cache gpio chip detection when properly initialized (#962)

This will prevent falling back to `/dev/gpiochip0` when `/dev/gpiochipX` was previously opened successfully.

This is designed to alert users when multiple processes are simultaneously using `/dev/gpiochipX`, but it should only happen if either
1. another process is spamming/hogging the gpiochipX.
2. the rare occurrence when RF24 needs to open gpiochipX while it is still used by another process.

utility/RPi/interrupt.cpp

@@ -28,8 +28,10 @@ void IrqChipCache::openDevice()
             msg += "; ";
             msg += strerror(errno);
             throw IRQException(msg);
+            return;
         }
     }
+    chipInitialized = true;
 }
 
 void IrqChipCache::closeDevice()
@@ -89,6 +91,10 @@ int attachInterrupt(rf24_gpio_pin_t pin, int mode, void (*function)(void))
         irqChipCache.openDevice();
     }
     catch (IRQException& exc) {
+        if (irqChipCache.chipInitialized) {
+            throw exc;
+            return 0;
+        }
         irqChipCache.chip = "/dev/gpiochip0";
         irqChipCache.openDevice();
     }

utility/RPi/interrupt.h

@@ -33,6 +33,7 @@ struct IrqChipCache
 {
     const char* chip = RF24_LINUX_GPIO_CHIP;
     int fd = -1;
+    bool chipInitialized = false;
 
     /// Open the File Descriptor for the GPIO chip
     void openDevice();

utility/SPIDEV/gpio.cpp

@@ -31,8 +31,10 @@ void GPIOChipCache::openDevice()
             msg += "; ";
             msg += strerror(errno);
             throw GPIOException(msg);
+            return;
         }
     }
+    chipInitialized = true;
 }
 
 void GPIOChipCache::closeDevice()
@@ -77,6 +79,10 @@ void GPIO::open(rf24_gpio_pin_t port, int DDR)
         gpioCache.openDevice();
     }
     catch (GPIOException& exc) {
+        if (gpioCache.chipInitialized) {
+            throw exc;
+            return;
+        }
         gpioCache.chip = "/dev/gpiochip0";
         gpioCache.openDevice();
     }

utility/SPIDEV/gpio.h

@@ -44,6 +44,7 @@ struct GPIOChipCache
 {
     const char* chip = RF24_LINUX_GPIO_CHIP;
     int fd = -1;
+    bool chipInitialized = false;
 
     /// Open the File Descriptor for the GPIO chip
     void openDevice();

utility/SPIDEV/interrupt.cpp

@@ -69,6 +69,10 @@ int attachInterrupt(rf24_gpio_pin_t pin, int mode, void (*function)(void))
         irqChipCache.openDevice();
     }
     catch (GPIOException& exc) {
+        if (irqChipCache.chipInitialized) {
+            throw exc;
+            return 0;
+        }
         irqChipCache.chip = "/dev/gpiochip0";
         irqChipCache.openDevice();
     }