Bug 526717 - remove non-working "Set as desktop background" from the UI for unsupported desktops. r=gavin

browser/base/content/nsContextMenu.js

@@ -267,7 +267,7 @@ nsContextMenu.prototype = {
     // Only enable Set as Desktop Background if we can get the shell service.
     var shell = getShellService();
     if (shell)
-      haveSetDesktopBackground = true;
+      haveSetDesktopBackground = shell.canSetDesktopBackground;
 #endif
     this.showItem("context-setDesktopBackground",
                   haveSetDesktopBackground && this.onLoadedImage);

browser/components/shell/public/nsIShellService.idl

@@ -74,6 +74,15 @@ interface nsIShellService : nsISupports
    */
   attribute boolean shouldCheckDefaultBrowser;
 
+  /**
+   * Used to determine whether or not to offer "Set as desktop background"
+   * functionality. Even if shell service is available it is not
+   * guaranteed that it is able to set the background for every desktop
+   * which is especially true for Linux with its many different desktop
+   * environments.
+   */
+  readonly attribute boolean canSetDesktopBackground;
+
   /** 
    * Flags for positioning/sizing of the Desktop Background image.
    */

browser/components/shell/src/nsGNOMEShellService.cpp

@@ -382,6 +382,21 @@ nsGNOMEShellService::SetShouldCheckDefaultBrowser(bool aShouldCheck)
   return NS_OK;
 }
 
+NS_IMETHODIMP
+nsGNOMEShellService::GetCanSetDesktopBackground(bool* aResult)
+{
+  // setting desktop background is currently only supported
+  // for Gnome or desktops using the same GSettings and GConf keys
+  const char* gnomeSession = getenv("GNOME_DESKTOP_SESSION_ID");
+  if (gnomeSession) {
+    *aResult = true;
+  } else {
+    *aResult = false;
+  }
+
+  return NS_OK;
+}
+
 static nsresult
 WriteImage(const nsCString& aPath, imgIContainer* aImage)
 {

browser/components/shell/src/nsMacShellService.cpp

@@ -156,6 +156,13 @@ nsMacShellService::SetShouldCheckDefaultBrowser(bool aShouldCheck)
   return NS_OK;
 }
 
+NS_IMETHODIMP
+nsMacShellService::GetCanSetDesktopBackground(bool* aResult)
+{
+  *aResult = true;
+  return NS_OK;
+}
+
 NS_IMETHODIMP
 nsMacShellService::SetDesktopBackground(nsIDOMElement* aElement, 
                                         PRInt32 aPosition)

browser/components/shell/src/nsWindowsShellService.cpp

@@ -561,6 +561,13 @@ nsWindowsShellService::IsDefaultBrowser(bool aStartupCheck,
   return NS_OK;
 }
 
+NS_IMETHODIMP
+nsWindowsShellService::GetCanSetDesktopBackground(bool* aResult)
+{
+  *aResult = true;
+  return NS_OK;
+}
+
 NS_IMETHODIMP
 nsWindowsShellService::SetDefaultBrowser(bool aClaimAllTypes, bool aForAllUsers)
 {