Avoid using X11 SHM for remote connections

This is a reland of [1], which was accidentally removed by [2].  The
only difference is the gethostname() check is removed to address [3].

> For remote connections, SHM doesn't work, but the MIT-SHM extension
> may still be available, so we need to do an explicit check before
> trying to use SHM.

[1] https://chromium.googlesource.com/chromium/src/+/633c011511bae4ff193ac007e3e8ba64287c1d89
[2] https://chromium.googlesource.com/chromium/src/+/12a86d61adf2520c177ed51aec6583f490e29f7d
[3] https://bugs.chromium.org/p/chromium/issues/detail?id=1035803#c12

R=nickdiego
BUG=1035803

Change-Id: I5b707de48a8c2991aaf73c347b29ef813a9c28b4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2062900
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Nick Yamane <nickdiego@igalia.com>
Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Nick Yamane <nickdiego@igalia.com>
Cr-Commit-Position: refs/heads/master@{#742312}

ui/base/x/x11_shm_image_pool.cc

@@ -18,6 +18,7 @@
 #include "base/strings/string_util.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "build/build_config.h"
+#include "net/base/url_util.h"
 #include "ui/events/platform/platform_event_dispatcher.h"
 #include "ui/events/platform/platform_event_source.h"
 #include "ui/gfx/geometry/rect.h"
@@ -55,7 +56,30 @@ std::size_t MaxShmSegmentSize() {
 }
 
 #if !defined(OS_CHROMEOS)
-bool ShouldUseMitShm() {
+bool IsRemoteHost(const std::string& name) {
+  if (name.empty())
+    return false;
+
+  return !net::HostStringIsLocalhost(name);
+}
+
+bool ShouldUseMitShm(XDisplay* display) {
+  // MIT-SHM may be available on remote connetions, but it will be unusable.  Do
+  // a best-effort check to see if the host is remote to disable the SHM
+  // codepath.  It may be possible in contrived cases for there to be a
+  // false-positive, but in that case we'll just fallback to the non-SHM
+  // codepath.
+  char* display_string = DisplayString(display);
+  char* host = nullptr;
+  int display_id = 0;
+  int screen = 0;
+  if (xcb_parse_display(display_string, &host, &display_id, &screen)) {
+    std::string name = host;
+    free(host);
+    if (IsRemoteHost(name))
+      return false;
+  }
+
   std::unique_ptr<base::Environment> env = base::Environment::Create();
 
   // Used by QT.
@@ -120,7 +144,7 @@ bool XShmImagePool::Resize(const gfx::Size& pixel_size) {
     return false;
 
 #if !defined(OS_CHROMEOS)
-  if (!ShouldUseMitShm())
+  if (!ShouldUseMitShm(display_))
     return false;
 #endif