gecko/parser/html/nsHtml5Parser.cpp

/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set sw=2 ts=2 et tw=79: */
/* ***** BEGIN LICENSE BLOCK *****
 * Version: MPL 1.1/GPL 2.0/LGPL 2.1
 *
 * The contents of this file are subject to the Mozilla Public License Version
 * 1.1 (the "License"); you may not use this file except in compliance with
 * the License. You may obtain a copy of the License at
 * http://www.mozilla.org/MPL/
 *
 * Software distributed under the License is distributed on an "AS IS" basis,
 * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
 * for the specific language governing rights and limitations under the
 * License.
 *
 * The Original Code is mozilla.org code.
 *
 * The Initial Developer of the Original Code is
 * Netscape Communications Corporation.
 * Portions created by the Initial Developer are Copyright (C) 1998
 * the Initial Developer. All Rights Reserved.
 *
 * Contributor(s):
 *   Pierre Phaneuf <pp@ludusdesign.com>
 *   Henri Sivonen <hsivonen@iki.fi>
 *
 * Alternatively, the contents of this file may be used under the terms of
 * either of the GNU General Public License Version 2 or later (the "GPL"),
 * or the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
 * in which case the provisions of the GPL or the LGPL are applicable instead
 * of those above. If you wish to allow use of your version of this file only
 * under the terms of either the GPL or the LGPL, and not to allow others to
 * use your version of this file under the terms of the MPL, indicate your
 * decision by deleting the provisions above and replace them with the notice
 * and other provisions required by the GPL or the LGPL. If you do not delete
 * the provisions above, a recipient may use your version of this file under
 * the terms of any one of the MPL, the GPL or the LGPL.
 *
 * ***** END LICENSE BLOCK ***** */

#include "nsCompatibility.h"
#include "nsScriptLoader.h"
#include "nsNetUtil.h"
#include "nsIStyleSheetLinkingElement.h"
#include "nsICharsetAlias.h"
#include "nsIWebShellServices.h"
#include "nsIDocShell.h"
#include "nsEncoderDecoderUtils.h"
#include "nsContentUtils.h"
#include "nsICharsetDetector.h"
#include "nsIScriptElement.h"
#include "nsIMarkupDocumentViewer.h"
#include "nsIDocShellTreeItem.h"
#include "nsIContentViewer.h"
#include "nsIScriptGlobalObjectOwner.h"
#include "nsIScriptSecurityManager.h"
#include "nsHtml5DocumentMode.h"
#include "nsHtml5Tokenizer.h"
#include "nsHtml5UTF16Buffer.h"
#include "nsHtml5TreeBuilder.h"
#include "nsHtml5Parser.h"
#include "nsHtml5AtomTable.h"
#include "nsICharsetAlias.h"

static NS_DEFINE_CID(kCharsetAliasCID, NS_CHARSETALIAS_CID);

NS_INTERFACE_TABLE_HEAD(nsHtml5Parser)
  NS_INTERFACE_TABLE2(nsHtml5Parser, nsIParser, nsISupportsWeakReference)
  NS_INTERFACE_TABLE_TO_MAP_SEGUE_CYCLE_COLLECTION(nsHtml5Parser)
NS_INTERFACE_MAP_END

NS_IMPL_CYCLE_COLLECTING_ADDREF(nsHtml5Parser)
NS_IMPL_CYCLE_COLLECTING_RELEASE(nsHtml5Parser)

NS_IMPL_CYCLE_COLLECTION_CLASS(nsHtml5Parser)

NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN(nsHtml5Parser)
  NS_IMPL_CYCLE_COLLECTION_TRAVERSE_NSCOMPTR_AMBIGUOUS(mExecutor, nsIContentSink)
  NS_IMPL_CYCLE_COLLECTION_TRAVERSE_NSCOMPTR_AMBIGUOUS(mStreamParser, nsIStreamListener)
NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END

NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(nsHtml5Parser)
  NS_IMPL_CYCLE_COLLECTION_UNLINK_NSCOMPTR(mExecutor)
  NS_IMPL_CYCLE_COLLECTION_UNLINK_NSCOMPTR(mStreamParser)
NS_IMPL_CYCLE_COLLECTION_UNLINK_END

nsHtml5Parser::nsHtml5Parser()
  : mFirstBuffer(new nsHtml5UTF16Buffer(0))
  , mLastBuffer(mFirstBuffer)
  , mExecutor(new nsHtml5TreeOpExecutor())
  , mTreeBuilder(new nsHtml5TreeBuilder(mExecutor, nsnull))
  , mTokenizer(new nsHtml5Tokenizer(mTreeBuilder))
  , mRootContextLineNumber(1)
{
  mAtomTable.Init(); // we aren't checking for OOM anyway...
  mTokenizer->setInterner(&mAtomTable);
  // There's a zeroing operator new for everything else
}

nsHtml5Parser::~nsHtml5Parser()
{
  mTokenizer->end();
  mFirstBuffer = nsnull;
}

NS_IMETHODIMP_(void)
nsHtml5Parser::SetContentSink(nsIContentSink* aSink)
{
  NS_ASSERTION(aSink == static_cast<nsIContentSink*> (mExecutor), 
               "Attempt to set a foreign sink.");
}

NS_IMETHODIMP_(nsIContentSink*)
nsHtml5Parser::GetContentSink(void)
{
  return static_cast<nsIContentSink*> (mExecutor);
}

NS_IMETHODIMP_(void)
nsHtml5Parser::GetCommand(nsCString& aCommand)
{
  aCommand.Assign("view");
}

NS_IMETHODIMP_(void)
nsHtml5Parser::SetCommand(const char* aCommand)
{
  NS_ASSERTION(!strcmp(aCommand, "view"), "Parser command was not view");
}

NS_IMETHODIMP_(void)
nsHtml5Parser::SetCommand(eParserCommands aParserCommand)
{
  NS_ASSERTION(aParserCommand == eViewNormal, 
               "Parser command was not eViewNormal.");
}

NS_IMETHODIMP_(void)
nsHtml5Parser::SetDocumentCharset(const nsACString& aCharset, PRInt32 aCharsetSource)
{
  NS_PRECONDITION(!mExecutor->HasStarted(),
                  "Document charset set too late.");
  NS_PRECONDITION(mStreamParser, "Tried to set charset on a script-only parser.");
  mStreamParser->SetDocumentCharset(aCharset, aCharsetSource);
  mExecutor->SetDocumentCharsetAndSource((nsACString&)aCharset, aCharsetSource);
  mCharsetSource = aCharsetSource; // used for the document.open() case only
}

NS_IMETHODIMP_(void)
nsHtml5Parser::SetParserFilter(nsIParserFilter* aFilter)
{
  NS_ERROR("Attempt to set a parser filter on HTML5 parser.");
}

NS_IMETHODIMP
nsHtml5Parser::GetChannel(nsIChannel** aChannel)
{
  if (mStreamParser) {
    return mStreamParser->GetChannel(aChannel);
  } else {
    return NS_ERROR_NOT_AVAILABLE;
  }
}

NS_IMETHODIMP
nsHtml5Parser::GetDTD(nsIDTD** aDTD)
{
  *aDTD = nsnull;
  return NS_OK;
}

NS_IMETHODIMP
nsHtml5Parser::GetStreamListener(nsIStreamListener** aListener)
{
  NS_IF_ADDREF(*aListener = mStreamParser);
  return NS_OK;
}

NS_IMETHODIMP
nsHtml5Parser::ContinueInterruptedParsing()
{
  NS_NOTREACHED("Don't call. For interface compat only.");
  return NS_ERROR_NOT_IMPLEMENTED;
}

NS_IMETHODIMP_(void)
nsHtml5Parser::BlockParser()
{
  mBlocked = PR_TRUE;
}

NS_IMETHODIMP_(void)
nsHtml5Parser::UnblockParser()
{
  mBlocked = PR_FALSE;
}

NS_IMETHODIMP_(PRBool)
nsHtml5Parser::IsParserEnabled()
{
  return !mBlocked;
}

NS_IMETHODIMP_(PRBool)
nsHtml5Parser::IsComplete()
{
  return mExecutor->IsComplete();
}

NS_IMETHODIMP
nsHtml5Parser::Parse(nsIURI* aURL, // legacy parameter; ignored
                     nsIRequestObserver* aObserver,
                     void* aKey,
                     nsDTDMode aMode) // legacy; ignored
{
  /*
   * Do NOT cause WillBuildModel to be called synchronously from here!
   * The document won't be ready for it until OnStartRequest!
   */
  NS_PRECONDITION(!mExecutor->HasStarted(), 
                  "Tried to start parse without initializing the parser properly.");
  NS_PRECONDITION(mStreamParser, 
                  "Can't call this variant of Parse() on script-created parser");
  mStreamParser->SetObserver(aObserver);
  mExecutor->SetStreamParser(mStreamParser);
  mExecutor->SetParser(this);
  mRootContextKey = aKey;
  return NS_OK;
}

NS_IMETHODIMP
nsHtml5Parser::Parse(const nsAString& aSourceBuffer,
                     void* aKey,
                     const nsACString& aContentType, // ignored
                     PRBool aLastCall,
                     nsDTDMode aMode) // ignored
{
  NS_PRECONDITION(!mExecutor->IsFragmentMode(), "Document.write called in fragment mode!");

  // Maintain a reference to ourselves so we don't go away
  // till we're completely done. The old parser grips itself in this method.
  nsCOMPtr<nsIParser> kungFuDeathGrip(this);
  
  // Gripping the other objects just in case, since the other old grip
  // required grips to these, too.
  nsRefPtr<nsHtml5StreamParser> streamKungFuDeathGrip(mStreamParser);
  nsRefPtr<nsHtml5TreeOpExecutor> treeOpKungFuDeathGrip(mExecutor);

  // Return early if the parser has processed EOF
  if (!mExecutor->HasStarted()) {
    NS_ASSERTION(!mStreamParser,
                 "Had stream parser but document.write started life cycle.");
    // This is the first document.write() on a document.open()ed document
    mExecutor->SetParser(this);
    mTokenizer->setEncodingDeclarationHandler(this);
    mTreeBuilder->setScriptingEnabled(mExecutor->IsScriptEnabled());
    mTokenizer->start();
    mExecutor->Start();
    /*
     * If you move the following line, be very careful not to cause 
     * WillBuildModel to be called before the document has had its 
     * script global object set.
     */
    mExecutor->WillBuildModel(eDTDMode_unknown);
  }
  if (mExecutor->IsComplete()) {
    return NS_OK;
  }
  if (aLastCall && aSourceBuffer.IsEmpty() && aKey == GetRootContextKey()) {
    // document.close()
    NS_ASSERTION(!mStreamParser,
                 "Had stream parser but got document.close().");
    mDocumentClosed = PR_TRUE;
    if (!mBlocked) {
      ParseUntilBlocked();
    }
    return NS_OK;
  }

  NS_PRECONDITION(IsInsertionPointDefined(), 
                  "Document.write called when insertion point not defined.");

  NS_PRECONDITION(!(mStreamParser && !aKey), "Got a null key in a non-script-created parser");

  if (aSourceBuffer.IsEmpty()) {
    return NS_OK;
  }

  nsRefPtr<nsHtml5UTF16Buffer> buffer = new nsHtml5UTF16Buffer(aSourceBuffer.Length());
  memcpy(buffer->getBuffer(), aSourceBuffer.BeginReading(), aSourceBuffer.Length() * sizeof(PRUnichar));
  buffer->setEnd(aSourceBuffer.Length());

  // The buffer is inserted to the stream here in case it won't be parsed
  // to completion.
  // The script is identified by aKey. If there's nothing in the buffer
  // chain for that key, we'll insert at the head of the queue.
  // When the script leaves something in the queue, a zero-length
  // key-holder "buffer" is inserted in the queue. If the same script
  // leaves something in the chain again, it will be inserted immediately
  // before the old key holder belonging to the same script.
  nsHtml5UTF16Buffer* prevSearchBuf = nsnull;
  nsHtml5UTF16Buffer* searchBuf = mFirstBuffer;
  if (aKey) { // after document.open, the first level of document.write has null key
    while (searchBuf != mLastBuffer) {
      if (searchBuf->key == aKey) {
        // found a key holder
        // now insert the new buffer between the previous buffer
        // and the key holder.
        buffer->next = searchBuf;
        if (prevSearchBuf) {
          prevSearchBuf->next = buffer;
        } else {
          mFirstBuffer = buffer;
        }
        break;
      }
      prevSearchBuf = searchBuf;
      searchBuf = searchBuf->next;
    }
    if (searchBuf == mLastBuffer) {
      // key was not found
      nsHtml5UTF16Buffer* keyHolder = new nsHtml5UTF16Buffer(aKey);
      keyHolder->next = mFirstBuffer;
      buffer->next = keyHolder;
      mFirstBuffer = buffer;
    }
  } else {
    // we have a first level document.write after document.open()
    // insert immediately before mLastBuffer
    while (searchBuf != mLastBuffer) {
      prevSearchBuf = searchBuf;
      searchBuf = searchBuf->next;
    }
    buffer->next = mLastBuffer;
    if (prevSearchBuf) {
      prevSearchBuf->next = buffer;
    } else {
      mFirstBuffer = buffer;
    }
  }

  if (!mBlocked) {
    while (buffer->hasMore()) {
      buffer->adjust(mLastWasCR);
      mLastWasCR = PR_FALSE;
      if (buffer->hasMore()) {

        PRInt32 lineNumberSave;
        PRBool inRootContext = (!mStreamParser && (aKey == mRootContextKey));
        if (inRootContext) {
          mTokenizer->setLineNumber(mRootContextLineNumber);
        } else {
          // we aren't the root context, so save the line number on the 
          // *stack* so that we can restore it.
          lineNumberSave = mTokenizer->getLineNumber();
        }

        mLastWasCR = mTokenizer->tokenizeBuffer(buffer);

        if (inRootContext) {
          mRootContextLineNumber = mTokenizer->getLineNumber();
        } else {
          mTokenizer->setLineNumber(lineNumberSave);
        }

        if (mTreeBuilder->HasScript()) {
          // No need to flush characters, because an end tag was tokenized last
          mTreeBuilder->Flush(); // Move ops to the executor
          mExecutor->FlushDocumentWrite(); // run the ops    
        }
        if (mBlocked) {
          break;
        }
        // Ignore suspension requests
      }
    }
  }

  if (!mBlocked) { // buffer was tokenized to completion
    NS_ASSERTION(!buffer->hasMore(), "Buffer wasn't tokenized to completion?");  
    // Scripting semantics require a forced tree builder flush here
    mTreeBuilder->flushCharacters(); // Flush trailing characters
    mTreeBuilder->Flush(); // Move ops to the executor
    mExecutor->FlushDocumentWrite(); // run the ops    
  }

  return NS_OK;
}

/**
 * This magic value is passed to the previous method on document.close()
 */
NS_IMETHODIMP_(void *)
nsHtml5Parser::GetRootContextKey()
{
  return mRootContextKey;
}

NS_IMETHODIMP
nsHtml5Parser::Terminate(void)
{
  // We should only call DidBuildModel once, so don't do anything if this is
  // the second time that Terminate has been called.
  if (mExecutor->IsComplete()) {
    return NS_OK;
  }
  // XXX - [ until we figure out a way to break parser-sink circularity ]
  // Hack - Hold a reference until we are completely done...
  nsCOMPtr<nsIParser> kungFuDeathGrip(this);
  nsRefPtr<nsHtml5StreamParser> streamKungFuDeathGrip(mStreamParser);
  nsRefPtr<nsHtml5TreeOpExecutor> treeOpKungFuDeathGrip(mExecutor);
  if (mStreamParser) {
    mStreamParser->Terminate();
  }
  return mExecutor->DidBuildModel(PR_TRUE);
}

NS_IMETHODIMP
nsHtml5Parser::ParseFragment(const nsAString& aSourceBuffer,
                             void* aKey,
                             nsTArray<nsString>& aTagStack,
                             PRBool aXMLMode,
                             const nsACString& aContentType,
                             nsDTDMode aMode)
{
  return NS_ERROR_NOT_IMPLEMENTED;
}

NS_IMETHODIMP
nsHtml5Parser::ParseFragment(const nsAString& aSourceBuffer,
                             nsISupports* aTargetNode,
                             nsIAtom* aContextLocalName,
                             PRInt32 aContextNamespace,
                             PRBool aQuirks)
{
  nsCOMPtr<nsIContent> target = do_QueryInterface(aTargetNode);
  NS_ASSERTION(target, "Target did not QI to nsIContent");

  nsIDocument* doc = target->GetOwnerDoc();
  NS_ENSURE_TRUE(doc, NS_ERROR_NOT_AVAILABLE);
  
  nsIURI* uri = doc->GetDocumentURI();
  NS_ENSURE_TRUE(uri, NS_ERROR_NOT_AVAILABLE);

  nsCOMPtr<nsISupports> container = doc->GetContainer();
  // Can be null if owner document is synthetic

  Initialize(doc, uri, container, nsnull);

  // Initialize() doesn't deal with base URI
  mExecutor->SetBaseUriFromDocument();
  mExecutor->SetParser(this);
  mExecutor->SetNodeInfoManager(target->GetOwnerDoc()->NodeInfoManager());

  nsIContent* weakTarget = target;
  mTreeBuilder->setFragmentContext(aContextLocalName, aContextNamespace, &weakTarget, aQuirks);
  mExecutor->EnableFragmentMode();
  
  NS_PRECONDITION(!mExecutor->HasStarted(), "Tried to start parse without initializing the parser properly.");
  mTreeBuilder->setScriptingEnabled(mExecutor->IsScriptEnabled());
  mTokenizer->start();
  mExecutor->Start(); // Don't call WillBuildModel in fragment case
  if (!aSourceBuffer.IsEmpty()) {
    PRBool lastWasCR = PR_FALSE;
    nsHtml5UTF16Buffer buffer(aSourceBuffer.Length());
    memcpy(buffer.getBuffer(), aSourceBuffer.BeginReading(), aSourceBuffer.Length() * sizeof(PRUnichar));
    buffer.setEnd(aSourceBuffer.Length());
    while (buffer.hasMore()) {
      buffer.adjust(lastWasCR);
      lastWasCR = PR_FALSE;
      if (buffer.hasMore()) {
        lastWasCR = mTokenizer->tokenizeBuffer(&buffer);
      }
    }
  }
  mTokenizer->eof();
  mTreeBuilder->StreamEnded();
  mTreeBuilder->Flush();
  mExecutor->FlushDocumentWrite();
  mTokenizer->end();
  mExecutor->DropParserAndPerfHint();
  mAtomTable.Clear();
  return NS_OK;
}

NS_IMETHODIMP
nsHtml5Parser::BuildModel(void)
{
  NS_NOTREACHED("Don't call this!");
  return NS_ERROR_NOT_IMPLEMENTED;
}

NS_IMETHODIMP
nsHtml5Parser::CancelParsingEvents()
{
  NS_NOTREACHED("Don't call this!");
  return NS_ERROR_NOT_IMPLEMENTED;
}

void
nsHtml5Parser::Reset()
{
  mExecutor->Reset();
  mLastWasCR = PR_FALSE;
  UnblockParser();
  mDocumentClosed = PR_FALSE;
  mStreamParser = nsnull;
  mRootContextLineNumber = 1;
  mParserInsertedScriptsBeingEvaluated = 0;
  mRootContextKey = nsnull;
  mAtomTable.Clear(); // should be already cleared in the fragment case anyway
  // Portable parser objects
  mFirstBuffer->next = nsnull;
  mFirstBuffer->setStart(0);
  mFirstBuffer->setEnd(0);
}

PRBool
nsHtml5Parser::CanInterrupt()
{
  return !mExecutor->IsFragmentMode();
}

PRBool
nsHtml5Parser::IsInsertionPointDefined()
{
  return !mExecutor->IsFlushing() &&
    (!mStreamParser || mParserInsertedScriptsBeingEvaluated);
}

void
nsHtml5Parser::BeginEvaluatingParserInsertedScript()
{
  ++mParserInsertedScriptsBeingEvaluated;
}

void
nsHtml5Parser::EndEvaluatingParserInsertedScript()
{
  --mParserInsertedScriptsBeingEvaluated;
}

void
nsHtml5Parser::MarkAsNotScriptCreated()
{
  NS_PRECONDITION(!mStreamParser, "Must not call this twice.");
  mStreamParser = new nsHtml5StreamParser(mExecutor, this);
}

PRBool
nsHtml5Parser::IsScriptCreated()
{
  return !mStreamParser;
}

/* End nsIParser  */

// not from interface
void
nsHtml5Parser::ParseUntilBlocked()
{
  NS_PRECONDITION(!mExecutor->IsFragmentMode(), "ParseUntilBlocked called in fragment mode.");

  if (mBlocked) {
    return;
  }

  if (mExecutor->IsComplete()) {
    return;
  }
  NS_ASSERTION(mExecutor->HasStarted(), "Bad life cycle.");

  for (;;) {
    if (!mFirstBuffer->hasMore()) {
      if (mFirstBuffer == mLastBuffer) {
        if (mExecutor->IsComplete()) {
          // something like cache manisfests stopped the parse in mid-flight
          return;
        }
        if (mDocumentClosed) {
          NS_ASSERTION(!mStreamParser,
                       "This should only happen with script-created parser.");
          mTokenizer->eof();
          mTreeBuilder->StreamEnded();
          mTreeBuilder->Flush();
          mExecutor->FlushDocumentWrite();
          mTokenizer->end();
          return;            
        } else {
          // never release the last buffer.
          NS_ASSERTION(!mLastBuffer->getStart(), 
            "Sentinel buffer had its indeces changed.");
          NS_ASSERTION(!mLastBuffer->getEnd(), 
            "Sentinel buffer had its indeces changed.");
          if (mStreamParser && 
              mReturnToStreamParserPermitted && 
              !mExecutor->IsScriptExecuting()) {
            mTreeBuilder->flushCharacters();
            mTreeBuilder->Flush();
            mReturnToStreamParserPermitted = PR_FALSE;
            mStreamParser->ContinueAfterScripts(mTokenizer, 
                                                mTreeBuilder, 
                                                mLastWasCR);
          }
          return; // no more data for now but expecting more
        }
      } else {
        mFirstBuffer = mFirstBuffer->next;
        continue;
      }
    }

    if (mBlocked || mExecutor->IsComplete()) {
      return;
    }

    // now we have a non-empty buffer
    mFirstBuffer->adjust(mLastWasCR);
    mLastWasCR = PR_FALSE;
    if (mFirstBuffer->hasMore()) {
      PRBool inRootContext = (!mStreamParser && (mFirstBuffer->key == mRootContextKey));
      if (inRootContext) {
        mTokenizer->setLineNumber(mRootContextLineNumber);
      }
      mLastWasCR = mTokenizer->tokenizeBuffer(mFirstBuffer);
      if (inRootContext) {
        mRootContextLineNumber = mTokenizer->getLineNumber();
      }
      if (mTreeBuilder->HasScript()) {
        mTreeBuilder->Flush();
        mExecutor->FlushDocumentWrite();
      }
      if (mBlocked) {
        return;
      }
    }
    continue;
  }
}

nsresult
nsHtml5Parser::Initialize(nsIDocument* aDoc,
                          nsIURI* aURI,
                          nsISupports* aContainer,
                          nsIChannel* aChannel)
{
  return mExecutor->Init(aDoc, aURI, aContainer, aChannel);
}

void
nsHtml5Parser::StartTokenizer(PRBool aScriptingEnabled) {
  mTreeBuilder->setScriptingEnabled(aScriptingEnabled);
  mTokenizer->start();
}

void
nsHtml5Parser::InitializeDocWriteParserState(nsAHtml5TreeBuilderState* aState, PRInt32 aLine)
{
  mTokenizer->resetToDataState();
  mTokenizer->setLineNumber(aLine);
  mTreeBuilder->loadState(aState, &mAtomTable);
  mLastWasCR = PR_FALSE;
  mReturnToStreamParserPermitted = PR_TRUE;
}

void
nsHtml5Parser::ContinueAfterFailedCharsetSwitch()
{
  NS_PRECONDITION(mStreamParser, 
    "Tried to continue after failed charset switch without a stream parser");
  mStreamParser->ContinueAfterFailedCharsetSwitch();
}

void
nsHtml5Parser::internalEncodingDeclaration(nsString* aEncoding)
{
  // Note: This handler is only installed when parsing a document.open()ed doc
  // See bug 539887 and bug 255820.
  if (mCharsetSource >= kCharsetFromMetaTag) { // this threshold corresponds to "confident" in the HTML5 spec
    return;
  }

  nsresult rv = NS_OK;
  nsCOMPtr<nsICharsetAlias> calias(do_GetService(kCharsetAliasCID, &rv));
  if (NS_FAILED(rv)) {
    NS_NOTREACHED("Charset alias service not available.");
    return;
  }
  nsCAutoString newEncoding;
  CopyUTF16toUTF8(*aEncoding, newEncoding);
  
  // XXX check HTML5 non-IANA aliases here
  
  nsCAutoString preferred;
  
  rv = calias->GetPreferred(newEncoding, preferred);
  if (NS_FAILED(rv)) {
    // the encoding name is bogus
    return;
  }
  
  mCharsetSource = kCharsetFromMetaTag;
  mTreeBuilder->SetDocumentCharset(preferred, mCharsetSource);
}