I'm of two minds about how to represent special errno's
defined in the interface between distinct modules (ceph
user space and the ceph kernel code).
Specifically, EOLDSNAPC is defined using ERESTART in
the kernel (and in the wireshark plugin) and using the
numeric value 85 in the user space code. Similarly,
EBLACKLISTED is defined as either ESHUTDOWN or 108.
Those numeric values typically match those standard
symbolic values.
But technically the symbolic values could change
what number they use to represent them.
In fact, in the kernel ERESTART is defined in five
different ways, dependent on hardware architecture
85 generic
91 mips
116 sparc
127 alpha,
175 parisc
Similarly, ESHUTDOWN can be:
58 alpha and sparc
108 generic
143 mips
236 parisc
So using a numeric value is preferable.
On the other hand, the numeric value might have a
different symbolic value. For example, numeric 85
(which we interpret as ERESTART/OLDSNAPC) represents
EPROTO on alpha.
The only good solution is to use known distinct values
for these special symbols at interface boundaries, and
translate between them and well-understood values for
internal use if necessary (i.e., if you need a valid
errno value).