[win] Make conflict module enumeration occur at most once.

chrome/browser/win/enumerate_modules_model.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/win/enumerate_modules_model.h"
 
 #include <softpub.h>
 #include <stddef.h>
 #include <stdint.h>
 #include <tlhelp32.h>
@@ skipping 370 matching lines @@
   BrowserThread::GetBlockingPool()->PostWorkerTaskWithShutdownBehavior(
       FROM_HERE,
       base::Bind(&ModuleEnumerator::ScanImpl,
                  base::Unretained(this)),
       base::SequencedWorkerPool::CONTINUE_ON_SHUTDOWN);
 }
 
 void ModuleEnumerator::ScanImpl() {
   base::TimeTicks start_time = base::TimeTicks::Now();
 
-  enumerated_modules_->clear();
+  // The provided destination for the enumerated modules should be empty, as it
+  // should only be populated once, by a single ModuleEnumerator instance. See
+  // EnumerateModulesModel::ScanNow for details.
+  DCHECK(enumerated_modules_->empty());
 
   // Make sure the path mapping vector is setup so we can collapse paths.
   PreparePathMappings();
 
   // Enumerating loaded modules must happen first since the other types of
   // modules check for duplication against the loaded modules.
   base::TimeTicks checkpoint = base::TimeTicks::Now();
   EnumerateLoadedModules();
   base::TimeTicks checkpoint2 = base::TimeTicks::Now();
   UMA_HISTOGRAM_TIMES("Conflicts.EnumerateLoadedModules",
@@ skipping 315 matching lines @@
         FROM_HERE,
         BrowserThread::GetTaskRunnerForThread(BrowserThread::UI),
         base::Bind(&EnumerateModulesModel::ScanNow, base::Unretained(this)));
     done = true;
   }
 }
 
 void EnumerateModulesModel::ScanNow() {
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
 
-  // If a module enumerator exists then a scan is already underway.
+  // |module_enumerator_| is used as a lock to know whether or not there are
+  // active/pending blocking pool tasks. If a module enumerator exists then a
+  // scan is already underway. Otherwise, either no scan has been completed or
+  // a scan has terminated.
   if (module_enumerator_)
     return;
 
+  // Only allow a single scan per process lifetime. Immediately notify any
+  // observers that the scan is complete. At this point |enumerated_modules_| is
+  // safe to access as no potentially racing blocking pool task can exist.
+  if (!enumerated_modules_.empty()) {
+    FOR_EACH_OBSERVER(Observer, observers_, OnScanCompleted());
+    return;
+  }
+
   // ScanNow does not block, rather it simply schedules a task.
   module_enumerator_.reset(new ModuleEnumerator(this));
   module_enumerator_->ScanNow(&enumerated_modules_);
 }
 
 base::ListValue* EnumerateModulesModel::GetModuleList() {
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
 
   // If a |module_enumerator_| is still around then scanning has not yet
   // completed, and it is unsafe to read from |enumerated_modules_|.
@@ skipping 120 matching lines @@
   module_enumerator_.reset();
 
   UMA_HISTOGRAM_COUNTS_100("Conflicts.SuspectedBadModules",
                            suspected_bad_modules_detected_);
   UMA_HISTOGRAM_COUNTS_100("Conflicts.ConfirmedBadModules",
                            confirmed_bad_modules_detected_);
 
   // Forward the callback to any registered observers.
   FOR_EACH_OBSERVER(Observer, observers_, OnScanCompleted());
 }