[Android Q&A, Help & Troubleshooting] joeymallone:
My Nexus 6P got stuck on a boot loop in the middle of a resource-intensive game. It was on the angler-opm6.171019.030.e1 firmware with a recent-ish version of Lineage OS.

In this state, I can get into the bootloader by keeping the volume down key pressed. However, I can't get into recovery from the bootloader, or do a factory reset, or do anything else. Choosing any option in the bootloader simply causes the phone to reboot.

However, while the bootloader is open, I am able to connect the phone to my computer via USB and run fastboot. At first I tried reflashing individual partitions like the bootloader, the vendor image, the recovery (TWRP) and eventually boot, in the hopes that reflashing one of them would fix the issue. Eventually, I downloaded the latest stock image from Google and reflashed the entire thing using the vendor-provided script:

fastboot flash bootloader bootloader-angler-angler-03.79.img
fastboot reboot-bootloader
sleep 5
fastboot flash radio radio-angler-angler-03.87.img
fastboot reboot-bootloader
sleep 5
fastboot -w update image-angler-opm6.171019.030.k1.zip

It was very thorough. As you can see by the mkfs, it even reformatted my storage (RIP dog pics):

$ ./flash-all.sh
Sending 'bootloader' (3552 KB) OKAY [ 0.128s]
Writing 'bootloader' OKAY [ 0.209s]
Finished. Total time: 0.366s
rebooting into bootloader OKAY [ 0.125s]
Finished. Total time: 0.126s
< waiting for any device >
Sending 'radio' (48728 KB) OKAY [ 1.303s]
Writing 'radio' OKAY [ 2.178s]
Finished. Total time: 3.512s
rebooting into bootloader OKAY [ 0.005s]
Finished. Total time: 0.006s
extracting android-info.txt (0 MB) to RAM...
--------------------------------------------
Bootloader Version...: angler-03.79
Baseband Version.....: angler-03.87
Serial Number........: 84B7N15A04004581
--------------------------------------------
Checking product OKAY [ 0.021s]
Checking version-bootloader OKAY [ 0.020s]
Checking version-baseband OKAY [ 0.020s]
extracting boot.img (11 MB) to disk... took 0.122s
archive does not contain 'boot.sig'
archive does not contain 'dtbo.img'
archive does not contain 'dt.img'
archive does not contain 'odm.img'
archive does not contain 'product.img'
extracting recovery.img (18 MB) to disk... took 0.084s
archive does not contain 'recovery.sig'
extracting system.img (1912 MB) to disk... took 11.444s
archive does not contain 'system.sig'
archive does not contain 'vbmeta.img'
extracting vendor.img (188 MB) to disk... took 1.259s
archive does not contain 'vendor.sig'
mke2fs 1.43.3 (04-Sep-2016)
Creating filesystem with 6694270 4k blocks and 1676080 inodes
Filesystem UUID: b506d1ce-64e3-40fc-a7ed-cb63d08a0704
Superblock backups stored on blocks:
32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208,
4096000

Allocating group tables: done
Writing inode tables: done
Creating journal (32768 blocks): done
Writing superblocks and filesystem accounting information: done

mke2fs 1.43.3 (04-Sep-2016)
Creating filesystem with 25600 4k blocks and 25600 inodes

Allocating group tables: done
Writing inode tables: done
Creating journal (1024 blocks): done
Writing superblocks and filesystem accounting information: done

Sending 'boot' (12089 KB) OKAY [ 0.338s]
Writing 'boot' OKAY [ 0.181s]
Sending 'recovery' (18961 KB) OKAY [ 0.510s]
Writing 'recovery' OKAY [ 0.275s]
Sending sparse 'system' 1/5 (482664 KB) OKAY [ 12.736s]
Writing sparse 'system' 1/5 OKAY [ 7.243s]
Sending sparse 'system' 2/5 (475570 KB) OKAY [ 12.729s]
Writing sparse 'system' 2/5 OKAY [ 6.745s]
Sending sparse 'system' 3/5 (474890 KB) OKAY [ 13.047s]
Writing sparse 'system' 3/5 OKAY [ 7.821s]
Sending sparse 'system' 4/5 (476548 KB) OKAY [ 12.622s]
Writing sparse 'system' 4/5 OKAY [ 6.947s]
Sending sparse 'system' 5/5 (49108 KB) OKAY [ 1.259s]
Writing sparse 'system' 5/5 OKAY [ 0.712s]
Sending 'vendor' (192565 KB) OKAY [ 5.124s]
Writing 'vendor' OKAY [ 3.262s]
Erasing 'userdata' OKAY [ 0.326s]
Sending 'userdata' (4412 KB) OKAY [ 0.163s]
Writing 'userdata' OKAY [ 0.095s]
Erasing 'cache' OKAY [ 0.024s]
Sending 'cache' (96 KB) OKAY [ 0.032s]
Writing 'cache' OKAY [ 0.030s]
Rebooting
Finished. Total time: 106.097s

Suffice to say this didn't fix the issue. The phone is still on a boot loop.

What else is there to try?

I suspect a hardware issue but I can't run any diagnostics since fastboot doesn't give me shell access.

Thanks in advance!