From 5ee2341be8330397aa39d14d71439aea5b9264a6 Mon Sep 17 00:00:00 2001
From: "Michael P. Soulier" <msoulier@digitaltorque.ca>
Date: Sat, 26 Mar 2022 08:11:50 -0400
Subject: Adding a test hook for network unreliability.

---
 t/test.py           |  5 +++++
 tftpy/TftpShared.py |  5 +++++
 tftpy/TftpStates.py | 25 +++++++++++++++++--------
 3 files changed, 27 insertions(+), 8 deletions(-)

diff --git a/t/test.py b/t/test.py
index 6ef27f7..e40be8d 100644
--- a/t/test.py
+++ b/t/test.py
@@ -280,6 +280,11 @@ class TestTftpyState(unittest.TestCase):
         self.clientServerDownloadOptions({})
         tftpy.TftpStates.DELAY_BLOCK = 0
 
+    def testClientServerNoOptionsUnreliable(self):
+        tftpy.TftpStates.NETWORK_UNRELIABILITY = 1000
+        self.clientServerDownloadOptions({})
+        tftpy.TftpStates.NETWORK_UNRELIABILITY = 0
+
     def testServerNoOptions(self):
         raddress = "127.0.0.2"
         rport = 10000
diff --git a/tftpy/TftpShared.py b/tftpy/TftpShared.py
index d464d4d..e727c94 100644
--- a/tftpy/TftpShared.py
+++ b/tftpy/TftpShared.py
@@ -12,6 +12,11 @@ DEF_TFTP_PORT = 69
 
 # A hook for deliberately introducing delay in testing.
 DELAY_BLOCK = 0
+# A hook to simulate a bad network
+NETWORK_UNRELIABILITY = 0
+# 0 is disabled, anything positive is the inverse of the percentage of
+# dropped traffic. For example, 1000 would cause 0.1% of DAT packets to
+# be skipped to simulate lost packets.
 
 
 def tftpassert(condition, msg):
diff --git a/tftpy/TftpStates.py b/tftpy/TftpStates.py
index 15d52f9..1f98682 100644
--- a/tftpy/TftpStates.py
+++ b/tftpy/TftpStates.py
@@ -12,6 +12,7 @@ error, in which case a TftpException is returned instead."""
 
 import logging
 import os
+import random
 
 from .TftpPacketTypes import *
 from .TftpShared import *
@@ -93,7 +94,6 @@ class TftpState:
         # Test hook
         if DELAY_BLOCK and DELAY_BLOCK == blocknumber:
             import time
-
             log.debug("Deliberately delaying 10 seconds...")
             time.sleep(10)
         dat = None
@@ -107,10 +107,14 @@ class TftpState:
         dat.data = buffer
         dat.blocknumber = blocknumber
         self.context.metrics.bytes += len(dat.data)
-        log.debug("Sending DAT packet %d", dat.blocknumber)
-        self.context.sock.sendto(
-            dat.encode().buffer, (self.context.host, self.context.tidport)
-        )
+        # Testing hook
+        if NETWORK_UNRELIABILITY > 0 and random.randrange(NETWORK_UNRELIABILITY) == 0:
+            log.warning("Skipping DAT packet %d for testing", dat.blocknumber)
+        else:
+            log.debug("Sending DAT packet %d", dat.blocknumber)
+            self.context.sock.sendto(
+                dat.encode().buffer, (self.context.host, self.context.tidport)
+            )
         if self.context.packethook:
             self.context.packethook(dat)
         self.context.last_pkt = dat
@@ -126,9 +130,13 @@ class TftpState:
         log.info("Sending ack to block %d" % blocknumber)
         ackpkt = TftpPacketACK()
         ackpkt.blocknumber = blocknumber
-        self.context.sock.sendto(
-            ackpkt.encode().buffer, (self.context.host, self.context.tidport)
-        )
+        # Testing hook
+        if NETWORK_UNRELIABILITY > 0 and random.randrange(NETWORK_UNRELIABILITY) == 0:
+            log.warning("Skipping ACK packet %d for testing", ackpkt.blocknumber)
+        else:
+            self.context.sock.sendto(
+                ackpkt.encode().buffer, (self.context.host, self.context.tidport)
+            )
         self.context.last_pkt = ackpkt
 
     def sendError(self, errorcode):
@@ -158,6 +166,7 @@ class TftpState:
 
     def resendLast(self):
         """Resend the last sent packet due to a timeout."""
+        assert( self.context.last_pkt is not None )
         log.warning(f"Resending packet {self.context.last_pkt} on sessions {self}")
         self.context.metrics.resent_bytes += len(self.context.last_pkt.buffer)
         self.context.metrics.add_dup(self.context.last_pkt)
-- 
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