+++ This bug was initially created as a clone of Bug #1265315 +++

Description of problem:

udev_device_get_sysattr_list_entry returns a list of attribute, value pairs.
The values are NULL, the attributes are said to be "available".

From the docs:
"""
 Retrieve the list of available sysattrs, with value being empty; This just return all available sysfs attributes for a particular device without reading their values. 
"""

I dunno what "available" is supposed to mean, but if these attributes are
further looked up by "udev_device_get_sysattr_value" they may have no value,
i.e., this method may return NULL. (This is not the same as having a value
like the empty string, which fits the specification just fine.)


Version-Release number of selected component (if applicable):

Any.

How reproducible:

Consistently, for a given device.

Steps to Reproduce:
1. Loop over all devices returned by udevadm.
2. List the attributes using udev_device_get_sysattr_list_entry().
3. Look up the listed attributes using udev_device_get_sysattr_value(). Observe that some of these attributes are NULL.

Actual results:

Some "available" attributes have no values. Generally speaking, these attributes seem to be symlinks to directories. It is not true that all attributes that correspond to symlinks to directories have no value, only some.

Expected results:

"Available" attributes should have a value or documentation should define clearly that "available" means fits some criteria which does not necessarily include actually having a value.

Additional info:

It is normal to think of attribute name/value pairs as being like keys in a map.
If they are not, that is, if a key can simultaneously be in the map and not be
in the map, that should get better documentation then by the single word "available".

Also, in documentation, the ';' is followed by a capital letter, should be lowercase.

--- Additional comment from mulhern on 2015-09-22 12:11:50 EDT ---

Output of new.c on my Fedora desktop. This is just an example, similar things
happen on my RHEL 7.2 box.

[mulhern@localhost reproducers]$ ./a.out
udev_list_entry_get_value()
Correct behavior, values are not obtained.
adr: (null)
path: (null)
subsystem: (null)
uevent: (null)
physical_node: (null)

udev_device_get_sysattr_value()
Incorrect behavior; physical_node should have a value.
adr: 0x00000000
path: \_SB_.PCI0.EHC2.HUBN
subsystem: acpi
uevent: 
physical_node: (null)

udev_device_get_sysattr_value(bogus)
correct behavior, value of non-existant attribute is null.
bogus: (null)

Should be like the first.
adr: (null)
path: (null)
subsystem: (null)
uevent: (null)
physical_node: (null)

--- Additional comment from mulhern on 2015-09-23 15:35 EDT ---



--- Additional comment from mulhern on 2015-09-23 17:03:36 EDT ---

The attached file printed SURPRISE a few times on both systems, indicating that some of the attributes with no value were not symlinked directories. What characteristics they do have I do not know.

-------------------------------------------------------------------------------

Since attributes inherits from Mapping, values method is defined by the Mapping class using in, like, "[s[key] for key in s]". Due to the libudev inconsistency described in #1265315, this is likely to raise an error when key is both in, in the second use of 'in', but not in in the first use of 'in'.