sftp: only rebuild when app task queue is empty

when multiple apptasks are scheduled, we end up with a sequence like this:
    - task1 finishes
    - task2 (uninstall) removes  appdata directory
    - sftp rebuild (from task1 finish)
    - task2 fails because sftp rebuild created empty appdata directory

a fix is to delay the sftp rebuild until all tasks are done. of course,
the same race is still there, if a user initiated another task immediately
but this seems unlikely. if that happens often, we can further add a sftpRebuildInProgress
flag inside apptaskmanager.

src/sftp.js

@@ -21,24 +21,11 @@ var apps = require('./apps.js'),
     volumes = require('./volumes.js'),
     _ = require('underscore');
 
-var gRebuildInProgress = false;
 function rebuild(serviceConfig, options, callback) {
     assert.strictEqual(typeof serviceConfig, 'object');
     assert.strictEqual(typeof options, 'object');
     assert.strictEqual(typeof callback, 'function');
 
-    if (gRebuildInProgress) {
-        debug('waiting for other rebuild to finish');
-        return setTimeout(function () { rebuild(serviceConfig, options, callback); }, 5000);
-    }
-
-    gRebuildInProgress = true;
-
-    function done(error) {
-        gRebuildInProgress = false;
-        callback(error);
-    }
-
     debug('rebuilding container');
 
     const force = !!options.force;
@@ -48,7 +35,7 @@ function rebuild(serviceConfig, options, callback) {
     const cloudronToken = hat(8 * 128);
 
     apps.getAll(function (error, result) {
-        if (error) return done(error);
+        if (error) return callback(error);
 
         let dataDirs = [];
         result.forEach(function (app) {
@@ -89,7 +76,7 @@ function rebuild(serviceConfig, options, callback) {
 
                         if (!force && _.isEqual(currentDataDirs, dataDirs)) {
                             debug('Skipping rebuild, no changes');
-                            return done();
+                            return callback();
                         }
                     }
                 }
@@ -119,7 +106,7 @@ function rebuild(serviceConfig, options, callback) {
                     shell.exec.bind(null, 'stopSftp', 'docker stop sftp || true'),
                     shell.exec.bind(null, 'removeSftp', 'docker rm -f sftp || true'),
                     shell.exec.bind(null, 'startSftp', cmd)
-                ], done);
+                ], callback);
             });
         });
     });