From 33ff5fd5abb714434acc1f8f3d663fe5b912af86 Mon Sep 17 00:00:00 2001
From: Shawn Nock <nock@nocko.se>
Date: Fri, 30 Dec 2016 20:58:58 -0500
Subject: [PATCH] Wait for sysfs node to ready before access (API BREAK)

---
 sysfs/gpio.py | 40 ++++++++++++++++++++++++++++++----------
 1 file changed, 30 insertions(+), 10 deletions(-)

diff --git a/sysfs/gpio.py b/sysfs/gpio.py
index 80b2cce..5c9875c 100755
--- a/sysfs/gpio.py
+++ b/sysfs/gpio.py
@@ -32,7 +32,7 @@
 import os
 import select
 
-from twisted.internet import reactor
+from twisted.internet import reactor, defer
 
 import logging
 
@@ -57,6 +57,7 @@
 SYSFS_GPIO_VALUE_HIGH  = '1'
 
 EPOLL_TIMEOUT = 1  # second
+SYSFS_GPIO_TIMEOUT = 5 # second
 
 # Public interface
 
@@ -74,6 +75,22 @@
 EDGES = (RISING, FALLING, BOTH)
 ACTIVE_LOW_MODES = (ACTIVE_LOW_ON, ACTIVE_LOW_OFF)
 
+def wait_for_file(path, timeout=SYSFS_GPIO_TIMEOUT):
+    d = defer.Deferred()
+    d.addTimeout(timeout, reactor)
+    def _exists(di):
+        if os.path.exists(path):
+            try:
+                with open(path, 'r+') as f:
+                    pass
+            except Exception as e:
+                pass
+            else:
+                di.callback(None)
+                return
+        reactor.callLater(0, _exists, di)
+    _exists(d)
+    return d
 
 class Pin(object):
     """
@@ -286,9 +303,6 @@ def stop(self):
 
     def alloc_pin(self, number, direction, callback=None, edge=None, active_low=0):
 
-        Logger.debug('SysfsGPIO: alloc_pin(%d, %s, %s, %s, %s)'
-                     % (number, direction, callback, edge, active_low))
-
         self._check_pin_validity(number)
 
         if direction not in DIRECTIONS:
@@ -298,19 +312,25 @@ def alloc_pin(self, number, direction, callback=None, edge=None, active_low=0):
         if callback and edge not in EDGES:
             raise Exception("Pin edge %s not in %s" % (edge, EDGES))
 
+        def _init_pin(_):
+            pin = Pin(number, direction, callback, edge, active_low)
+            Logger.debug('SysfsGPIO: alloc_pin(%d, %s, %s, %s, %s)'
+                     % (number, direction, callback, edge, active_low))
+            if direction is INPUT:
+                self._poll_queue_register_pin(pin)
+            self._allocated_pins[number] = pin
+            return pin
+
         if not self._check_pin_already_exported(number):
             with open(SYSFS_EXPORT_PATH, 'w') as export:
                 export.write('%d' % number)
         else:
             Logger.debug("SysfsGPIO: Pin %d already exported" % number)
 
-        pin = Pin(number, direction, callback, edge, active_low)
-
-        if direction is INPUT:
-            self._poll_queue_register_pin(pin)
+        d = wait_for_file(SYSFS_GPIO_VALUE_PATH % number)
+        d.addCallback(_init_pin)
 
-        self._allocated_pins[number] = pin
-        return pin
+        return d
 
     def _poll_queue_register_pin(self, pin):
         ''' Pin responds to fileno(), so it's pollable. '''