Add a BufferConsumer primitive for wire [de]serialization

BufferConsumer wraps a buffer pointer and size and exposes a
limited number of operations to get data while decrementing
the remaining available size. This makes it so that code
reading or writing into a buffer cannot easily consume more
bytes than available.

This CL guards against serialization overflows using
BufferConsumer, and it implements GetPtrFromBuffer
(for deserialization) on top of BufferConsumer. A future patch
will make the rest of the deserialization code use BufferConsumer.

Bug: dawn:680
Change-Id: Ic2bd6e7039e83ce70307c2ff47aaca9891c16d91
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/41780
Commit-Queue: Austin Eng <enga@chromium.org>
Reviewed-by: Stephen White <senorblanco@chromium.org>

src/dawn_wire/server/ServerBuffer.cpp

@@ -13,6 +13,7 @@
 // limitations under the License.
 
 #include "common/Assert.h"
+#include "dawn_wire/WireCmd_autogen.h"
 #include "dawn_wire/server/Server.h"
 
 #include <memory>
@@ -242,11 +243,15 @@ namespace dawn_wire { namespace server {
                 data->readHandle->SerializeInitialDataSize(readData, data->size);
         }
 
-        SerializeCommand(cmd, cmd.readInitialDataInfoLength, [&](char* cmdSpace) {
+        SerializeCommand(cmd, cmd.readInitialDataInfoLength, [&](SerializeBuffer* serializeBuffer) {
             if (isSuccess) {
                 if (isRead) {
+                    if (serializeBuffer->AvailableSize() != cmd.readInitialDataInfoLength) {
+                        return false;
+                    }
                     // Serialize the initialization message into the space after the command.
-                    data->readHandle->SerializeInitialData(readData, data->size, cmdSpace);
+                    data->readHandle->SerializeInitialData(readData, data->size,
+                                                           serializeBuffer->Buffer());
                     // The in-flight map request returned successfully.
                     // Move the ReadHandle so it is owned by the buffer.
                     bufferData->readHandle = std::move(data->readHandle);
@@ -261,6 +266,7 @@ namespace dawn_wire { namespace server {
                         data->size);
                 }
             }
+            return true;
         });
     }