name (with hash function indicator) is permitted where an old object
name is permitted. A single object refers to all the objects it
references by the same hash function; in general this might be a
-different hash function to the hash function by this particular object
-was itself referenced or obtained.
+different hash function to the hash function by which this particular
+object was itself referenced or obtained.
As an exception, it is forbidden to refer to a tree object by a name
other than the hash function it uses to name its subtrees. If this
existing objects and notes that this hash function is now
`ENABLED PRESENT'.
+ If a hash collision is detected, we crash immediately.
+
* OBSOLESCENT: Every object in the object store has its hash
calculated using H. However, H is known to possibly have collisions
which we try to tolerate. When a collision occurs, the object text
which is currently in the object store is preferred and the "new"
- object is thrown away. Local creation of new objects with
- references using H is forbidden.
+ object is thrown away.
+
+ Local creation of new objects with references using H is
+ discouraged. Specifically, if another hash function is ENABLED, we
+ will use that instead.
This is used as part of a gradual desupport strategy. When the hash
function is in this stage, existing history in all existing object
This allows us to finally retire a hash function entirely. We
effectively throw away all the history which uses H.
-During transfer protocols, the receiver will say which hashes are
-obsolete or forgotten, and the sender will not follow such references
-when computing the set of objects to send. So receivers will not
-receive the objects which were named only by obsolete or forgotten
-names.
+During transfer protocols, the receiver will say which hashes it
+thinks are obsolete or forgotten, and the sender will not follow such
+references when computing the set of objects to send. So receivers
+will not receive the objects which were named only by obsolete or
+forgotten names.
Naming in newly-generated objects, queries, etc.
This (together with the `forbidden' state, above) ensures that
switching a project to use a new hash function is a deliberate
decision: the default hash function needs to be changed to make the
-first first commit with the new hash function. After that, provided
+first commit with the new hash function. After that, provided
the server accepts it, it's infectious.
A ref refers to an object by one of its names. However, operations
like git-show-ref convert that name to the default format (see above).
-git-gc rewrites ref names to the default format.
+git-gc rewrites ref names to the default format iff that is newer.
Remote protocol
-During the negotation, a client needs to specify what names it
-understands, and which it prefers (its default).
+During the negotation, a receiver needs to specify what hashes it
+understands.
+
+When the sender is listing its refs, the names are converted to a
+hash understood by the client if necessary. If this is not necessary,
+they are left unchanged.
-When the server is listing its refs, the names are converted to the
-client's preferred format.
+When a receiver is updating refs, it should by follow the sender's
+idea of a hash change iff it's an upgrade (and the new function is
+ENABLED). That is, if the sender sends name H2 for some ref, and the
+receiver has H1, but these refer to the same object, then the receiver
+should update its own ref name from H1 to H2 iff H2 uses a newer hash
+function.
Equality testing
This is going to be quite annoying.
+We should provide a convenient utility which tests whether two object
+names refer to the same object.
+
Note that semantically identical trees may (now) have different tree
objects because those tree objects might contain different object
-names. So tree comparison cannot any longer be done by comparing
-names; rather an invocation of git diff is needed.
+names. So (in some contexts at least) tree comparison cannot any
+longer be done by comparing names; rather an invocation of git diff is
+needed, or explicit generation of a tree object with the right name.
Transition plan
Y0: Implement all of the above. Test it.
Default configuration:
- SHA-1 is ENABLED and is default HEAD hash
-
+ SHA-1 is ENABLED
SHA-512 is FORBIDDEN in bare repos
SHA-512 is ENABLED in trees with working trees
+ default HEAD hash is SHA-1
+
+ Effects:
+
+ Existing projects will not switch to SHA-512 willy-nilly.
+ New projects will still use SHA-1.
+
+ Incompatible new-style commits cannot be pushed without server
+ admin effort (or until future upgrade).
+
+ So all old git clients still work.
+
+Y4: SHA-512 by default for new projects.
+ Conversion enabled for existing projects.
+ Old git software is now pretty firmly deprecated.
+
+ Default configuration change:
+
+ When creating a new bare tree, a configuration dropping is left
+ (in `config') which specifies that SHA-1 is OBSOLESCENT
+
+ Default status for SHA-512 is FORBIDDEN if SHA-1 is ENABLED,
+ or ENABLED if SHA-1 is OBSOLESCENT.
+
+ default HEAD hash is newest ENABLED hash.
+
+ Effects:
+
+ When creating a new working tree, it starts using SHA-512.
+ A new server tree will accept SHA-512.
+
+ Existing server trees do not yet accept SHA-512. They publish
+ their SHA-1 hashes, so clients make commits with SHA-1.
+
+ To convert a project, an administrator would set SHA-1 to
+ OBSOLESCENT on the server. All clones after that will have HEAD
+ with a SHA-512 name. Fetches and pulls will update to SHA-512
+ names.
+
+will , and push one SHA-512 commit to
+ mainline.
+
+
+
+ Default configuration change:
+
+ Effects:
+
+ When creating a new tree with working tree with git init (ie, no
+ HEAD), the default HEAD hash is set to SHA-512 (because SHA-1 is
+ OBSOLESCENT in a new tree and therefore SHA-512 is the only
+ ENABLED hash and is the default).
+
+ Newly minted server trees accept SHA-512.
+
+
+ start using SHA-512 by default.
+
+Y6: Existing projects start being converted infectiously.
+ It is hard to stop this happening.
+ Old git software is firmly stuffed.
+
+ Default configuration change:
+ SHA-1 is OBSOLESCENT
+ (default for SHA-512, and HEAD hash, computed as in Y4)
+
+ Result is that by default all software
+
+ (Projects which do not want to convert need to set SHA-1 to
+ ENABLED, explicitly, on their
-Y5: New projects should start using SHA-512.
+Y6: Existing projects start using SHA-512.
Default configuration change:
+ SHA-512 is ENABLED
+ SHA-1 is OBSOLESCENT
+ (default default HEAD hash is already SHA-512)
- SHA-512 becomes ENABLED in *new* bare repos but remains
- FORBIDDEN in existing ones
-
+ In existing repositories where no special action
--
~ian / git-hash-transition-plan.git / commitdiff