`_prepare_persistent_project_dir` keys the persistent build dir on
(idf_target, variant), but the lock was keyed on the FQBN. Several FQBNs
resolve to one variant — arduino-esp32's boards.txt gives both `esp32` and
`esp32cam` build.variant=esp32 — so those two were free to build in the SAME
directory at the same time.
Measured against staging, compiling both concurrently:
esp32 285504 bytes sha f65ca909… contains its own marker
esp32cam 285504 bytes sha f65ca909… contains the OTHER sketch's marker
Byte-identical firmware, and the esp32cam caller got the esp32 sketch's
binary. In the product that is two gallery tabs open at once: the first tab
prints the second example's serial log and does nothing itself. It made
esp32-pwm-led-rgb look broken (dark LED, every SPICE node 0 V) when the
example is fine — it was running someone else's program.
Key the lock on the same identity as the directory. Boards with genuinely
separate build dirs still compile in parallel up to the semaphore cap; an
unresolvable FQBN falls back to the default pair, which is the safe
direction (serialising more only costs throughput).
Also in this commit: battery-voltage-monitor ships the 9 V battery its own
description promises. The divider's top end was wired to nothing, so A0 read
0 V and the sketch printed "Battery: 0.0 V / WARNING: Battery low!" forever.
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