diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -1306,6 +1306,7 @@
     'HarbormasterLeaseWorkingCopyBuildStepImplementation' => 'applications/harbormaster/step/HarbormasterLeaseWorkingCopyBuildStepImplementation.php',
     'HarbormasterLintMessagesController' => 'applications/harbormaster/controller/HarbormasterLintMessagesController.php',
     'HarbormasterLintPropertyView' => 'applications/harbormaster/view/HarbormasterLintPropertyView.php',
+    'HarbormasterLogWorker' => 'applications/harbormaster/worker/HarbormasterLogWorker.php',
     'HarbormasterManagementArchiveLogsWorkflow' => 'applications/harbormaster/management/HarbormasterManagementArchiveLogsWorkflow.php',
     'HarbormasterManagementBuildWorkflow' => 'applications/harbormaster/management/HarbormasterManagementBuildWorkflow.php',
     'HarbormasterManagementRestartWorkflow' => 'applications/harbormaster/management/HarbormasterManagementRestartWorkflow.php',
@@ -6610,6 +6611,7 @@
     'HarbormasterLeaseWorkingCopyBuildStepImplementation' => 'HarbormasterBuildStepImplementation',
     'HarbormasterLintMessagesController' => 'HarbormasterController',
     'HarbormasterLintPropertyView' => 'AphrontView',
+    'HarbormasterLogWorker' => 'HarbormasterWorker',
     'HarbormasterManagementArchiveLogsWorkflow' => 'HarbormasterManagementWorkflow',
     'HarbormasterManagementBuildWorkflow' => 'HarbormasterManagementWorkflow',
     'HarbormasterManagementRestartWorkflow' => 'HarbormasterManagementWorkflow',
diff --git a/src/applications/harbormaster/management/HarbormasterManagementWriteLogWorkflow.php b/src/applications/harbormaster/management/HarbormasterManagementWriteLogWorkflow.php
--- a/src/applications/harbormaster/management/HarbormasterManagementWriteLogWorkflow.php
+++ b/src/applications/harbormaster/management/HarbormasterManagementWriteLogWorkflow.php
@@ -39,7 +39,6 @@
           $target_id));
     }
 
-
     $log = HarbormasterBuildLog::initializeNewBuildLog($target);
     $log->openBuildLog();
 
@@ -50,6 +49,12 @@
     $content = file_get_contents('php://stdin');
     $log->append($content);
 
+    echo tsprintf(
+      "%s\n",
+      pht('Write completed. Closing log...'));
+
+    PhabricatorWorker::setRunAllTasksInProcess(true);
+
     $log->closeBuildLog();
 
     echo tsprintf(
diff --git a/src/applications/harbormaster/storage/build/HarbormasterBuildLog.php b/src/applications/harbormaster/storage/build/HarbormasterBuildLog.php
--- a/src/applications/harbormaster/storage/build/HarbormasterBuildLog.php
+++ b/src/applications/harbormaster/storage/build/HarbormasterBuildLog.php
@@ -52,17 +52,24 @@
       throw new Exception(pht('This build log is not open!'));
     }
 
-    if ($this->canCompressLog()) {
-      $this->compressLog();
-    }
-
     $start = $this->getDateCreated();
     $now = PhabricatorTime::getNow();
 
-    return $this
+    $this
       ->setDuration($now - $start)
       ->setLive(0)
       ->save();
+
+    PhabricatorWorker::scheduleTask(
+      'HarbormasterLogWorker',
+      array(
+        'logPHID' => $this->getPHID(),
+      ),
+      array(
+        'objectPHID' => $this->getPHID(),
+      ));
+
+    return $this;
   }
 
 
@@ -201,7 +208,7 @@
     return implode('', $full_text);
   }
 
-  private function canCompressLog() {
+  public function canCompressLog() {
     return function_exists('gzdeflate');
   }
 
diff --git a/src/applications/harbormaster/worker/HarbormasterLogWorker.php b/src/applications/harbormaster/worker/HarbormasterLogWorker.php
new file mode 100644
--- /dev/null
+++ b/src/applications/harbormaster/worker/HarbormasterLogWorker.php
@@ -0,0 +1,50 @@
+<?php
+
+final class HarbormasterLogWorker extends HarbormasterWorker {
+
+  protected function doWork() {
+    $viewer = $this->getViewer();
+
+    $data = $this->getTaskData();
+    $log_phid = idx($data, 'logPHID');
+
+    $log = id(new HarbormasterBuildLogQuery())
+      ->setViewer($viewer)
+      ->withPHIDs(array($log_phid))
+      ->executeOne();
+    if (!$log) {
+      throw new PhabricatorWorkerPermanentFailureException(
+        pht('Invalid build log PHID "%s".', $log_phid));
+    }
+
+    $phid_key = PhabricatorHash::digestToLength($log_phid, 14);
+    $lock_key = "build.log({$phid_key})";
+    $lock = PhabricatorGlobalLock::newLock($lock_key);
+
+    try {
+      $lock->lock();
+    } catch (PhutilLockException $ex) {
+      throw new PhabricatorWorkerYieldException(15);
+    }
+
+    $caught = null;
+    try {
+      $this->finalizeBuildLog($log);
+    } catch (Exception $ex) {
+      $caught = $ex;
+    }
+
+    $lock->unlock();
+
+    if ($caught) {
+      throw $caught;
+    }
+  }
+
+  private function finalizeBuildLog(HarbormasterBuildLog $log) {
+    if ($log->canCompressLog()) {
+      $log->compressLog();
+    }
+  }
+
+}