Fix DCHECK-only race condition in ProxyResolverV8TracingImpl

Bug: 699562
Change-Id: I9c818a0c0c490b2d32929a9867d5d3bebf6e8ca9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1618016
Auto-Submit: David Benjamin <davidben@chromium.org>
Reviewed-by: Eric Roman <eroman@chromium.org>
Commit-Queue: David Benjamin <davidben@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661014}

build/sanitizers/tsan_suppressions.cc

@@ -183,10 +183,6 @@ char kTSanDefaultSuppressions[] =
     // http://crbug.com/633145
     "race:third_party/libjpeg_turbo/simd/jsimd_x86_64.c\n"
 
-    // http://crbug.com/v8/6065
-    "race:net::(anonymous namespace)::ProxyResolverV8TracingImpl::RequestImpl"
-    "::~RequestImpl()\n"
-
     // http://crbug.com/691029
     "deadlock:libGLX.so*\n"
 

net/proxy_resolution/proxy_resolver_v8_tracing.cc

@@ -406,7 +406,12 @@ void Job::Cancel() {
 
   ReleaseCallback();
 
-  pending_dns_.reset();
+  // Note we only mutate |pending_dns_| if it is non-null. If it is null, the
+  // worker thread may be about to request a new DNS resolution. This avoids a
+  // race condition with the DCHECK in PostDnsOperationAndWait().
+  // See https://crbug.com/699562.
+  if (pending_dns_)
+    pending_dns_.reset();
 
   // The worker thread might be blocked waiting for DNS.
   event_.Signal();
@@ -695,7 +700,9 @@ bool Job::ResolveDnsNonBlocking(const std::string& host,
 bool Job::PostDnsOperationAndWait(const std::string& host,
                                   ProxyResolveDnsOperation op,
                                   bool* completed_synchronously) {
-  // Post the DNS request to the origin thread.
+  // Post the DNS request to the origin thread. It is safe to mutate
+  // |pending_dns_host_| and |pending_dns_op_| because there cannot be another
+  // DNS operation in progress or scheduled.
   DCHECK(!pending_dns_);
   pending_dns_host_ = host;
   pending_dns_op_ = op;