[gparted] Switch to POSIX open() in useable_device() (!46)

[Curtis Gedak <gedakc src gnome org>]

commit e5898e5813af7add0292495c6665cef87296a81b
Author: Mike Fleetwood <mike fleetwood googlemail com>
Date:   Wed Jul 10 10:49:43 2019 +0100

    Switch to POSIX open() in useable_device() (!46)
    
    For every device that GParted scans, it determines if it can be used by
    attempting to read the first sector.  This is the sequence of events,
    as reported in the previous two commit messages:
    
      gparted    |set_devices_thread(pdevices)  pdevices->["/dev/sdb"]
      ...
      gparted    |  useable_device(lp_device)  lp_device->path="/dev/sdb"
      gparted    |    ped_device_open()
      libparted  |      open("/dev/sdb", O_RDWR) = 8
      gparted    |    ped_device_read()
      gparted    |    ped_device_close()
      libparted  |      close(8)
      udev(async)|        KERNEL change /devices/.../sdb (block)
      ...
      udev(async)|        UDEV   change /devices/.../sdb (block)
    
    Note that these udev block device change events occur before the ones
    waited for in the first commit "Wait for udev change on /dev/DISK when
    querying whole device FS (!46)" so these were already waited for.
    
    So because libparted only opens devices read-write this triggers a set
    of udev change events for the whole block device, and if the device is
    partitioned, also remove and re-add events for all the partitions.
    
    Switch to using POSIX open(), read() and close() calls so read-only
    access can be specified to avoid the unnecessary udev change events.
    
    Closes !46 - Whole device FAT32 file system reports device busy warning
                 from mlabel

 src/GParted_Core.cc | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)
---
diff --git a/src/GParted_Core.cc b/src/GParted_Core.cc
index 88078d1d..44739c57 100644
--- a/src/GParted_Core.cc
+++ b/src/GParted_Core.cc
@@ -60,6 +60,7 @@
 #include <sys/types.h>
 #include <stdlib.h>
 #include <unistd.h>
+#include <fcntl.h>
 #include <glibmm/miscutils.h>
 #include <glibmm/fileutils.h>
 #include <glibmm/shell.h>
@@ -4051,12 +4052,12 @@ bool GParted_Core::useable_device( PedDevice * lp_device )
        // Must be able to read from the first sector before the disk device is considered
        // useable in GParted.
        bool success = false;
-       if ( ped_device_open( lp_device )            &&
-            ped_device_read( lp_device, buf, 0, 1 )    )
+       int fd = open(lp_device->path, O_RDONLY|O_NONBLOCK);
+       if (fd >= 0)
        {
-               success = true;
-
-               ped_device_close( lp_device );
+               ssize_t bytes_read = read(fd, buf, lp_device->sector_size);
+               success = (bytes_read == lp_device->sector_size);
+               close(fd);
        }
 
        free( buf );