Merge branch 'feature_updater' into develop

5 files changed, 952 insertions, 0 deletions

diff --git a/logic/net/FileDownload.cpp b/logic/net/FileDownload.cpp
index 239af351..38f0b9c2 100644
--- a/logic/net/FileDownload.cpp
+++ b/logic/net/FileDownload.cpp
@@ -63,6 +63,15 @@ void FileDownload::start()
 	request.setRawHeader(QString("If-None-Match").toLatin1(), m_expected_md5.toLatin1());
 	request.setHeader(QNetworkRequest::UserAgentHeader, "MultiMC/5.0 (Uncached)");
 
+	// Go ahead and try to open the file.
+	// If we don't do this, empty files won't be created, which breaks the updater.
+	// Plus, this way, we don't end up starting a download for a file we can't open.
+	if (!m_output_file.open(QIODevice::WriteOnly))
+	{
+		emit failed(index_within_job);
+		return;
+	}
+
 	auto worker = MMC->qnam();
 	QNetworkReply *rep = worker->get(request);
 
diff --git a/logic/updater/DownloadUpdateTask.cpp b/logic/updater/DownloadUpdateTask.cpp
new file mode 100644
index 00000000..ef975c93
--- /dev/null
+++ b/logic/updater/DownloadUpdateTask.cpp
@@ -0,0 +1,398 @@
+/* Copyright 2013 MultiMC Contributors
+ *
+ * Licensed under the Apache License, Version 2.0 (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.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "DownloadUpdateTask.h"
+
+#include "MultiMC.h"
+#include "logic/updater/UpdateChecker.h"
+#include "logic/net/NetJob.h"
+#include "pathutils.h"
+
+#include <QFile>
+#include <QTemporaryDir>
+#include <QCryptographicHash>
+
+#include <QDomDocument>
+
+
+DownloadUpdateTask::DownloadUpdateTask(QString repoUrl, int versionId, QObject* parent) :
+	Task(parent)
+{
+	m_cVersionId = MMC->version().build;
+
+	m_nRepoUrl = repoUrl;
+	m_nVersionId = versionId;
+
+	m_updateFilesDir.setAutoRemove(false);
+}
+
+void DownloadUpdateTask::executeTask()
+{
+	// GO!
+	// This will call the next step when it's done.
+	findCurrentVersionInfo();
+}
+
+void DownloadUpdateTask::findCurrentVersionInfo()
+{
+	setStatus(tr("Finding information about the current version."));
+
+	auto checker = MMC->updateChecker();
+
+	// This runs after we've tried loading the channel list.
+	// If the channel list doesn't need to be loaded, this will be called immediately.
+	// If the channel list does need to be loaded, this will be called when it's done.
+	auto processFunc = [this, &checker] () -> void
+	{
+		// Now, check the channel list again.
+		if (checker->hasChannels())
+		{ 
+			// We still couldn't load the channel list. Give up. Call loadVersionInfo and return.
+			QLOG_INFO() << "Reloading the channel list didn't work. Giving up.";
+			loadVersionInfo();
+			return;
+		}
+
+		QList<UpdateChecker::ChannelListEntry> channels = checker->getChannelList();
+		QString channelId = MMC->version().channel;
+
+		// Search through the channel list for a channel with the correct ID.
+		for (auto channel : channels)
+		{
+			if (channel.id == channelId)
+			{
+				QLOG_INFO() << "Found matching channel.";
+				m_cRepoUrl = channel.url;
+				break;
+			}
+		}
+
+		// Now that we've done that, load version info.
+		loadVersionInfo();
+	};
+
+	if (checker->hasChannels())
+	{
+		// Load the channel list and wait for it to finish loading.
+		QLOG_INFO() << "No channel list entries found. Will try reloading it.";
+
+		QObject::connect(checker.get(), &UpdateChecker::channelListLoaded, processFunc);
+		checker->updateChanList();
+	}
+	else
+	{
+		processFunc();
+	}
+}
+
+void DownloadUpdateTask::loadVersionInfo()
+{
+	setStatus(tr("Loading version information."));
+
+	// Create the net job for loading version info.
+	NetJob* netJob = new NetJob("Version Info");
+	
+	// Find the index URL.
+	QUrl newIndexUrl = QUrl(m_nRepoUrl).resolved(QString::number(m_nVersionId) + ".json");
+	
+	// Add a net action to download the version info for the version we're updating to.
+	netJob->addNetAction(ByteArrayDownload::make(newIndexUrl));
+
+	// If we have a current version URL, get that one too.
+	if (!m_cRepoUrl.isEmpty())
+	{
+		QUrl cIndexUrl = QUrl(m_cRepoUrl).resolved(QString::number(m_cVersionId) + ".json");
+		netJob->addNetAction(ByteArrayDownload::make(cIndexUrl));
+	}
+
+	// Connect slots so we know when it's done.
+	QObject::connect(netJob, &NetJob::succeeded, this, &DownloadUpdateTask::vinfoDownloadFinished);
+	QObject::connect(netJob, &NetJob::failed, this, &DownloadUpdateTask::vinfoDownloadFailed);
+
+	// Store the NetJob in a class member. We don't want to lose it!
+	m_vinfoNetJob.reset(netJob);
+
+	// Finally, we start the network job and the thread's event loop to wait for it to finish.
+	netJob->start();
+}
+
+void DownloadUpdateTask::vinfoDownloadFinished()
+{
+	// Both downloads succeeded. OK. Parse stuff.
+	parseDownloadedVersionInfo();
+}
+
+void DownloadUpdateTask::vinfoDownloadFailed()
+{
+	// Something failed. We really need the second download (current version info), so parse downloads anyways as long as the first one succeeded.
+	if (m_vinfoNetJob->first()->m_status != Job_Failed)
+	{
+		parseDownloadedVersionInfo();
+		return;
+	}
+
+	// TODO: Give a more detailed error message.
+	QLOG_ERROR() << "Failed to download version info files.";
+	emitFailed(tr("Failed to download version info files."));
+}
+
+void DownloadUpdateTask::parseDownloadedVersionInfo()
+{
+	setStatus(tr("Reading file lists."));
+
+	parseVersionInfo(NEW_VERSION, &m_nVersionFileList);
+
+	// If there is a second entry in the network job's list, load it as the current version's info.
+	if (m_vinfoNetJob->size() >= 2 && m_vinfoNetJob->operator[](1)->m_status != Job_Failed)
+	{
+		parseVersionInfo(CURRENT_VERSION, &m_cVersionFileList);
+	}
+
+	// We don't need this any more.
+	m_vinfoNetJob.reset();
+
+	// Now that we're done loading version info, we can move on to the next step. Process file lists and download files.
+	processFileLists();
+}
+
+void DownloadUpdateTask::parseVersionInfo(VersionInfoFileEnum vfile, VersionFileList* list)
+{
+	if (vfile == CURRENT_VERSION)  setStatus(tr("Reading file list for current version."));
+	else if (vfile == NEW_VERSION) setStatus(tr("Reading file list for new version."));
+
+	QLOG_DEBUG() << "Reading file list for" << (vfile == NEW_VERSION ? "new" : "current") << "version.";
+	
+	QByteArray data;
+	{
+		ByteArrayDownloadPtr dl = std::dynamic_pointer_cast<ByteArrayDownload>(
+				vfile == NEW_VERSION ? m_vinfoNetJob->first() : m_vinfoNetJob->operator[](1));
+		data = dl->m_data;
+	}
+
+	QJsonParseError jsonError;
+	QJsonDocument jsonDoc = QJsonDocument::fromJson(data, &jsonError);
+	if (jsonError.error != QJsonParseError::NoError)
+	{
+		QLOG_ERROR() << "Failed to parse version info JSON:" << jsonError.errorString() << "at" << jsonError.offset;
+		return;
+	}
+
+	QJsonObject json = jsonDoc.object();
+
+	QLOG_DEBUG() << "Loading version info from JSON.";
+	QJsonArray filesArray = json.value("Files").toArray();
+	for (QJsonValue fileValue : filesArray)
+	{
+		QJsonObject fileObj = fileValue.toObject();
+
+		VersionFileEntry file{
+			fileObj.value("Path").toString(),
+			fileObj.value("Perms").toVariant().toInt(),
+			FileSourceList(),
+			fileObj.value("MD5").toString(),
+		};
+		QLOG_DEBUG() << "File" << file.path << "with perms" << file.mode;
+		
+		QJsonArray sourceArray = fileObj.value("Sources").toArray();
+		for (QJsonValue val : sourceArray)
+		{
+			QJsonObject sourceObj = val.toObject();
+
+			QString type = sourceObj.value("SourceType").toString();
+			if (type == "http")
+			{
+				file.sources.append(FileSource("http", sourceObj.value("Url").toString()));
+			}
+			else if (type == "httpc")
+			{
+				file.sources.append(FileSource("httpc", sourceObj.value("Url").toString(), sourceObj.value("CompressionType").toString()));
+			}
+			else
+			{
+				QLOG_WARN() << "Unknown source type" << type << "ignored.";
+			}
+		}
+
+		QLOG_DEBUG() << "Loaded info for" << file.path;
+
+		list->append(file);
+	}
+}
+
+void DownloadUpdateTask::processFileLists()
+{
+	setStatus(tr("Processing file lists. Figuring out how to install the update."));
+
+	// First, if we've loaded the current version's file list, we need to iterate through it and 
+	// delete anything in the current one version's list that isn't in the new version's list.
+	for (VersionFileEntry entry : m_cVersionFileList)
+	{
+		for (VersionFileEntry newEntry : m_nVersionFileList)
+		{ 
+			if (newEntry.path == entry.path)
+				continue;
+		}
+		// If the loop reaches the end, we didn't find a match. Delete the file.
+		m_operationList.append(UpdateOperation::DeleteOp(entry.path));
+	}
+
+	// Create a network job for downloading files.
+	NetJob* netJob = new NetJob("Update Files");
+
+	// Next, check each file in MultiMC's folder and see if we need to update them.
+	for (VersionFileEntry entry : m_nVersionFileList)
+	{
+		// TODO: Let's not MD5sum a ton of files on the GUI thread. We should probably find a way to do this in the background.
+		QString fileMD5;
+		QFile entryFile(entry.path);
+		if (entryFile.open(QFile::ReadOnly))
+		{
+			QCryptographicHash hash(QCryptographicHash::Md5);
+			hash.addData(entryFile.readAll());
+			fileMD5 = hash.result().toHex();
+		}
+
+		if (!entryFile.exists() || fileMD5.isEmpty() || fileMD5 != entry.md5)
+		{
+			QLOG_DEBUG() << "Found file" << entry.path << "that needs updating.";
+
+			// Go through the sources list and find one to use.
+			// TODO: Make a NetAction that takes a source list and tries each of them until one works. For now, we'll just use the first http one.
+			for (FileSource source : entry.sources)
+			{
+				if (source.type == "http")
+				{
+					QLOG_DEBUG() << "Will download" << entry.path << "from" << source.url;
+
+					// Download it to updatedir/<filepath>-<md5> where filepath is the file's path with slashes replaced by underscores.
+					QString dlPath = PathCombine(m_updateFilesDir.path(), QString(entry.path).replace("/", "_"));
+
+					// We need to download the file to the updatefiles folder and add a task to copy it to its install path.
+					FileDownloadPtr download = FileDownload::make(source.url, dlPath);
+					download->m_check_md5 = true;
+					download->m_expected_md5 = entry.md5;
+					netJob->addNetAction(download);
+
+					// Now add a copy operation to our operations list to install the file.
+					m_operationList.append(UpdateOperation::CopyOp(dlPath, entry.path, entry.mode));
+				}
+			}
+		}
+	}
+
+	// Add listeners to wait for the downloads to finish.
+	QObject::connect(netJob, &NetJob::succeeded, this, &DownloadUpdateTask::fileDownloadFinished);
+	QObject::connect(netJob, &NetJob::progress, this, &DownloadUpdateTask::fileDownloadProgressChanged);
+	QObject::connect(netJob, &NetJob::failed, this, &DownloadUpdateTask::fileDownloadFailed);
+
+	// Now start the download.
+	setStatus(tr("Downloading %1 update files.").arg(QString::number(netJob->size())));
+	QLOG_DEBUG() << "Begin downloading update files to" << m_updateFilesDir.path();
+	m_filesNetJob.reset(netJob);
+	netJob->start();
+
+	writeInstallScript(m_operationList, PathCombine(m_updateFilesDir.path(), "file_list.xml"));
+}
+
+void DownloadUpdateTask::writeInstallScript(UpdateOperationList& opsList, QString scriptFile)
+{
+	// Build the base structure of the XML document.
+	QDomDocument doc;
+
+	QDomElement root = doc.createElement("update");
+	root.setAttribute("version", "3");
+	doc.appendChild(root);
+
+	QDomElement installFiles = doc.createElement("install");
+	root.appendChild(installFiles);
+
+	QDomElement removeFiles = doc.createElement("uninstall");
+	root.appendChild(removeFiles);
+
+	// Write the operation list to the XML document.
+	for (UpdateOperation op : opsList)
+	{
+		QDomElement file = doc.createElement("file");
+
+		switch (op.type)
+		{
+			case UpdateOperation::OP_COPY:
+				{
+				// Install the file.
+				QDomElement name = doc.createElement("source");
+				QDomElement path = doc.createElement("dest");
+				QDomElement mode = doc.createElement("mode");
+				name.appendChild(doc.createTextNode(op.file));
+				path.appendChild(doc.createTextNode(op.dest));
+				// We need to add a 0 at the beginning here, because Qt doesn't convert to octal correctly.
+				mode.appendChild(doc.createTextNode("0" + QString::number(op.mode, 8)));
+				file.appendChild(name);
+				file.appendChild(path);
+				file.appendChild(mode);
+				installFiles.appendChild(file);
+				QLOG_DEBUG() << "Will install file" << op.file;
+				}
+				break;
+
+			case UpdateOperation::OP_DELETE:
+				{
+				// Delete the file.
+				file.appendChild(doc.createTextNode(op.file));
+				removeFiles.appendChild(file);
+				QLOG_DEBUG() << "Will remove file" << op.file;
+				}
+				break;
+
+			default:
+				QLOG_WARN() << "Can't write update operation of type" << op.type << "to file. Not implemented.";
+				continue;
+		}
+	}
+
+	// Write the XML document to the file.
+	QFile outFile(scriptFile);
+
+	if (outFile.open(QIODevice::WriteOnly))
+	{
+		outFile.write(doc.toByteArray());
+	}
+	else
+	{
+		emitFailed(tr("Failed to write update script file."));
+	}
+}
+
+void DownloadUpdateTask::fileDownloadFinished()
+{
+	emitSucceeded();
+}
+
+void DownloadUpdateTask::fileDownloadFailed()
+{
+	// TODO: Give more info about the failure.
+	QLOG_ERROR() << "Failed to download update files.";
+	emitFailed(tr("Failed to download update files."));
+}
+
+void DownloadUpdateTask::fileDownloadProgressChanged(qint64 current, qint64 total)
+{
+	setProgress((int)(((float)current / (float)total)*100));
+}
+
+QString DownloadUpdateTask::updateFilesDir()
+{
+	return m_updateFilesDir.path();
+}
+
diff --git a/logic/updater/DownloadUpdateTask.h b/logic/updater/DownloadUpdateTask.h
new file mode 100644
index 00000000..f5b23d12
--- /dev/null
+++ b/logic/updater/DownloadUpdateTask.h
@@ -0,0 +1,192 @@
+/* Copyright 2013 MultiMC Contributors
+ *
+ * Licensed under the Apache License, Version 2.0 (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.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include "logic/tasks/Task.h"
+#include "logic/net/NetJob.h"
+
+/*!
+ * The DownloadUpdateTask is a task that takes a given version ID and repository URL,
+ * downloads that version's files from the repository, and prepares to install them.
+ */
+class DownloadUpdateTask : public Task
+{
+	Q_OBJECT
+
+public:
+	explicit DownloadUpdateTask(QString repoUrl, int versionId, QObject* parent=0);
+
+	/*!
+	 * Gets the directory that contains the update files.
+	 */
+	QString updateFilesDir();
+	
+protected:
+	// TODO: We should probably put these data structures into a separate header...
+
+	/*!
+	 * Struct that describes an entry in a VersionFileEntry's `Sources` list.
+	 */
+	struct FileSource
+	{
+		FileSource(QString type, QString url, QString compression="")
+		{
+			this->type = type;
+			this->url = url;
+			this->compressionType = compression;
+		}
+
+		QString type;
+		QString url;
+		QString compressionType;
+	};
+
+	typedef QList<FileSource> FileSourceList;
+
+	/*!
+	 * Structure that describes an entry in a GoUpdate version's `Files` list.
+	 */
+	struct VersionFileEntry
+	{
+		QString path;
+		int mode;
+		FileSourceList sources;
+		QString md5;
+	};
+
+	typedef QList<VersionFileEntry> VersionFileList;
+
+
+	/*!
+	 * Structure that describes an operation to perform when installing updates.
+	 */
+	struct UpdateOperation
+	{
+		static UpdateOperation CopyOp(QString fsource, QString fdest, int fmode=0644) { return UpdateOperation{OP_COPY, fsource, fdest, fmode}; }
+		static UpdateOperation MoveOp(QString fsource, QString fdest, int fmode=0644) { return UpdateOperation{OP_MOVE, fsource, fdest, fmode}; }
+		static UpdateOperation DeleteOp(QString file) { return UpdateOperation{OP_DELETE, file, "", 0644}; }
+		static UpdateOperation ChmodOp(QString file, int fmode) { return UpdateOperation{OP_CHMOD, file, "", fmode}; }
+
+		//! Specifies the type of operation that this is.
+		enum Type
+		{
+			OP_COPY,
+			OP_DELETE,
+			OP_MOVE,
+			OP_CHMOD,
+		} type;
+
+		//! The file to operate on. If this is a DELETE or CHMOD operation, this is the file that will be modified.
+		QString file;
+
+		//! The destination file. If this is a DELETE or CHMOD operation, this field will be ignored.
+		QString dest;
+
+		//! The mode to change the source file to. Ignored if this isn't a CHMOD operation.
+		int mode;
+
+		// Yeah yeah, polymorphism blah blah inheritance, blah blah object oriented. I'm lazy, OK?
+	};
+
+	typedef QList<UpdateOperation> UpdateOperationList;
+
+	/*!
+	 * Used for arguments to parseVersionInfo and friends to specify which version info file to parse.
+	 */
+	enum VersionInfoFileEnum { NEW_VERSION, CURRENT_VERSION };
+
+
+	//! Entry point for tasks.
+	virtual void executeTask();
+
+	/*!
+	 * Attempts to find the version ID and repository URL for the current version.
+	 * The function will look up the repository URL in the UpdateChecker's channel list.
+	 * If the repository URL can't be found, this function will return false.
+	 */
+	virtual void findCurrentVersionInfo();
+
+	/*!
+	 * Downloads the version info files from the repository.
+	 * The files for both the current build, and the build that we're updating to need to be downloaded.
+	 * If the current version's info file can't be found, MultiMC will not delete files that 
+	 * were removed between versions. It will still replace files that have changed, however.
+	 * Note that although the repository URL for the current version is not given to the update task,
+	 * the task will attempt to look it up in the UpdateChecker's channel list.
+	 * If an error occurs here, the function will call emitFailed and return false.
+	 */
+	virtual void loadVersionInfo();
+
+	/*!
+	 * This function is called when version information is finished downloading.
+	 * This handles parsing the JSON downloaded by the version info network job and then calls processFileLists.
+	 * Note that this function will sometimes be called even if the version info download emits failed. If
+	 * we couldn't download the current version's info file, we can still update. This will be called even if the 
+	 * current version's info file fails to download, as long as the new version's info file succeeded.
+	 */
+	virtual void parseDownloadedVersionInfo();
+
+	/*!
+	 * Loads the file list from the given version info JSON object into the given list.
+	 */
+	virtual void parseVersionInfo(VersionInfoFileEnum vfile, VersionFileList* list);
+
+	/*!
+	 * Takes a list of file entries for the current version's files and the new version's files
+	 * and populates the downloadList and operationList with information about how to download and install the update.
+	 */
+	virtual void processFileLists();
+
+	/*!
+	 * Takes the operations list and writes an install script for the updater to the update files directory.
+	 */
+	virtual void writeInstallScript(UpdateOperationList& opsList, QString scriptFile);
+
+	VersionFileList m_downloadList;
+	UpdateOperationList m_operationList;
+
+	VersionFileList m_nVersionFileList;
+	VersionFileList m_cVersionFileList;
+
+	//! Network job for downloading version info files.
+	NetJobPtr m_vinfoNetJob;
+	
+	//! Network job for downloading update files.
+	NetJobPtr m_filesNetJob;
+
+	// Version ID and repo URL for the new version.
+	int m_nVersionId;
+	QString m_nRepoUrl;
+
+	// Version ID and repo URL for the currently installed version.
+	int m_cVersionId;
+	QString m_cRepoUrl;
+
+	/*!
+	 * Temporary directory to store update files in.
+	 * This will be set to not auto delete. Task will fail if this fails to be created.
+	 */
+	QTemporaryDir m_updateFilesDir;
+
+protected slots:
+	void vinfoDownloadFinished();
+	void vinfoDownloadFailed();
+
+	void fileDownloadFinished();
+	void fileDownloadFailed();
+	void fileDownloadProgressChanged(qint64 current, qint64 total);
+};
+
diff --git a/logic/updater/UpdateChecker.cpp b/logic/updater/UpdateChecker.cpp
new file mode 100644
index 00000000..5ff1898e
--- /dev/null
+++ b/logic/updater/UpdateChecker.cpp
@@ -0,0 +1,247 @@
+/* Copyright 2013 MultiMC Contributors
+ *
+ * Licensed under the Apache License, Version 2.0 (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.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "UpdateChecker.h"
+
+#include "MultiMC.h"
+
+#include "config.h"
+#include "logger/QsLog.h"
+
+#include <QJsonObject>
+#include <QJsonArray>
+#include <QJsonValue>
+
+#define API_VERSION 0
+#define CHANLIST_FORMAT 0
+
+UpdateChecker::UpdateChecker()
+{
+	m_currentChannel = VERSION_CHANNEL;
+	m_channelListUrl = CHANLIST_URL;
+	m_updateChecking = false;
+	m_chanListLoading = false;
+	m_checkUpdateWaiting = false;
+	m_chanListLoaded = false;
+}
+
+QList<UpdateChecker::ChannelListEntry> UpdateChecker::getChannelList() const
+{
+	return m_channels;
+}
+
+bool UpdateChecker::hasChannels() const
+{
+	return m_channels.isEmpty();
+}
+
+void UpdateChecker::checkForUpdate()
+{
+	QLOG_DEBUG() << "Checking for updates.";
+
+	// If the channel list hasn't loaded yet, load it and defer checking for updates until later.
+	if (!m_chanListLoaded)
+	{
+		QLOG_DEBUG() << "Channel list isn't loaded yet. Loading channel list and deferring update check.";
+		m_checkUpdateWaiting = true;
+		updateChanList();
+		return;
+	}
+
+	if (m_updateChecking)
+	{
+		QLOG_DEBUG() << "Ignoring update check request. Already checking for updates.";
+		return;
+	}
+
+	m_updateChecking = true;
+
+	// Get the URL for the channel we're using.
+	// TODO: Allow user to select channels. For now, we'll just use the current channel.
+	QString updateChannel = m_currentChannel;
+
+	// Find the desired channel within the channel list and get its repo URL. If if cannot be found, error.
+	m_repoUrl = "";
+	for (ChannelListEntry entry : m_channels)
+	{
+		if (entry.id == updateChannel)
+			m_repoUrl = entry.url;
+	}
+
+	// If we didn't find our channel, error.
+	if (m_repoUrl.isEmpty())
+	{
+		emit updateCheckFailed();
+		return;
+	}
+
+	QUrl indexUrl = QUrl(m_repoUrl).resolved(QUrl("index.json"));
+
+	auto job = new NetJob("GoUpdate Repository Index");
+	job->addNetAction(ByteArrayDownload::make(indexUrl));
+	connect(job, SIGNAL(succeeded()), SLOT(updateCheckFinished()));
+	connect(job, SIGNAL(failed()), SLOT(updateCheckFailed()));
+	indexJob.reset(job);
+	job->start();
+}
+
+void UpdateChecker::updateCheckFinished()
+{
+	QLOG_DEBUG() << "Finished downloading repo index. Checking for new versions.";
+
+	QJsonParseError jsonError;
+	QByteArray data;
+	{
+		ByteArrayDownloadPtr dl = std::dynamic_pointer_cast<ByteArrayDownload>(indexJob->first());
+		data = dl->m_data;
+		indexJob.reset();
+	}
+
+	QJsonDocument jsonDoc = QJsonDocument::fromJson(data, &jsonError);
+	if (jsonError.error != QJsonParseError::NoError || !jsonDoc.isObject())
+	{
+		QLOG_ERROR() << "Failed to parse GoUpdate repository index. JSON error" << jsonError.errorString() << "at offset" << jsonError.offset;
+		return;
+	}
+
+	QJsonObject object = jsonDoc.object();
+
+	bool success = false;
+	int apiVersion = object.value("ApiVersion").toVariant().toInt(&success);
+	if (apiVersion != API_VERSION || !success)
+	{
+		QLOG_ERROR() << "Failed to check for updates. API version mismatch. We're using" << API_VERSION << "server has" << apiVersion;
+		return;
+	}
+
+	QLOG_DEBUG() << "Processing repository version list.";
+	QJsonObject newestVersion;
+	QJsonArray versions = object.value("Versions").toArray();
+	for (QJsonValue versionVal : versions)
+	{
+		QJsonObject version = versionVal.toObject();
+		if (newestVersion.value("Id").toVariant().toInt() < version.value("Id").toVariant().toInt())
+		{
+			QLOG_DEBUG() << "Found newer version with ID" << version.value("Id").toVariant().toInt();
+			newestVersion = version;
+		}
+	}
+
+	// We've got the version with the greatest ID number. Now compare it to our current build number and update if they're different.
+	int newBuildNumber = newestVersion.value("Id").toVariant().toInt();
+	if (newBuildNumber != MMC->version().build)
+	{
+		// Update!
+		emit updateAvailable(m_repoUrl, newestVersion.value("Name").toVariant().toString(), newBuildNumber);
+	}
+
+	m_updateChecking = false;
+}
+
+void UpdateChecker::updateCheckFailed()
+{
+	// TODO: log errors better
+	QLOG_ERROR() << "Update check failed for reasons unknown.";
+}
+
+void UpdateChecker::updateChanList()
+{
+	QLOG_DEBUG() << "Loading the channel list.";
+
+	if (m_channelListUrl.isEmpty())
+	{
+		QLOG_ERROR() << "Failed to update channel list. No channel list URL set."
+					 << "If you'd like to use MultiMC's update system, please pass the channel list URL to CMake at compile time.";
+		return;
+	}
+
+	m_chanListLoading = true;
+	NetJob* job = new NetJob("Update System Channel List");
+	job->addNetAction(ByteArrayDownload::make(QUrl(m_channelListUrl)));
+	QObject::connect(job, &NetJob::succeeded, this, &UpdateChecker::chanListDownloadFinished);
+	QObject::connect(job, &NetJob::failed, this, &UpdateChecker::chanListDownloadFailed);
+	chanListJob.reset(job);
+	job->start();
+}
+
+void UpdateChecker::chanListDownloadFinished()
+{
+	QByteArray data;
+	{
+		ByteArrayDownloadPtr dl = std::dynamic_pointer_cast<ByteArrayDownload>(chanListJob->first());
+		data = dl->m_data;
+		chanListJob.reset();
+	}
+
+	QJsonParseError jsonError;
+	QJsonDocument jsonDoc = QJsonDocument::fromJson(data, &jsonError);
+	if (jsonError.error != QJsonParseError::NoError)
+	{
+		// TODO: Report errors to the user.
+		QLOG_ERROR() << "Failed to parse channel list JSON:" << jsonError.errorString() << "at" << jsonError.offset;
+		return;
+	}
+	
+	QJsonObject object = jsonDoc.object();
+
+	bool success = false;
+	int formatVersion = object.value("format_version").toVariant().toInt(&success);
+	if (formatVersion != CHANLIST_FORMAT || !success)
+	{
+		QLOG_ERROR() << "Failed to check for updates. Channel list format version mismatch. We're using" << CHANLIST_FORMAT << "server has" << formatVersion;
+		return;
+	}
+
+	// Load channels into a temporary array.
+	QList<ChannelListEntry> loadedChannels;
+	QJsonArray channelArray = object.value("channels").toArray();
+	for (QJsonValue chanVal : channelArray)
+	{
+		QJsonObject channelObj = chanVal.toObject();
+		ChannelListEntry entry{
+				channelObj.value("id").toVariant().toString(),
+				channelObj.value("name").toVariant().toString(),
+				channelObj.value("description").toVariant().toString(),
+				channelObj.value("url").toVariant().toString()
+			};
+		if (entry.id.isEmpty() || entry.name.isEmpty() || entry.url.isEmpty())
+		{
+			QLOG_ERROR() << "Channel list entry with empty ID, name, or URL. Skipping.";
+			continue;
+		}
+		loadedChannels.append(entry);
+	}
+
+	// Swap  the channel list we just loaded into the object's channel list.
+	m_channels.swap(loadedChannels);
+
+	m_chanListLoading = false;
+	m_chanListLoaded = true;
+	QLOG_INFO() << "Successfully loaded UpdateChecker channel list.";
+
+	// If we're waiting to check for updates, do that now.
+	if (m_checkUpdateWaiting)
+		checkForUpdate();
+
+	emit channelListLoaded();
+}
+
+void UpdateChecker::chanListDownloadFailed()
+{
+	m_chanListLoading = false;
+	QLOG_ERROR() << "Failed to download channel list.";
+	emit channelListLoaded();
+}
+
diff --git a/logic/updater/UpdateChecker.h b/logic/updater/UpdateChecker.h
new file mode 100644
index 00000000..59fb8e47
--- /dev/null
+++ b/logic/updater/UpdateChecker.h
@@ -0,0 +1,106 @@
+/* Copyright 2013 MultiMC Contributors
+ *
+ * Licensed under the Apache License, Version 2.0 (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.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include "logic/net/NetJob.h"
+
+#include <QUrl>
+
+class UpdateChecker : public QObject
+{
+	Q_OBJECT
+
+public:
+	UpdateChecker();
+	void checkForUpdate();
+
+	/*!
+	 * Causes the update checker to download the channel list from the URL specified in config.h (generated by CMake).
+	 * If this isn't called before checkForUpdate(), it will automatically be called.
+	 */
+	void updateChanList();
+
+	/*!
+	 * An entry in the channel list.
+	 */
+	struct ChannelListEntry
+	{
+		QString id;
+		QString name;
+		QString description;
+		QString url;
+	};
+
+	/*!
+	 * Returns a the current channel list.
+	 * If the channel list hasn't been loaded, this list will be empty.
+	 */
+	QList<ChannelListEntry> getChannelList() const;
+
+	/*!
+	 * Returns true if the channel list is empty.
+	 */
+	bool hasChannels() const;
+
+signals:
+	//! Signal emitted when an update is available. Passes the URL for the repo and the ID and name for the version.
+	void updateAvailable(QString repoUrl, QString versionName, int versionId);
+
+	//! Signal emitted when the channel list finishes loading or fails to load.
+	void channelListLoaded();
+
+private slots:
+	void updateCheckFinished();
+	void updateCheckFailed();
+
+	void chanListDownloadFinished();
+	void chanListDownloadFailed();
+
+private:
+	NetJobPtr indexJob;
+	NetJobPtr chanListJob;
+
+	QString m_repoUrl;
+
+	QString m_channelListUrl;
+	QString m_currentChannel;
+
+	QList<ChannelListEntry> m_channels;
+
+	/*!
+	 * True while the system is checking for updates.
+	 * If checkForUpdate is called while this is true, it will be ignored.
+	 */
+	bool m_updateChecking;
+
+	/*!
+	 * True if the channel list has loaded.
+	 * If this is false, trying to check for updates will call updateChanList first.
+	 */
+	bool m_chanListLoaded;
+
+	/*!
+	 * Set to true while the channel list is currently loading.
+	 */
+	bool m_chanListLoading;
+
+	/*!
+	 * Set to true when checkForUpdate is called while the channel list isn't loaded.
+	 * When the channel list finishes loading, if this is true, the update checker will check for updates.
+	 */
+	bool m_checkUpdateWaiting;
+};
+