[PATCH] shadow_copy2: Allow configurable prefix for snapshots

[Rajesh Joseph]

Hi all,

Current shadow_copy2 module provide a configurable time format
(shadow:format) to specify snapshot names. But this seems little
restrictive in nature where your snapshot name can only be a time string.

I updated shadow_copy2 module to take a new prefix option. With this now
user can also provide a configurable prefix along with time format.

Implementation detail:
Let's say the underlying file-system creates snapshots in the following
format
<snap_name>_GMT-%Y.%m.%d-%H.%M.%S

Windows expect snapshot names to be in the following format
@GMT-%Y.%m.%d-%H.%M.%S

VFS using shadow_copy2 module converts underlying file-system format  to
the Windows format and vice-versa.

Earlier with just a time and the shadow:format you can create a unique
file-name and access that file. With the addition of prefix this is no more
valid. Now with given time we also need to find a unique file which will
match the prefix.

One way to address this is to scan the directory every time we access
snapshot. Another way, which I have chosen, is to store the list of
snapshot on the first scan and use this list for subsequent access.

The implementation currently uses a global list to save the list of
snapshot names so that we
can get actual snapshot name with a given time-stamp. Currently I am using
VFS handle's private
variable to store this list.

This approach was already discussed with Michael Adam (obnox) and Ira
Cooper. I got some internal review comments from Ira, which I addressed.

Ira also raised a question on this approach and how well it can work
durable handles and persistent handles. I am saving or creating the
snapshot list in shadow_copy2_get_shadow_copy_data function. Ira suspected
that there is no guarantee shadow_copy2_get_shadow_copy_data function will
be called before shadow_copy2_get_real_filename, and other routines like
it. Especially in case of a reconnect.

Since I am not very well versed with this part of code, I like to have
opinion of others here.

Also review the patch and let me know if you have any comments.

Thanks & Regards,
Rajesh
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