extmod/zephyr_ble: Fix soft reset crash due to net_buf pool state.

After a MicroPython soft reset, Zephyr net_buf pools retain stale runtime
state (free list, uninit_count) from the previous BLE session. When
bt_enable() runs again, it assumes pools are in their initial state with
all buffers uninitialized and the free list empty. This mismatch causes
a HardFault when allocating or freeing buffers.

Add mp_net_buf_pool_state_reset() to restore all net_buf pools to their
initial state before BLE reinitialization. The function iterates over
all statically-defined pools and resets:
- .free lifo (set head/tail to NULL)
- .uninit_count (reset to buf_count)
- .lock spinlock

Tested with 8+ consecutive BLE init/deinit + soft_reset cycles on Pico W.

Signed-off-by: Andrew Leech <andrew.leech@planetinnovation.com.au>

Author: pi-anl

extmod/zephyr_ble/modbluetooth_zephyr.c

@@ -40,6 +40,7 @@
 #include <zephyr/device.h>
 #include "extmod/modbluetooth.h"
 #include "extmod/zephyr_ble/hal/zephyr_ble_work.h"
+#include "extmod/zephyr_ble/net_buf_pool_registry.h"
 
 // Access Zephyr's internal bt_dev for force-reset on deinit failure
 // The include path should have lib/zephyr/subsys/bluetooth/host already
@@ -465,6 +466,12 @@ int mp_bluetooth_init(void) {
         // This must also be started before bt_enable() to process work items
         mp_bluetooth_zephyr_work_thread_start();
 
+        // Reset net_buf pool state before BLE initialization.
+        // After a soft reset, pools retain stale runtime state (free list,
+        // uninit_count) from the previous session. This causes crashes when
+        // bt_enable() tries to allocate buffers from corrupted pools.
+        mp_net_buf_pool_state_reset();
+
         // Enter init phase - work will be processed synchronously in this loop
         extern void mp_bluetooth_zephyr_init_phase_enter(void);
         mp_bluetooth_zephyr_init_phase_enter();

extmod/zephyr_ble/net_buf_pool_registry.c

@@ -102,6 +102,49 @@ void mp_net_buf_pool_reset(void) {
 // The registry system (mp_net_buf_pool_register/get) is retained for potential future use
 // if dynamic pool registration is needed, but is not currently used.
 
+// ============================================================================
+// Pool State Reset for Soft Reset Support
+// ============================================================================
+// Net_buf pools are statically initialized but maintain runtime state:
+// - .free: k_lifo containing free buffers
+// - .uninit_count: number of uninitialized buffers remaining
+// - .lock: spinlock for concurrent access
+//
+// After a MicroPython soft reset, these pools retain stale state from the
+// previous BLE session. When bt_enable() runs again, it assumes pools are
+// in their initial state (all buffers uninitialized, free list empty).
+// This mismatch causes crashes when allocating/freeing buffers.
+//
+// mp_net_buf_pool_state_reset() restores pools to their initial state,
+// allowing clean BLE reinitialization after soft reset.
+
+#include <zephyr/net_buf.h>
+#include <zephyr/sys/iterable_sections.h>
+
+void mp_net_buf_pool_state_reset(void) {
+    // Iterate over all statically-defined net_buf pools
+    STRUCT_SECTION_FOREACH(net_buf_pool, pool) {
+        // Reset the free list (k_lifo) to empty
+        // k_lifo contains a k_queue with sys_slist_t data_q
+        pool->free._queue.data_q.head = NULL;
+        pool->free._queue.data_q.tail = NULL;
+        pool->free._queue.lock.unused = 0;
+
+        // Reset uninit_count to buf_count so all buffers are re-initialized
+        // on first allocation (this is how pools start after static init)
+        pool->uninit_count = pool->buf_count;
+
+        // Reset the pool spinlock
+        pool->lock.unused = 0;
+
+#if defined(CONFIG_NET_BUF_POOL_USAGE)
+        // Reset usage tracking counters
+        atomic_set(&pool->avail_count, pool->buf_count);
+        pool->max_used = 0;
+#endif
+    }
+}
+
 // ============================================================================
 // Note on Zephyr iterable sections
 // ============================================================================

extmod/zephyr_ble/net_buf_pool_registry.h

@@ -45,4 +45,9 @@ void mp_net_buf_pool_update_end(void);
 // Reset registration (for testing)
 void mp_net_buf_pool_reset(void);
 
+// Reset all net_buf pool runtime state for soft reset support.
+// This must be called before bt_enable() when reinitializing BLE after
+// a soft reset, to restore pools to their initial state.
+void mp_net_buf_pool_state_reset(void);
+
 #endif // MICROPY_INCLUDED_EXTMOD_ZEPHYR_BLE_NET_BUF_POOL_REGISTRY_H