Replace /proc/mounts grep with Mount_Info cache reload and query (!89)

Creating a grep process to check if a particular mount is still mounted
is an unnecessary overhead.  All that is needed is for the Mount_Info
module to refresh it's copy of /proc/mounts and query that.

To keep the code as simple as possible just reload the whole of the
Mount_Info module and query the mount cache to determine if the
particular block device is still mounted at this particular mount point.
This therefore re-reads /proc/mounts (necessary) and /proc/swaps and
/etc/fstab (unnecessary).  This is still much less overhead than
creating a separate grep process.

Closes !89 - Fix unmount error when unmounting below a bind mount point

include/Mount_Info.h

@@ -49,6 +49,7 @@ public:
 	static void load_cache();
 	static bool is_dev_mounted( const Glib::ustring & path );
 	static bool is_dev_mounted( const BlockSpecial & bs );
+	static bool is_dev_mounted_at(const Glib::ustring& path, const Glib::ustring& mountpoint);
 	static bool is_dev_mounted_readonly( const Glib::ustring & path );
 	static bool is_dev_mounted_readonly( const BlockSpecial & bs );
 	static std::vector<Glib::ustring> get_all_mountpoints();

src/Mount_Info.cc

@@ -136,6 +136,18 @@ const std::vector<Glib::ustring> & Mount_Info::get_fstab_mountpoints( const Glib
 	return find( fstab_info, path ).mountpoints;
 }
 
+
+// Return whether the device path, such as /dev/sda3, is mounted at mount point or not
+bool Mount_Info::is_dev_mounted_at(const Glib::ustring& path, const Glib::ustring& mountpoint)
+{
+	const std::vector<Glib::ustring>& mountpoints = get_mounted_mountpoints(path);
+	for (unsigned i = 0; i < mountpoints.size(); i++)
+		if (mountpoint == mountpoints[i])
+			return true;
+	return false;
+}
+
+
 // Private methods
 
 void Mount_Info::read_mountpoints_from_file( const Glib::ustring & filename, MountMapping & map )

src/Win_GParted.cc

@@ -2847,14 +2847,16 @@ bool Win_GParted::unmount_partition( const Partition & partition, Glib::ustring
 		}
 		else
 		{
-			Glib::ustring checkmount = "grep -w " + Glib::shell_quote(fs_mountpoints[i]) + " /proc/mounts";
-			Glib::ustring cmd = "umount -v " + Glib::shell_quote( fs_mountpoints[i] );
-			Glib::ustring dummy;
-			Glib::ustring umount_error;
-
-			// Check mount point is still mounted
-			if (! Utils::execute_command(checkmount, dummy, umount_error))
+			// Unmounting below a duplicating bind mount, unmounts all copies
+			// in one go so check if the file system is still mounted at this
+			// mount point before trying to unmount it.
+			Mount_Info::load_cache();
+			if (Mount_Info::is_dev_mounted_at(partition.get_path(), fs_mountpoints[i]))
 			{
+				Glib::ustring cmd = "umount -v " + Glib::shell_quote(fs_mountpoints[i]);
+				Glib::ustring dummy;
+				Glib::ustring umount_error;
+
 				if (Utils::execute_command(cmd, dummy, umount_error))
 					umount_errors.push_back("# " + cmd + "\n" + umount_error);
 			}