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Sharing one physical device between several workers

scope: generic · severity: technique · confidence: proven · subsystem: method

There is one phone. There may be several agents, or an agent and a human.

Every command that touches it goes through the mutex:

Terminal window
TK_AGENT=<yourname> tools/ph-device.sh <command>

flock, not a hand-rolled lockfile: it is race-free across processes and the kernel releases it when the holder dies. A hand-rolled lockfile gets exactly that case wrong, and a crashed agent then wedges everyone until someone notices.

Declare the state you need, so you fail in a second rather than queueing ten minutes for a device that was never going to answer: the-lock-says-who-not-what.

Name yourself. TK_AGENT goes in the holder file, so the next worker’s timeout message says who has it and what the device was doing.

Two timeouts, and they are different things. How long you wait for the lock (exit 75) versus how long you may hold it (exit 124). The hold ceiling exists to break wedges, not to discipline long measurements — a CPU benchmark legitimately held the device for well over ten minutes. A ceiling that kills a valid measurement is a worse bug than the one it fixes, so the default is generous and a caller that needs longer says so explicitly.

Hand back what is not yours. Finding the device in the bootloader usually means someone is mid-experiment, not that something is broken.