Allow CertNetFetcher to be shutdown from the network thread

net/cert/cert_net_fetcher.h

Index: net/cert/cert_net_fetcher.h
diff --git a/net/cert/cert_net_fetcher.h b/net/cert/cert_net_fetcher.h
index 56a74948117db4f6ae95432dd9a721967a2cc0f3..f6eff1220ee30c59adf9b4a9cc88bfd4ad88cecd 100644
--- a/net/cert/cert_net_fetcher.h
+++ b/net/cert/cert_net_fetcher.h
@@ -20,16 +20,15 @@ class GURL;
 namespace net {
 
 // CertNetFetcher is a synchronous interface for fetching AIA URLs and CRL
-// URLs.
+// URLs. It is shared between a caller thread (which starts and waits for
+// fetches), and a network thread (which does the actual fetches). It can be
+// shutdown from the network thread to cancel outstanding requests.
 //
 // A Request object is returned when starting a fetch. The consumer can
 // use this as a handle for aborting the request (by freeing it), or reading
 // the result of the request (WaitForResult)
-//
-// This interface is expected to be operated from a single thread.
-//
-// The CertNetFetcher must outlive all Request objects it creates.
-class NET_EXPORT CertNetFetcher {
+class NET_EXPORT CertNetFetcher
+    : public base::RefCountedThreadSafe<CertNetFetcher> {
  public:
   class Request {
    public:
@@ -47,7 +46,10 @@ class NET_EXPORT CertNetFetcher {
 
   CertNetFetcher() {}
 
-  virtual ~CertNetFetcher() {}
+  // Shuts down the CertNetFetcher and cancels outstanding network
+  // requests. It is not guaranteed that any outstanding or subsequent
+  // Request::WaitForResult() calls will be completed.

[eroman, 2017/01/11 01:56:59]

Could you add some comments that answer the questions:

 * What thread(s) can this be called on
 * Is it safe to call more than once?
 * Does it need to be called before destruction of CertNetFetcher, or does
destruction implicitly Shutdown?

Some of this is duplication with the class level comments, but having it here is
useful.

[estark, 2017/01/12 01:06:30]

Done.

+  virtual void Shutdown() = 0;
 
   // The Fetch*() methods start a request which can be cancelled by
   // deleting the returned Request. Here is the meaning of the common
@@ -76,7 +78,11 @@ class NET_EXPORT CertNetFetcher {
       int timeout_milliseconds,
       int max_response_bytes) = 0;
 
+ protected:
+  virtual ~CertNetFetcher() {}
+
  private:
+  friend class base::RefCountedThreadSafe<CertNetFetcher>;
   DISALLOW_COPY_AND_ASSIGN(CertNetFetcher);
 };