cloudstorage: try to pick between multiple cloud servers

The backend infrastructure will soon be able to support more than one
cloud server which automagically stay in sync with each other.

One critical requirement for that to work is that once a session was
started with one of the servers, the complete session happens with that
server - we must not switch from server to server while doing a git
transaction. To make sure that's the case, we aren't trying to use DNS
tricks to make this load balancing scheme work, but instead try to
determine at program start which server is the best one to use.

Right now this is super simplistic. Two servers, one in the US, one in
Europe. By default we use the European server (most of our users appear
to be in Europe), but if we can figure out that the client is actually
in the Americas, use the US server. We might improve that heuristic over
time, but as a first attempt it seems not entirely bogus.

The way this is implemented is a simple combination of two free
webservices that together appear to give us a very reliable estimate
which continent the user is located on.

api.ipify.org gives us our external IP address
ip-api.com gives us the continent that IP address is on

If any of this fails or takes too long to respond, we simply ignore it
since either server will work. One oddity is that if we decide to change
servers we only change the settings that are stored on disk, not the
runtime preferences. This goes back to the comment above that we have to
avoid changing servers in mid sync.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

1 files changed, 77 insertions, 0 deletions

diff --git a/core/checkcloudconnection.cpp b/core/checkcloudconnection.cpp
index b9a7d3b0e..714b62836 100644
--- a/core/checkcloudconnection.cpp
+++ b/core/checkcloudconnection.cpp
@@ -4,11 +4,14 @@
 #include <QNetworkAccessManager>
 #include <QNetworkReply>
 #include <QEventLoop>
+#include <QHostAddress>
 
 #include "pref.h"
 #include "qthelper.h"
 #include "git-access.h"
 #include "errorhelper.h"
+#include "core/subsurface-string.h"
+#include "core/settings/qPrefCloudStorage.h"
 
 #include "checkcloudconnection.h"
 
@@ -19,6 +22,10 @@ CheckCloudConnection::CheckCloudConnection(QObject *parent) :
 
 }
 
+// two free APIs to figure out where we are
+#define GET_EXTERNAL_IP_API "http://api.ipify.org"
+#define GET_CONTINENT_API "http://ip-api.com/line/%1?fields=continent"
+
 // our own madeup API to make sure we are talking to a Subsurface cloud server
 #define TEAPOT "/make-latte?number-of-shots=3"
 #define HTTP_I_AM_A_TEAPOT 418
@@ -84,6 +91,76 @@ void CheckCloudConnection::sslErrors(const QList<QSslError> &errorList)
 		qDebug() << err.errorString();
 }
 
+void CheckCloudConnection::pickServer()
+{
+	QNetworkRequest request(QString(GET_EXTERNAL_IP_API));
+	request.setRawHeader("Accept", "text/plain");
+	request.setRawHeader("User-Agent", getUserAgent().toUtf8());
+	QNetworkAccessManager *mgr = new QNetworkAccessManager();
+	connect(mgr, &QNetworkAccessManager::finished, this, &CheckCloudConnection::gotIP);
+	mgr->get(request);
+}
+
+void CheckCloudConnection::gotIP(QNetworkReply *reply)
+{
+	if (reply->error() != QNetworkReply::NoError) {
+		// whatever, just use the default host
+		if (verbose)
+			qDebug() << __FUNCTION__ << "got error reply from ip webservice - not changing cloud host";
+		return;
+	}
+	QString addressString = reply->readAll();
+	// use the QHostAddress constructor as a convenient way to validate that this is indeed an IP address
+	// but then don't do annything with the QHostAdress - we need the address string...
+	QHostAddress addr(addressString);
+	if (addr.isNull()) {
+		// this isn't an address, don't try to update the cloud host
+		if (verbose)
+			qDebug() << __FUNCTION__ << "returned address doesn't appear to be valid (" << addressString << ") - not changing cloud host";
+		return;
+	}
+	if (verbose)
+		qDebug() << "IP used for cloud server access" << addressString;
+	// now figure out which continent we are on
+	QNetworkRequest request(QString(GET_CONTINENT_API).arg(addressString));
+	request.setRawHeader("Accept", "text/plain");
+	request.setRawHeader("User-Agent", getUserAgent().toUtf8());
+	QNetworkAccessManager *mgr = new QNetworkAccessManager();
+	connect(mgr, &QNetworkAccessManager::finished, this, &CheckCloudConnection::gotContinent);
+	mgr->get(request);
+}
+
+void CheckCloudConnection::gotContinent(QNetworkReply *reply)
+{
+	if (reply->error() != QNetworkReply::NoError) {
+		// whatever, just use the default host
+		if (verbose)
+			qDebug() << __FUNCTION__ << "got error reply from ip location webservice - not changing cloud host";
+		return;
+	}
+	QString continentString = reply->readAll();
+	// in most cases this response comes back too late for us - we may already have
+	// started to talk to the cloud server (this certinaly seems to be the case when
+	// we use the cloud storage as default file). So instead of potentially changing
+	// the server that is used in mid connection, let's just update what's stored in
+	// our settings so the next time we'll use the server that's closer.
+
+	// of course, right now the logic for that is very simplistic. Use the US server
+	// when in the Americas, the EU server otherwise. This may need a better algorithm
+	// at some point, but for now it seems good enough
+
+	const char *base_url;
+	if (continentString.contains("America", Qt::CaseInsensitive))
+		base_url = "https://" CLOUD_HOST_US "/";
+	else
+		base_url = "https://" CLOUD_HOST_EU "/";
+	if (!same_string(base_url, prefs.cloud_base_url)) {
+		if (verbose)
+			qDebug() << "remember cloud server" << base_url << "based on IP location in " << continentString;
+		qPrefCloudStorage::instance()->store_cloud_base_url(base_url);
+	}
+}
+
 // helper to be used from C code
 extern "C" bool canReachCloudServer()
 {