Add DCHECK that HistoryURLProvider is on the main thread

chrome/browser/autocomplete/history_url_provider.cc

Index: chrome/browser/autocomplete/history_url_provider.cc
diff --git a/chrome/browser/autocomplete/history_url_provider.cc b/chrome/browser/autocomplete/history_url_provider.cc
index d901d4d9aa74456f65ef1be8adf1400fd7037d61..e79651c7a1bc9132090b69751a9a78b9a2d010fe 100644
--- a/chrome/browser/autocomplete/history_url_provider.cc
+++ b/chrome/browser/autocomplete/history_url_provider.cc
@@ -26,6 +26,7 @@
 #include "chrome/common/chrome_switches.h"
 #include "chrome/common/pref_names.h"
 #include "chrome/common/url_constants.h"
+#include "content/public/browser/browser_thread.h"
 #include "googleurl/src/gurl.h"
 #include "googleurl/src/url_parse.h"
 #include "googleurl/src/url_util.h"
@@ -347,6 +348,17 @@ HistoryURLProvider::HistoryURLProvider(ACProviderListener* listener,
       LOG(ERROR) << "Making automatic.";
     }
     // Automatic means eligible for the field trial.
+    // For the field trial stuff to work correctly, we must be running

[Peter Kasting, 2012/02/09 21:51:19]

I wonder if the field trial is the only requirement imposition here.  It seems
like to use the in-memory data structures we also need to be on the main thread?
 Maybe this check should move to the top of the constructor?  Might be good to
ask Mike.

[mrossetti, 2012/02/09 21:58:58]

I was considering suggesting that the DCHECK be placed in the
AutocompleteProvider ctor.

On 2012/02/09 21:51:19, Peter Kasting wrote:

[Mark P, 2012/02/09 22:17:47]

I think we should pepper our code with these DCHECKs where we think it's
appropriate.  I think this place is a good stop for this one because we have a
good reason to put one here.  I'm not opposed to adding others (in other
changelists).

On 2012/02/09 21:58:58, mrossetti wrote:

+    // on the same thread as the thread that created the field trial,
+    // which happens via a call to AutocompleteFieldTrial::Active in
+    // chrome_browser_main.cc on the main thread.  Let's check this to
+    // be sure.  We check "if we've heard of the UI thread then we'd better
+    // be on it."  The first part is necessary so unit tests pass.  (Many
+    // unit tests don't set up the threading naming system; hence
+    // CurrentlyOn(UI thread will fail.)

[Peter Kasting, 2012/02/09 21:51:19]

Nit: Missing paren after "thread"

[Mark P, 2012/02/09 22:17:47]

Fixed.

+    DCHECK(!content::BrowserThread::IsWellKnownThread(
+               content::BrowserThread::UI) ||
+           content::BrowserThread::CurrentlyOn(content::BrowserThread::UI));
     if (AutocompleteFieldTrial::InAggressiveHUPFieldTrial()) {
       if (AutocompleteFieldTrial::InAggressiveHUPFieldTrialExperimentGroup()) {
         enable_aggressive_scoring_ = true;