Fix issue where message is assumed to contain correct length byte (leading to process exit) (#1050)

Do not assume that message contains correct length to prevent calling GetString with invalid parameters.

Author: mnmr

src/NetMQ.Tests/MechanismTests.cs

@@ -0,0 +1,55 @@
+using System.Text;
+using NetMQ.Core.Mechanisms;
+using Xunit;
+
+namespace NetMQ.Tests
+{
+    public class MechanismTests
+    {
+        private Msg CreateMsg(string data, int lengthDiff)
+        {
+            Assert.NotNull(data);
+            Assert.True(data.Length > 1);
+            var length = data.Length + lengthDiff;
+            Assert.True(length > 0);
+            Assert.True(length < byte.MaxValue - 1);
+            
+            var msg = new Msg();
+            msg.InitGC(new byte[data.Length + 1], 0, data.Length + 1);
+            msg.SetFlags(MsgFlags.Command);
+            msg.Put((byte)length);
+            msg.Put(Encoding.ASCII, data, 1);
+            return msg;
+        }
+
+        [Fact]
+        public void IsCommandShouldReturnTrueForValidCommand()
+        {
+            var mechanism = new NullMechanism(null!, null!);
+            var msg = CreateMsg("READY", 0);
+            Assert.True(mechanism.IsCommand("READY", ref msg));
+        }
+
+        [Fact]
+        public void IsCommandShouldReturnFalseForInvalidCommand()
+        {
+            var mechanism = new NullMechanism(null!, null!);
+            var msg = CreateMsg("READY", -1);
+            Assert.False(mechanism.IsCommand("READY", ref msg));
+            msg = CreateMsg("READY", 1);
+            Assert.False(mechanism.IsCommand("READY", ref msg));
+            // this test case would fail due to an exception being throw (in 4.0.1.10 and prior)
+            msg = CreateMsg("READY", 2);
+            Assert.False(mechanism.IsCommand("READY", ref msg));
+        }
+
+        // this test was used to validate the behavior prior to changing the validation logic in Mechanism.IsCommand
+        // [Fact]
+        // public void IsCommandShouldThrowWhenLengthByteExceedsSize()
+        // {
+        //     var mechanism = new NullMechanism(null, null);
+        //     var msg = CreateMsg("READY", 2);
+        //     Assert.Throws<ArgumentOutOfRangeException>(() => mechanism.IsCommand("READY", ref msg));
+        // }        
+    }
+}

src/NetMQ/Core/Mechanisms/Mechanism.cs

@@ -348,15 +348,13 @@ protected bool CheckBasicCommandStructure(ref Msg msg)
             return true;
         }
 
-        protected bool IsCommand(string command, ref Msg msg)
+        internal protected bool IsCommand(string command, ref Msg msg)
         {
-            if (msg.Size >= command.Length + 1)
+            if (msg.Size >= command.Length + 1 && msg[0] == command.Length)
             {
-                string msgCommand = msg.GetString(Encoding.ASCII, 1, msg[0]);
-                return msgCommand == command && msg[0] == command.Length;
+                return msg.GetString(Encoding.ASCII, 1, msg[0]) == command;
             }
-
             return false;
         }
     }
-}
+}