[tahoe-dev] [tahoe-lafs] #924: stop writing mtime and ctime fields (except for "tahoe backup")

[tahoe-lafs]

#924: stop writing mtime and ctime fields (except for "tahoe backup")
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 Reporter:  zooko                                                                   |           Owner:  warner    
     Type:  enhancement                                                             |          Status:  new       
 Priority:  major                                                                   |       Milestone:  eventually
Component:  code-dirnodes                                                           |         Version:  1.5.0     
 Keywords:  backward-compatibility forward-compatibility unix windows tahoe-backup  |   Launchpad_bug:            
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 In Tahoe-LAFS v1.3 metadata fields named {{{mtime}}} and {{{ctime}}} were
 written onto links both by "tahoe backup" and by any operation which
 created or updated a link.  In Tahoe-LAFS v1.4 we started putting data
 with clarified semantics in fields named {{{linkmotime}}} and
 {{{linkcrtime}}} in a sub-dict of the link metadata named {{{tahoe}}}.

 For backwards compatibility, we continued to write {{{mtime}}} and
 {{{ctime}}} into the original fields whenever a link was
 This situation, piled on top of the ambiguity and mistakes inherent in
 Python's notion of "ctime" (see #628 and #897) led to a rather complicated
 semantics for the {{{mtime}}} and {{{ctime}}} fields. Interpreting what a
 {{{ctime}}} field means require both figuring out whether "tahoe backup"
 or a different operation was the last operation to touch that link as well
 as whether that operation was running on Windows or another operating
 system.  See [source:docs/frontends/webapi.txt at 4112#L552 the doc about
 those fields] which takes up 86 lines in webapi.txt.

 In some future release of Tahoe-LAFS (for example, maybe Tahoe-LAFS v1.7)
 we should make it so that normal link creations or updates (i.e.,
 excluding the "tahoe backup" command) no longer write to the old
 {{{mtime}}} and {{{ctime}}} fields, but only to the new
 {{{tahoe:linkmotime}}} and {{{tahoe:linkcrtime}}} fields.

 This will greatly simplify the semantics of {{{mtime}}} and {{{ctime}}} --
 they will then mean only what "tahoe backup" intends them to mean (see
 #897 for what that is), provided that only new clients (>= v1.7) have ever
 written to them.  (Hm, that suggests that "tahoe backup" ought to choose
 new names, such as creating a new sub-dict named {{{backup}}} or
 {{{original-file-metadata}}}, containing keys named something like
 {{{mtime}}} and {{{posix-change-time}}} and {{{creation-time}}}, and we
 should abandon the old {{{mtime}}} completely...)

 This will mean that Tahoe-LAFS v1.3 clients which examine those
 directories will not see any mtime or ctime info but they will still be
 able to use the links to access file contents.

-- 
Ticket URL: <http://allmydata.org/trac/tahoe/ticket/924>
tahoe-lafs <http://allmydata.org>
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