util: Move packet trace file read to protolib

This patch moves the code for opening an input protobuf packet trace into
a function defined in the protobuf library. This is because the code is
commonly used in decode scripts and is independent of the src protobuf
message.

util/protolib.py

@@ -71,8 +71,33 @@
 # with protobuf python messages. For eg, the decode scripts for different
 # types of proto objects can use the same function to decode a single message
 
+import gzip
 import struct
 
+def openFileRd(in_file):
+    """
+    This opens the file passed as argument for reading using an appropriate
+    function depending on if it is gzipped or not. It returns the file
+    handle.
+    """
+    try:
+        # First see if this file is gzipped
+        try:
+            # Opening the file works even if it is not a gzip file
+            proto_in = gzip.open(in_file, 'rb')
+
+            # Force a check of the magic number by seeking in the
+            # file. If we do not do it here the error will occur when
+            # reading the first message.
+            proto_in.seek(1)
+            proto_in.seek(0)
+        except IOError:
+            proto_in = open(in_file, 'rb')
+    except IOError:
+        print "Failed to open ", in_file, " for reading"
+        exit(-1)
+    return proto_in
+
 def DecodeVarint(in_file):
     """
     The decoding of the Varint32 is copied from