urbit/bin/solid.pill
Philip Monk d9276cd51c
clay: Add support for representing tombstones
This adds support for tombstoned files to clay.  It does not include any
way to actually tombstone them; that is left for later.

This allows tombstoning at the level of a file.  Precisely, this expands
+blob:clay by adding a %dead case:

    +$  blob                                              ::  fs blob
      $%  [%delta p=lobe q=[p=mark q=lobe] r=page]        ::  delta on q
          [%direct p=lobe q=page]                         ::  immediate
          [%dead p=lobe ~]                                ::  tombstone
      ==                                                  ::

Thus, we maintain the invariant that every lobe corresponds to a blob,
but now a blob may be an explicit tombstone.

Details:
- This has not been tested at all, except that it compiles and boots.
- This does not have a state adapter from master.  The only state change
  is the definition of +cach.
- Additionally, out-of-date ships may unexpectedly receive a %dead blob
  from a foreign clay which would interfere with their ability to download
  that desk.  No code changes necessary, but sponsors should avoid
  tombstoning files in %base for a while so their children can get the
  update.
- A merge will only fail if the tombstoned file conflicts with another
  change.  Note that as written, merging from a past desk *can* bring a
  tombstoned file to the head of a desk.  Possibly this shouldn't be
  allowed.

This also includes a couple refactors that were made possible by ford
fusion (since everything is synchronous now) but never got done.  In
both cases we get to remove a monad, which simplifies the code
considerably.

- refactor +merge's error handling to use !!/mule instead of threading
  through errors
- refactor all +read-* functions and related parts of +try-fill-sub to
  eagerly convert lobes to cages.

We also add support reading %a/b/c/e/f/r/x from past and foreign desks,
when possible.  Apologies that all of these are in one commit, it was
all a single chunk of work.

This is a draft until we have a way to tombstone.  I suspect we'll want
to have a mechanism of keeping track of gc roots and trace to remove,
but this PR doesn't suggest any particular strategy.
2021-11-07 22:17:02 -08:00

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