk-manager
$ adb install MagiskManager-v7.5.1.apk
// inside bonito-pq3b.190801.002-factory-61a2836d.zip in Step 2
$ unzip image-sargo-qq1a.200105.002.zip
$ adb push image-sargo-pq3b.190801.002/boot.img  /sdcard/Download/

Open Magisk Manager
Click on the first "INSTALL" -> INSTALL -> "Select and Patch a File" > /sdcard/Download/boot.img
Wait and we will get the magisk_patched.img file.

$ adb pull /storage/emulated/0/Download/magisk_patched.img .
$ adb reboot bootloader
$ fastboot flash boot magisk_patched.img
$ fastboot reboot

5. Disable Auto-Update (Insecure. For Testing Device Only.)
Settings > System (advanced) > Developer Options > Disable "Automatic System Updates"

Ref: My Pixel 1 Magisk Guide
https://gist.github.com/pich4ya/c3565c97cb6a845c8e543301e665b40e