Avoid race conditions in I2C(Wire) callbacks (#995)

Fixes #979

Make sure to read the last byte of I2C data in the case where the IRQ happens
and the STOP signal is also asserted.

Also ensure all branches of the IRQ handler look at the same point in time
value for the IRQ status.

Author: earlephilhower

libraries/Wire/src/Wire.cpp

@@ -141,7 +141,22 @@ void TwoWire::begin(uint8_t addr) {
 }
 
 void TwoWire::onIRQ() {
-    if (_i2c->hw->intr_stat & (1 << 12)) {
+    // Make a local copy of the IRQ status up front.  If it changes while we're
+    // running the IRQ callback will fire again after returning.  Avoids potential
+    // race conditions
+    volatile uint32_t irqstat = _i2c->hw->intr_stat;
+
+    // First, pull off any data available
+    if (irqstat & (1 << 2)) {
+        // RX_FULL
+        if (_slaveStartDet && (_buffLen < (int)sizeof(_buff))) {
+            _buff[_buffLen++] = _i2c->hw->data_cmd & 0xff;
+        } else {
+            _i2c->hw->data_cmd;
+        }
+    }
+    // RESTART_DET
+    if (irqstat & (1 << 12)) {
         if (_onReceiveCallback && _buffLen) {
             _onReceiveCallback(_buffLen);
         }
@@ -150,13 +165,15 @@ void TwoWire::onIRQ() {
         _slaveStartDet = false;
         _i2c->hw->clr_restart_det;
     }
-    if (_i2c->hw->intr_stat & (1 << 10)) {
+    // START_DET
+    if (irqstat & (1 << 10)) {
         _buffLen = 0;
         _buffOff = 0;
         _slaveStartDet = true;
         _i2c->hw->clr_start_det;
     }
-    if (_i2c->hw->intr_stat & (1 << 9)) {
+    // STOP_DET
+    if (irqstat & (1 << 9)) {
         if (_onReceiveCallback && _buffLen) {
             _onReceiveCallback(_buffLen);
         }
@@ -165,25 +182,17 @@ void TwoWire::onIRQ() {
         _slaveStartDet = false;
         _i2c->hw->clr_stop_det;
     }
-    if (_i2c->hw->intr_stat & (1 << 6)) {
-        // TX_ABRT
+    // TX_ABRT
+    if (irqstat & (1 << 6)) {
         _i2c->hw->clr_tx_abrt;
     }
-    if (_i2c->hw->intr_stat & (1 << 5)) {
-        // RD_REQ
+    // RD_REQ
+    if (irqstat & (1 << 5)) {
         if (_onRequestCallback) {
             _onRequestCallback();
         }
         _i2c->hw->clr_rd_req;
     }
-    if (_i2c->hw->intr_stat & (1 << 2)) {
-        // RX_FULL
-        if (_slaveStartDet && (_buffLen < (int)sizeof(_buff))) {
-            _buff[_buffLen++] = _i2c->hw->data_cmd & 0xff;
-        } else {
-            _i2c->hw->data_cmd;
-        }
-    }
 }
 
 void TwoWire::end() {