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Three crashes reproduced live (gdb + ASan) on every device/channel/buffer change on a Focusrite Scarlett Solo, all upstream JUCE 9.0.1 bugs with open, unmerged fixes already filed (same author, all dated the day before this investigation started): - ALSAThread::close() waits a fixed 400ms for the audio thread to exit before force-closing the ALSA handles (closeNow()) out from under it. A large buffer at a low sample rate needs longer than 400ms for one i/o period, so a thread that's merely mid-read gets torn down as if it were stuck, use-after-freeing whatever it was reading from. Confirmed live: closeNow() firing with numCallbacks == callbacksToStop (no callback landed during the wait - not actually stuck). juce-framework/JUCE#1707 - ALSAThread::open() builds the active-channel bitset and the callback's channel-pointer array in one loop over the requested mask, then widens the bitset alone up to the hardware's channel-count floor afterward - so on a device fixed at 2 channels, a mono request leaves the bitset claiming 2 active channels with only 1 real pointer behind it. juce-framework/JUCE#1706 - AudioDeviceManager's CallbackMaxSizeEnforcer sizes its stored channel vectors from that (now too-wide) bitset, fills in only the channels the callback actually has, and forwards the full vector size regardless - so the unfilled tail is a null pointer that gets dereferenced downstream (AudioProcessorPlayer, LevelMeter::updateLevel). This is the direct segfault: confirmed live that JUCE's own jassert in this exact spot fires the moment a channel-mask change lands. juce-framework/JUCE#1708 All three applied as idempotent configure-time source patches, matching the existing pattern in this file. Drop each once its PR lands upstream. Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
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A device that passes null pointers for channels it was not opened with crashes on the audio thread, in AudioDeviceManager::LevelMeter::updateLevel, which dereferences them unconditionally. AudioIODeviceCallback documents that those pointers may be null. The ALSA case in #1706 is one way to get there.
CallbackMaxSizeEnforcer can produce the same situation on its own. It sizes its stored channel vectors from the device's active channel bitsets when audioDeviceAboutToStart runs, but fills in only the number of channels the device actually passes to the callback, and then forwards the vectors' full size. If a device's bitsets and channel counts disagree, the tail of those vectors still holds the null pointers that resize() default constructed. The assertions catch it in a debug build, but a release build passes the nulls straight through to the inner callback. This forwards the counts that were really filled in instead.