Add a histogram (Net.PacResultForStrippedUrl) that estimates how often PAC scripts depend on the UR…

net/proxy/proxy_resolver_v8_tracing.h

Index: net/proxy/proxy_resolver_v8_tracing.h
diff --git a/net/proxy/proxy_resolver_v8_tracing.h b/net/proxy/proxy_resolver_v8_tracing.h
index f5f66cf7486797d0c6282f7dab361b2942cfa283..058b6b88ec55ce32ede0d47b3debf4a296cfbf49 100644
--- a/net/proxy/proxy_resolver_v8_tracing.h
+++ b/net/proxy/proxy_resolver_v8_tracing.h
@@ -89,6 +89,50 @@ class NET_EXPORT ProxyResolverV8TracingFactory {
   DISALLOW_COPY_AND_ASSIGN(ProxyResolverV8TracingFactory);
 };
 
+// TODO(eroman): Remove when done gathering data for crbug.com/593759
+enum class PacResultForStrippedUrl {
+  // Did NOT measure the impact of running FindProxyForURL() with a modified
+  // URL path, because the original URL could not complete using the
+  // non-blocking DNS mode.
+  SKIPPED_FALLBACK_BLOCKING_DNS = 0,
+
+  // The result of running FindProxyForURL() with a modified URL path appears
+  // to be indistinguishable. (Although there may have been sideffects to the
+  // script state that won't manifest until later invocations).
+  SUCCESS = 1,
+
+  // Calling FindProxyForURL() with a modified URL path returned the same proxy
+  // list, but had measurable sideffects in calls to alert().

[cbentzel, 2016/03/18 18:54:53]

I had no idea you could alert in PAC eval.

[eroman, 2016/03/18 20:20:16]

tl;dr: Yes, alert() basically just exists for printf() style debugging of PAC
scripts.
It is not actually window.alert() but specific to the PAC context.

...

For funzies, alert() has a pretty insane implementation in Internet Explorer -
it will pop a model dialog in pretty much any context. If you edit your PAC
script to contain alert(url) you can get it to pop modals while at the Windows
login screen!!!!!! You can make the system completely unusable by virtue of
having modal dialogs continually popping up at random (not just while using IE).
And there is no way to dismiss them from recurring.

In Chrome we map alert() to just emitting into the NetLog, so it cannot be used
for user-visible interactions or modal blocking.

With ProxyResolverV8Tracing we try hard to make it appear as if the script
executes linearly just once and hide the shenanigans we are doing to script
execution and restart.

To accomplish that, alerts() are buffered (to a point) until the final restart,
otherwise you would see a very confusing sequence of alerts. There is fallback
to a blocking mode when buffering exceeds a memory threashold, do instead can
stream the data directly to NetLog.

+  SUCCESS_DIFFERENT_ALERTS = 2,
+
+  // Calling FindProxyForURL() with a modified URL path returned the same proxy
+  // list, but invoked a different sequence of DNS resolutions. This would
+  // require a rather unusual script to trigger.
+  SUCCESS_DIFFERENT_NUM_DNS = 3,
+
+  // Calling FindProxyForURL() with a modified URL path resulted in a different
+  // set of DNS dependencies.
+  FAIL_ABANDONED = 4,
+
+  // Calling FindProxyForURL() with a modified URL path caused a different
+  // execution flow. Whereas with the original URL it succeeded with
+  // non-blocking DNS, this attempt requires fallback to blocking DNS (and was
+  // not attempted).
+  FAIL_FALLBACK_BLOCKING_DNS = 5,
+
+  // Calling FindProxyForURL() with a modified URL path caused a script error.
+  FAIL_ERROR = 6,
+
+  // Calling FindProxyForURL() with a modified URL path returned a different
+  // proxy list.
+  FAIL_DIFFERENT_PROXY_LIST = 7,
+
+  MAX_VALUE,
+};
+
+// TODO(eroman): Remove when done gathering data for crbug.com/593759
+extern NET_EXPORT_PRIVATE const char kHistogramPacResultForStrippedUrl[];
+
 }  // namespace net
 
 #endif  // NET_PROXY_PROXY_RESOLVER_V8_TRACING_H_