[1603] updated the behavior of setCompressMode(): now disallow it in rendering.

the new implementation doesn't allow such flexibility anymore (as the
hash value would change).  added comment about that, and updated the test
accordingly.

src/lib/dns/messagerenderer.cc

@@ -184,6 +184,10 @@ MessageRenderer::getCompressMode() const {
 
 void
 MessageRenderer::setCompressMode(const CompressMode mode) {
+    if (!impl_->names_.empty()) {
+        isc_throw(isc::InvalidParameter,
+                  "compress mode cannot be changed during rendering");
+    }
     impl_->compress_mode_ = mode;
 }
 

src/lib/dns/messagerenderer.h

@@ -359,7 +359,17 @@ public:
     virtual CompressMode getCompressMode() const;
     virtual void setTruncated();
     virtual void setLengthLimit(size_t len);
+
+    /// This implementation does not allow this call in the middle of
+    /// rendering (i.e. after at least one name is rendered) due to
+    /// restriction specific to the internal implementation.  Such attempts
+    /// will result in an \c isc::InvalidParameter exception.
+    ///
+    /// This shouldn't be too restrictive in practice; there's no known
+    /// practical case for such a mixed compression policy in a single
+    /// message.
     virtual void setCompressMode(CompressMode mode);
+
     virtual void clear();
     virtual void writeName(const Name& name, bool compress = true);
 private:

src/lib/dns/tests/messagerenderer_unittest.cc

@@ -142,13 +142,15 @@ TEST_F(MessageRendererTest, writeNameMixedCaseCompress) {
     renderer.writeName(Name("a.example.com."));
     renderer.writeName(Name("b.eXample.com."));
 
-    // Change the compression mode in the middle of rendering.  This is an
-    // unusual operation and is unlikely to happen in practice, but is still
-    // allowed in this API.
-    renderer.setCompressMode(MessageRenderer::CASE_INSENSITIVE);
-    renderer.writeName(Name("c.b.EXAMPLE.com."));
-    EXPECT_PRED_FORMAT4(UnitTestUtil::matchWireData, renderer.getData(),
-                        renderer.getLength(), &data[0], data.size());
+    // Change the compression mode in the middle of rendering.  This is not
+    // allowed in this implementation.
+    EXPECT_THROW(renderer.setCompressMode(MessageRenderer::CASE_INSENSITIVE),
+                 isc::InvalidParameter);
+
+    // Once the renderer is cleared, it's okay again.
+    renderer.clear();
+    EXPECT_NO_THROW(renderer.setCompressMode(
+                        MessageRenderer::CASE_INSENSITIVE));
 }
 
 TEST_F(MessageRendererTest, writeRootName) {