Far more advanced poll scheduler. Now it remembers that finish time

of the last successful periodic mail and computes the next start time
as an offset from that successful finish.  The ramifications of this
new method is that changing polling interval on an account does not
force delaying all accounts to poll next in the future by the new
interval.  Instead, K-9 Mail now adjusts the next poll time based on
what the next poll time should be based on the last poll finish and
the new interval.  

Example 1: In the old way, if the old polling interval was 1 hour, and
the next poll was 50 minutes away (10 minutes have passed), and you
changed the interval to 15 minutes, the poll would happen 15 minutes
from now.  In the new way, the next poll will happen only 5 minutes
from now, which is 15 minutes since the last poll.

Example 2: In the old way, if the old polling interval was 1 hour, and
the next poll was 10 minutes away (50 minutes have passed), and you
changed the interval to 30 minutes, the poll would happen 30 minutes
from now.  The next poll would then happen actually 80 minutes after
the previous poll completed.  In the new way, it'll actually happen
immediately, because the time for the next poll, based on the new
schedule, has already passed.

Similar scenarios happen when a loss of network connectivity
occurs. In the old way, polling would resume using the restoration of
connectivity as the starting point.  Each time network connectivity
was lost and restored, the next poll would be further delayed.  *If
connectivity was lost and restored frequently, a poll might never
happen!* In the new way, the next poll is rescheduled based on the
time of the last successful poll, so will be rescheduled just like it
was before the loss of connectivity.  If the time has already been
passed, the poll will happen immediately.

src/com/fsck/k9/Account.java

@@ -765,15 +765,7 @@ public class Account implements Serializable
         int newInterval = automaticCheckIntervalMinutes;
         this.mAutomaticCheckIntervalMinutes = automaticCheckIntervalMinutes;
         
-        if (oldInterval == -1 && newInterval != -1)
-        {
-            return true;
-        }
-        if (newInterval < oldInterval)
-        {
-            return true;
-        }
-        return false;
+        return (oldInterval != newInterval);
     }
 
     /**

src/com/fsck/k9/service/MailService.java

@@ -9,6 +9,7 @@ import android.app.NotificationManager;
 import android.app.PendingIntent;
 import android.content.Context;
 import android.content.Intent;
+import android.content.SharedPreferences;
 import android.net.ConnectivityManager;
 import android.net.NetworkInfo;
 import android.net.NetworkInfo.State;
@@ -147,8 +148,7 @@ public class MailService extends CoreService
                 {
                     PollService.startService(this);
                 }
-
-                reschedule(hasConnectivity, doBackground, startIdObj);
+                reschedulePoll(hasConnectivity, doBackground, startIdObj, false);
                 startIdObj = null;
             }
             else if (ACTION_CANCEL.equals(intent.getAction()))
@@ -179,7 +179,7 @@ public class MailService extends CoreService
             {
                 if (K9.DEBUG)
                     Log.v(K9.LOG_TAG, "***** MailService *****: rescheduling poll");
-                reschedule(hasConnectivity, doBackground, startIdObj);
+                reschedulePoll(hasConnectivity, doBackground, startIdObj, true);
                 startIdObj = null;
 
             }
@@ -218,7 +218,7 @@ public class MailService extends CoreService
 
     private void rescheduleAll(final boolean hasConnectivity, final boolean doBackground, final Integer startId)
     {
-        reschedule(hasConnectivity, doBackground, null);
+        reschedulePoll(hasConnectivity, doBackground, null, true);
         reschedulePushers(hasConnectivity, doBackground, startId);
         
     }
@@ -271,7 +271,23 @@ public class MailService extends CoreService
         BootReceiver.cancelIntent(this, i);
     }
 
-    private void reschedule(final boolean hasConnectivity, final boolean doBackground, Integer startId)
+    private final static String PREVIOUS_INTERVAL = "MailService.previousInterval";
+    private final static String LAST_CHECK_END = "MailService.lastCheckEnd";
+    
+    public static void saveLastCheckEnd(Context context)
+    {
+        
+        long lastCheckEnd = System.currentTimeMillis();
+        if (K9.DEBUG)
+            Log.i(K9.LOG_TAG, "Saving lastCheckEnd = " + new Date(lastCheckEnd));
+        Preferences prefs = Preferences.getPreferences(context);
+        SharedPreferences sPrefs = prefs.getPreferences();
+        SharedPreferences.Editor editor = sPrefs.edit();
+        editor.putLong(LAST_CHECK_END, lastCheckEnd);
+        editor.commit();
+    }
+   
+    private void reschedulePoll(final boolean hasConnectivity, final boolean doBackground, Integer startId, final boolean considerLastCheckEnd)
     {
         if (hasConnectivity && doBackground)
         {
@@ -280,8 +296,12 @@ public class MailService extends CoreService
                 public void run()
                 {
                     int shortestInterval = -1;
-    
-                    for (Account account : Preferences.getPreferences(MailService.this).getAccounts())
+                    
+                    Preferences prefs = Preferences.getPreferences(MailService.this);
+                    SharedPreferences sPrefs = prefs.getPreferences();
+                    int previousInterval = sPrefs.getInt(PREVIOUS_INTERVAL, -1);
+                    long lastCheckEnd = sPrefs.getLong(LAST_CHECK_END, -1);
+                    for (Account account : prefs.getAccounts())
                     {
                         if (account.getAutomaticCheckIntervalMinutes() != -1
                                 && account.getFolderSyncMode() != FolderMode.NONE
@@ -290,7 +310,10 @@ public class MailService extends CoreService
                             shortestInterval = account.getAutomaticCheckIntervalMinutes();
                         }
                     }
-    
+                    SharedPreferences.Editor editor = sPrefs.edit();
+                    editor.putInt(PREVIOUS_INTERVAL, shortestInterval);
+                    editor.commit();
+                    
                     if (shortestInterval == -1)
                     {
                         nextCheck = -1;
@@ -301,8 +324,14 @@ public class MailService extends CoreService
                     else
                     {
                         long delay = (shortestInterval * (60 * 1000));
-    
-                        long nextTime = System.currentTimeMillis() + delay;
+                        long base = (previousInterval == -1 || lastCheckEnd == -1 || considerLastCheckEnd == false ? System.currentTimeMillis() : lastCheckEnd);
+                        long nextTime = base + delay;
+                        if (K9.DEBUG)
+                            Log.i(K9.LOG_TAG, 
+                                    "previousInterval = " + previousInterval 
+                                    + ", shortestInterval = " + shortestInterval
+                                    + ", lastCheckEnd = " + new Date(lastCheckEnd)
+                                    + ", considerLastCheckEnd = " + considerLastCheckEnd);
                         nextCheck = nextTime;
                         try
                         {

src/com/fsck/k9/service/PollService.java

@@ -2,6 +2,7 @@ package com.fsck.k9.service;
 
 import android.content.Context;
 import android.content.Intent;
+import android.content.SharedPreferences;
 import android.os.IBinder;
 import android.os.PowerManager;
 import android.os.PowerManager.WakeLock;
@@ -140,6 +141,8 @@ public class PollService extends CoreService
 
         private void release()
         {
+            MailService.saveLastCheckEnd(getApplication());
+            
             MessagingController controller = MessagingController.getInstance(getApplication());
             controller.setCheckMailListener(null);
             MailService.actionReschedulePoll(PollService.this, null);