[3286] Make fromWire() always start reading from position 0 in the passed buffer

This fixes a bug which occurs if parseHeader() is called separately from
fromWire().

src/lib/dns/message.cc

@@ -620,6 +620,10 @@ Message::parseHeader(InputBuffer& buffer) {
                   "Message parse attempted in non parse mode");
     }
 
+    if (impl_->header_parsed_) {
+        return;
+    }
+
     if ((buffer.getLength() - buffer.getPosition()) < HEADERLEN) {
         isc_throw(MessageTooShort, "Malformed DNS message (short length): "
                   << buffer.getLength() - buffer.getPosition());
@@ -648,9 +652,8 @@ Message::fromWire(InputBuffer& buffer, ParseOptions options) {
     // Clear any old parsed data
     clear(Message::PARSE);
 
-    if (!impl_->header_parsed_) {
-        parseHeader(buffer);
-    }
+    buffer.setPosition(0);
+    parseHeader(buffer);
 
     impl_->counts_[SECTION_QUESTION] = impl_->parseQuestion(buffer);
     impl_->counts_[SECTION_ANSWER] =

src/lib/dns/message.h

@@ -607,6 +607,10 @@ public:
     };
 
     /// \brief Parse the header section of the \c Message.
+    ///
+    /// NOTE: If the header has already been parsed by a previous call
+    /// to this method, this method simply returns (i.e., it does not
+    /// read from the \c buffer).
     void parseHeader(isc::util::InputBuffer& buffer);
 
     /// \brief (Re)build a \c Message object from wire-format data.
@@ -642,7 +646,8 @@ public:
     /// \exception std::bad_alloc Memory allocation failure
     /// \exception Others \c Name, \c Rdata, and \c EDNS classes can also throw
     ///
-    /// \param buffer A input buffer object that stores the wire data
+    /// \param buffer A input buffer object that stores the wire
+    /// data. This method reads from position 0 in the passed buffer.
     /// \param options Parse options
     void fromWire(isc::util::InputBuffer& buffer, ParseOptions options
         = PARSE_DEFAULT);

src/lib/dns/tests/message_unittest.cc

@@ -623,6 +623,18 @@ TEST_F(MessageTest, fromWireMultiple) {
     factoryFromFile(message_parse, "message_fromWire1");
     factoryFromFile(message_parse, "message_fromWire1");
     checkMessageFromWire(message_parse, test_name);
+
+    // Calling parseHeader() directly before fromWire() should not cause
+    // any problems.
+    received_data.clear();
+    UnitTestUtil::readWireData("message_fromWire1", received_data);
+
+    InputBuffer buffer(&received_data[0], received_data.size());
+    message_parse.parseHeader(buffer);
+    message_parse.fromWire(buffer);
+    message_parse.parseHeader(buffer);
+    message_parse.fromWire(buffer);
+    checkMessageFromWire(message_parse, test_name);
 }
 
 TEST_F(MessageTest, fromWireShortBuffer) {