[XProto] Obtain Xcursor path from libXcursor

Some distros (eg. Gentoo) use a non-default libXcursor path, so the
path (which was previously hardcoded) must now be obtained from
libXcursor.  This patch uses dlopen()/dlsym() to get the path when
available.

BUG=1127712
R=sky

Change-Id: Ib99dff0448c521c1a6cf8ad03bfe981572b9627c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2410305
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807158}

ui/base/x/x11_cursor_loader.cc

@@ -4,6 +4,8 @@
 
 #include "ui/base/x/x11_cursor_loader.h"
 
+#include <dlfcn.h>
+
 #include <limits>
 #include <string>
 
@@ -13,6 +15,7 @@
 #include "base/files/file_util.h"
 #include "base/memory/ref_counted_memory.h"
 #include "base/memory/scoped_refptr.h"
+#include "base/no_destructor.h"
 #include "base/sequence_checker.h"
 #include "base/stl_util.h"
 #include "base/strings/string_number_conversions.h"
@@ -28,6 +31,10 @@
 #include "ui/gfx/x/connection.h"
 #include "ui/gfx/x/xproto.h"
 
+extern "C" {
+const char* XcursorLibraryPath(void);
+}
+
 namespace ui {
 
 namespace {
@@ -120,14 +127,41 @@ std::string GetEnv(const std::string& var) {
   return value;
 }
 
-std::string CursorPath() {
+std::string CursorPathFromLibXcursor() {
+  struct DlCloser {
+    void operator()(void* ptr) const { dlclose(ptr); }
+  };
+
+  std::unique_ptr<void, DlCloser> lib(dlopen("libXcursor.so.1", RTLD_LAZY));
+  if (!lib)
+    return "";
+
+  if (auto* sym = reinterpret_cast<decltype(&XcursorLibraryPath)>(
+          dlsym(lib.get(), "XcursorLibraryPath"))) {
+    if (const char* path = sym())
+      return path;
+  }
+  return "";
+}
+
+std::string CursorPathImpl() {
   constexpr const char kDefaultPath[] =
       "~/.local/share/icons:~/.icons:/usr/share/icons:/usr/share/pixmaps:"
       "/usr/X11R6/lib/X11/icons";
+
+  auto libxcursor_path = CursorPathFromLibXcursor();
+  if (!libxcursor_path.empty())
+    return libxcursor_path;
+
   std::string path = GetEnv("XCURSOR_PATH");
   return path.empty() ? kDefaultPath : path;
 }
 
+const std::string& CursorPath() {
+  static base::NoDestructor<std::string> path(CursorPathImpl());
+  return *path;
+}
+
 x11::Render::PictFormat GetRenderARGBFormat(
     const x11::Render::QueryPictFormatsReply& formats) {
   for (const auto& format : formats.formats) {