df: output placeholder values for inaccessible mount points

A system provided mount entry may be unavailable due to TOCTOU race,
or if another device has been over-mounted at that position, or due to
access permissions.  In all these cases output "-" placeholder values
rather than either producing an error, or in the over-mount case
outputting values for the wrong device.

* src/df.c (device_list): A new global list now updated by
filter_mount_list().
(filter_mount_list): Adjust to take a parameter as to whether
update the global mount list, or only the mount <-> device ID mapping.
(get_dev): Use the device ID mapping to ensure we're not outputting
stats for the wrong device.  Also output placeholder values when we
can't access a system specified mount point.
(get_all_entries): Set the DEVICE_ONLY param for filter_mount_list().
(devname_for_dev): A new function to search the mount <-> dev mapping.
* test/df/skip-duplicates.sh: Adjust accordingly.
* NEWS: Mention the bug fixes.

Discussed at: http://bugs.gnu.org/16539

1 files changed, 4 insertions, 2 deletions

diff --git a/tests/df/skip-duplicates.sh b/tests/df/skip-duplicates.sh
index a620e7320..9f0f749fe 100755
--- a/tests/df/skip-duplicates.sh
+++ b/tests/df/skip-duplicates.sh
@@ -96,8 +96,8 @@ test -f x || skip_ "internal test failure: maybe LD_PRELOAD doesn't work?"
 LD_PRELOAD=./k.so df -T >out || fail=1
 test $(wc -l <out) -eq $(expr 1 + $unique_entries) || { fail=1; cat out; }
 
-# Ensure we fail when unable to stat invalid entries
-LD_PRELOAD=./k.so CU_TEST_DUPE_INVALID=1 df -T >out && fail=1
+# Ensure we don't fail when unable to stat (currently) unavailable entries
+LD_PRELOAD=./k.so CU_TEST_DUPE_INVALID=1 df -T >out || fail=1
 test $(wc -l <out) -eq $(expr 1 + $unique_entries) || { fail=1; cat out; }
 
 # df should also prefer "/fsname" over "fsname"
@@ -113,6 +113,8 @@ test $(grep -c 'virtfs2.*fstype2' <out) -eq 1 || { fail=1; cat out; }
 # Ensure that filtering duplicates does not affect -a processing.
 LD_PRELOAD=./k.so df -a >out || fail=1
 test $(wc -l <out) -eq 6 || { fail=1; cat out; }
+# Ensure placeholder "-" values used for the eclipsed "virtfs"
+test $(grep -c 'virtfs *-' <out) -eq 1 || { fail=1; cat out; }
 
 # Ensure that filtering duplicates does not affect
 # argument processing (now without the fake getmntent()).