[Bugs] [Bug 1521042] New: Make a DHT subvolume go read-only when a subvolume crashes

[bugzilla at redhat.com]

https://bugzilla.redhat.com/show_bug.cgi?id=1521042

            Bug ID: 1521042
           Summary: Make a DHT subvolume go read-only when a subvolume
                    crashes
           Product: GlusterFS
           Version: mainline
         Component: distribute
          Keywords: Triaged
          Assignee: bugs at gluster.org
          Reporter: srangana at redhat.com
                CC: bugs at gluster.org



Description of problem:

When subvolumes crash, users get messages like "No such file or directory" or
"I/O Error" when doing operations that are cluster-wide, i.e., operations that
touch the subvolume that has crashed. These include operations like mkdir() and
rmdir() which are cluster-wide, as well as reads/writes/creates that hash to
the dead subvolume. DHT does the right thing by disallowing operations to the
subvolume -- it is effectively putting the subvolume in "read-only" mode to
protect data, but it does not return the correct error. As a result, users of
the filesystem think that the data is gone (in the case of "No such file or
directory", or worse a blanket error that means nothing in the case of EIO).
DHT sets the errno to ENOENT, which while makes sense in the context of DHT (No
subvolume entry, hence ENOENT), the error it should bubble up to the user is
EROFS, since it is putting the system in read-only mode. This diff changes the
error messages to EROFS so the users get a more clear message of what is going
on. Test Plan: Tested by downing a subvolume and checking error codes. Also ran
other prove tests to make sure they pass.

Patch on 3.8 FB branch: https://review.gluster.org/#/c/17952/

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