dbhub: Ask user what to do when trying to open an updated database

If there is a local clone of a remote database but the remote database
has been updated to a newer version in the meantime, we're now asking
the user what to do. The choice is between downloading the newer version
and overwriting the local one with all the changes it might have
contained or opening the local version and not downloading anything.

See issue #1096.

src/RemoteDatabase.cpp

@@ -619,17 +619,30 @@ QString RemoteDatabase::localExists(const QUrl& url, QString identity)
     // is newer, or the local commit id is newer.
     if(local_commit_id == url_commit_id)
     {
-        // Both commit ids are the same. That's the perfect match, so we can build the full path to where the file should be
-        QString full_path = Settings::getValue("remote", "clonedirectory").toString() + "/" + local_file;
+        // Both commit ids are the same. That's the perfect match, so we can download the local file if it still exists
+        return localCheckFile(local_file);
+    } else {
+        // The commit ids differ. That means we have another version locally checked out than we're trying to download. Because the commit ids are
+        // only calculated on the server side and we're currently always checking out the latest version this can only mean that the remote version has
+        // been updated, i.e. is newer than the local version.
 
-        // Check if the database still exists. If so return its path, if not return an empty string to redownload it
-        if(QFile::exists(full_path))
+        // TODO Support multiple checkouts of the same database at different versions at the same time. For this we need to be more intelligent with
+        // comparing the commit ids.
+
+        // Ask the user what to do: open the local version or updating to the new remote version
+        if(QMessageBox::question(nullptr, qApp->applicationName(),
+                                 tr("The remote database has been updated since the last checkout. Do you want to update the local database to the newest version? Note "
+                                    "that this discards any changes you have made locally! If you don't want to lose local changes, click No to open the local version."),
+                                 QMessageBox::Yes | QMessageBox::No, QMessageBox::No) == QMessageBox::Yes)
         {
-            return full_path;
-        } else {
-            // Remove the apparently invalid entry from the local clones database to avoid future lookups and confusions. The file column
-            // should be unique for the entire table because the files are all in the same directory and their names need to be unique because
-            // of this.
+            // User wants to download the newest version. So delete the entry from the clones database and delete the local database copy and return an empty
+            // string to indicate a redownload request.
+
+            // Build full path to database file and delete it
+            QFile::remove(Settings::getValue("remote", "clonedirectory").toString() + "/" + local_file);
+
+            // Remove the old entry from the local clones database to enforce a redownload. The file column should be unique for the entire table because the
+            // files are all in the same directory and their names need to be unique because of this.
             QString sql = QString("DELETE FROM local WHERE file=?");
             sqlite3_stmt* stmt;
             if(sqlite3_prepare_v2(m_dbLocal, sql.toUtf8(), -1, &stmt, 0) != SQLITE_OK)
@@ -642,16 +655,43 @@ QString RemoteDatabase::localExists(const QUrl& url, QString identity)
             sqlite3_step(stmt);
             sqlite3_finalize(stmt);
 
-            // Return empty string to indicate a redownload request
+            // Return an empty string to indicate a redownload request
+            return QString();
+        } else {
+            // User wants to open the local version. So build the full path and return it if the file still exists.
+            return localCheckFile(local_file);
+        }
+    }
+}
+
+QString RemoteDatabase::localCheckFile(const QString& local_file)
+{
+    // This function takes the file name of a locally cloned database and checks if this file still exists. If it has been deleted in the meantime it returns
+    // an empty string and deletes the file from the clone database. If the file still exists, it returns the full path to the file.
+
+    // Build the full path to where the file should be
+    QString full_path = Settings::getValue("remote", "clonedirectory").toString() + "/" + local_file;
+
+    // Check if the database still exists. If so return its path, if not return an empty string to redownload it
+    if(QFile::exists(full_path))
+    {
+        return full_path;
+    } else {
+        // Remove the apparently invalid entry from the local clones database to avoid future lookups and confusions. The file column should
+        // be unique for the entire table because the files are all in the same directory and their names need to be unique because of this.
+        QString sql = QString("DELETE FROM local WHERE file=?");
+        sqlite3_stmt* stmt;
+        if(sqlite3_prepare_v2(m_dbLocal, sql.toUtf8(), -1, &stmt, 0) != SQLITE_OK)
+            return QString();
+        if(sqlite3_bind_text(stmt, 1, local_file.toUtf8(), local_file.toUtf8().length(), SQLITE_TRANSIENT))
+        {
+            sqlite3_finalize(stmt);
             return QString();
         }
-    } else {
-        // In all the other cases just treat the remote database as a completely new database for now.
-
-        // TODO Add some way to update the local clone here. Maybe ask the user what to do because I don't really know what the
-        // most sensible way to go is in the two remaining cases. We can use the local_id variable (see above) to update the
-        // record afterwards.
+        sqlite3_step(stmt);
+        sqlite3_finalize(stmt);
 
+        // Return empty string to indicate a redownload request
         return QString();
     }
 }

src/RemoteDatabase.h

@@ -73,6 +73,7 @@ private:
     void localAssureOpened();
     void localAdd(QString filename, QString identity, const QUrl& url);
     QString localExists(const QUrl& url, QString identity);
+    QString localCheckFile(const QString& local_file);
 
     // Helper functions for building multi-part HTTP requests
     void addPart(QHttpMultiPart* multipart, const QString& name, const QString& value);