Wrap database update in an explicit transaction

All changes to the database in SQLite must be done within a
transaction, so if there isn't one in progress one will be started
implicitly, i.e. each change will be done in a separate transaction.

Since committing a transaction is a fairly expensive operation - on
some filesystems ridiculously expensive - it should be done as
sparingly as possible.

In tests on my Galaxy S, this change makes the update between 2500%
and 4500% faster (for slightly over 100 applications).

src/org/fdroid/fdroid/DB.java

@@ -491,6 +491,10 @@ public class DB {
         updateApps = getApps(null, null, true);
         Log.d("FDroid", "AppUpdate: " + updateApps.size()
                 + " apps before starting.");
+        // Wrap the whole update in a transaction. Make sure to call
+        // either endUpdate or cancelUpdate to commit or discard it,
+        // respectively.
+        db.beginTransaction();
     }
 
     // Called when a repo update ends. Any applications that have not been
@@ -521,12 +525,20 @@ public class DB {
                 }
             }
         }
+        // Commit updates to the database.
+        db.setTransactionSuccessful();
+        db.endTransaction();
         Log.d("FDroid", "AppUpdate: " + updateApps.size()
                 + " apps on completion.");
         updateApps = null;
         return;
     }
 
+    // Called instead of endUpdate if the update failed.
+    public void cancelUpdate() {
+        db.endTransaction();
+    }
+
     // Called during update to supply new details for an application (or
     // details of a completely new one). Calls to this must be wrapped by
     // a call to beginUpdate and a call to endUpdate.

src/org/fdroid/fdroid/RepoXMLHandler.java

@@ -223,6 +223,7 @@ public class RepoXMLHandler extends DefaultHandler {
 
     public static boolean doUpdates(Context ctx, DB db) {
         long startTime = System.currentTimeMillis();
+        boolean success = true;
         db.beginUpdate();
         Vector<DB.Repo> repos = db.getRepos();
         for (DB.Repo repo : repos) {
@@ -262,12 +263,12 @@ public class RepoXMLHandler extends DefaultHandler {
                         jar.close();
                         if (certs == null) {
                             Log.d("FDroid", "No signature found in index");
-                            return false;
+                            return success = false;
                         }
                         if (certs.length != 1) {
                             Log.d("FDroid", "Expected one signature - found "
                                     + certs.length);
-                            return false;
+                            return success = false;
                         }
 
                         byte[] sig = certs[0].getEncoded();
@@ -285,7 +286,7 @@ public class RepoXMLHandler extends DefaultHandler {
 
                         if (!ssig.equals(repo.pubkey)) {
                             Log.d("FDroid", "Index signature mismatch");
-                            return false;
+                            return success = false;
                         }
 
                     } else {
@@ -324,10 +325,12 @@ public class RepoXMLHandler extends DefaultHandler {
                 } catch (Exception e) {
                     Log.e("FDroid", "Exception updating from " + repo.address
                             + ":\n" + Log.getStackTraceString(e));
-                    return false;
+                    return success = false;
                 } finally {
                     ctx.deleteFile("tempindex.xml");
                     ctx.deleteFile("tempindex.jar");
+                    if (!success)
+                        db.cancelUpdate();
                 }
 
             }