-From: Ian Jackson <ijackson@chiark.greenend.org.uk>
-To: ijackson@chiark.greenend.org.uk
Subject: Transition plan for git to move to a new hash function
-Date: Thu, 20 Oct 2016 19:26:44 +0100
-Basic principle: Every object will have two (or more) names,
-corresponding to different hash functions. It may be named by any of
-its names, in every context.
+BASIC PRINCIPLE
+
+We run multiple object name subnamespaces in parallel, one for each
+hash function. Each object lives in exactly one subnamespace.
+Objects with identical content in the different object stores, named
+by different hash functions, are different objects.
+
+Objects may refer to objects living in different subnamespaces (ie,
+named by a different hash function) to their own.
+
+Packfiles need to be extended to be able to contain objects named by
+new hash functions. Blob objects with identical contents but living
+in different subnamespaces would ideally share storage.
Every program that invokes git or speaks git protocols will need to
-understand the extended object name syntax, and understand that
-objects have multiple names.
+understand the extended object name syntax.
Safety catches preferent accidental incorporation into a project of
-objects which contain references by incompatibly-new or
-deprecatedly-old names. This allows for incremental deployment.
+incompatibly-new objects, or additional deprecatedly-old objects.
+This allows for incremental deployment.
+
+
+TEXTUAL SYNTAX
+
+The object name textual syntax is extended as follows:
+
+We declare that the object name syntax is henceforth
+ [A-Z]+[0-9a-z]+ | [0-9a-f]+
+and that names [A-Z].* are deprecated as ref name components.
+
+ Rationale:
+
+ Full backwards compatibility is impossible, because the hash
+ function needs to be evident in the name, so the new names
+ must be disjoint from all old SHA-1 names.
+ We want a short but extensible syntax. The syntax should impose
+ minimal extra requirements on existing git users. In most
+ contexts where existing git users use hashes, ASCII alphanumeric
+ object names will fit. Use of punctuation such as : or even _
+ may give trouble to existing users, who are already using
+ such things as delimiters.
-Syntax:
+ In existing deployments, refnames that differ only in case are
+ generally avoided (because they are troublesome on
+ case-insensitive filesystems). And conventionally refnames are
+ lower case. So names starting with an upper case letter will be
+ disjoint from most existing ref name components.
-The object name syntax is extended as follows: object names using sha1
-are as current. Object names starting with lowercase ASCII letters h
-or later refer to new hash functions. (`g' is reserved because of the
-way that many programs write `g<objectname>'. Programs that use
-`g<objectname>' should be changed to show `h<hash>' for hash function
-`h' rather than `gh<hash>'.)
+ Even though we probably want to keep using hex, it is a good
+ idea to reserve the flexibility to use a more compact encoding,
+ while not excessively widening the existing permissible
+ character set.
-Object names h<hex> are SHA-512 hashes. Remaining letters are
-reserved. `x' `y' `z' are reserved for private experiments; we
-declare that public releases of git will never accept such names.
+Object names using SHA-1 are represented, in text, as at present.
+
+Object names starting with uppercase ASCII letters H or later refer to
+new hash functions. Programs that use `g<objectname>' should ideally
+be changed to show `H<hash>' for hash function `H' rather than
+`gH<hash>'.)
+
+ Rationale:
+
+ Object names starting with A-F might look like hex. G is
+ reserved because of the way that many programs write
+ `g<objectname>'.
+
+ This gives us 19 new hash function values until we have to
+ starting using two-letter hash function prefixes, or decide to
+ use A-F after all.
+
+(Truncated object names work as they do at the moment.)
+
+Initially we define and assign one new hash function (and textual
+object name encoding):
+
+ H<hex> where <hex> is the BLAKE2b hash of the object
+ (in lowercase)
+
+We also reserve the following syntax for private experiments:
+ E[A-Z]+[0-9a-z]+
+We declare that public releases of git will never accept such
+object names.
Everywhere in the git object formats and git protocols, a new object
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.
+name is permitted.
+
+A single object may refer to other objects by its own hash functon, or
+by other hash functions. Ie, object references cross subnamespaces.
+During all git operations, subnamespace boundaries in the object graph
+are traversed freely.
-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
-seems necessary, the tree object must be recursively rewritten instead
-to use the desired object name.
+Two additional restrictions: a tree object may be referenced only by
+objects in the same subnamespace; and, a tree object may reference
+blobs in its own subnamespace.
In binary protocols, where a SHA-1 object name in binary form was
previously used, a new codepoint must be allocated in a containing
structure (eg a new typecode). Usually, the new-format binary object
-will have a new typecode and also an additional name hash indicator.
-15 of the hash indicator values correspond to the lowercase letters
-reserved above.
+will have a new typecode and also an additional name hash indicator,
+and it will also need a length field (as new hashes may be of
+different lengths).
+
+Whenever a new hash function textual syntax is defined, corresponding
+binary format codepoint(s) are assigned. (Implementation details such
+as the binary format specification is outside the scope of this
+transition plan.)
+
+
+ORDERING
+
+Hash functions are partially ordered, from `worse' to `better'.
+The ordering is configurable. For details of the defaults,
+see _Transition Plan_.
+
+
+CHOICE OF SUBNAMESPACE
+
+Whenever objects are created, it is necessary to choose the
+subnamespace to use (ie, the hash function).
+
+Each ref may also have a subnamespace hint associated with it.
+
+
+Commits
+
+A commit is made (by default) as new as the newest of
+ (i) each of its parents
+ (ii) if applicable, the subnamespace hint for the ref to which the
+ new commit is to be written
+
+Implicitly this normally means that if HEAD refers to a new commit,
+further new commits will be generated on top of it.
+
+The subnamespace of an origin commit is controlled by the hint left in
+.git by git checkout --orphan or git init.
+
+At boundaries between old and new history, new commit(s) will refer to
+old parent(s).
+
+
+Tags
+
+A tag is created (by default) in the same subnamespace as the object
+to which it refers.
-Object store:
+Trees
-The object store knows which hash functions are enabled. Each hash
-function H has one of the following statuses, which are configured by
-the user:
+Trees are only referenced by objects in their own subnamespace.
-* ENABLED:
+To satisfy this rule, occasionally a tree object from one subnamespace
+must be recursively rewritten into another subnamespace.
- As far as the user is concerned every object in the object store is
- accessible using H. Objects which use H names can be received and
- stored.
+When a tree refers to a commit, it may refer to one in a different
+subnamespace.
- This is actually two states, depending on whether any objects exist
- in the store which use these names. If no such objects exist yet,
- we say that the hash function is `ENABLED PROSPECTIVE'. The H names
- for the objects have not yet been calculated.
+ Rationale: we want to avoid new commits and tags relying on weak
+ hashes. But we must avoid demanding that commits be rewritten.
- When the first object which names another object using H is received
- (or, on demand), the object store calculates the H names for all
- existing objects and notes that this hash function is now
- `ENABLED PRESENT'.
-* 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.
+Blobs
- This is used as part of a gradual desupport strategy. When the hash
- function is in this stage, existing history in all existing object
- stores is safe and cannot be corrupted or modified by receiving
- colliding objects.
+Blobs are normally referred to by trees. Trees always refer to blobs
+in the same subnamespace.
- New object stores which receive their data from a trustworthy sender
+Where a blob is created in other circumstances, the caller should
+specify the subnamespace.
+
+
+Ref hints
+
+As noted above, each ref may also have a subnamespace hint associated
+with it.
+
+The subnamespace hint is (by default) copied, when the ref value is
+copied. So for exmple if `git checkout foo' makes refs/heads/foo out
+of refs/remotes/origin/foo, it will copy the subnamespace hint (or
+lack of one) from refs/remotes/origin/foo.
+
+Likewise, the subnamespace hint is conveyed by `git fetch' (by
+default) and can be updated with `git push' (though this is not done
+by default).
+
+The ref subnamespace hint may be set explicitly. That is how an
+individual branch is upgraded. git checkout --orphan sets it to the
+subnamespace (or hint) of the previous HEAD.
+
+When a commit is made and stored in a ref, the subnamespace hint for
+that ref is removed iff the commit's subnamespace and the hint's
+subnamespace are the same.
+
+
+OBJECT STORE BEHAVIOUR
+
+The object store has configuration to specify which hash functions are
+enabled. Each hash function H has a combination of the following
+behaviours, according to configuration:
+
+* Collision behaviour:
+
+ What to do if we encounter an object we already have (eg as part of
+ a pack, or with hash-object) but with different contents.
+
+ (a) fail: print a scary message and abort operation (on the
+ basis that the source of the colliding object probably intended
+ the preimage that they provided, or is conducting an attack).
+
+ (b) tolerate: prefer our own data; print a message, but treat
+ the reference as referring to our version of the object.
+
+ In both cases we keep a copy of the second preimage in our .git, for
+ forensic purposes.
+
+ This is used as part of a gradual desupport strategy. Existing
+ history in all existing object stores is safe and cannot be
+ corrupted or modified by receiving colliding objects.
+
+ New trees which receive their initial data from a trustworthy sender
over a trustworthy channel will receive correct data. Bad object
stores or untrustworthy channels could exploit collisions, but not
in new regions of the history which are presumably using new names.
Merging previously-unrelated histories does introduce a collision
hazard, but the collision would have had to have been introduced
- while H was still a "live" hash function in at least one of the two
- projects.
+ while the colliding hash function was still a live hash function
+ in at least one of the two projects.
-* FORBIDDEN: Objects do not have their hashes calculated using this
- hash function. Attempts to reference an object by such a name
- fail. Optionally the user may specify a tolerant mode where:
- a commit which refers to parents by obsolete names is taken to
- simply not have those parents; a commit which refers to a tree by
- an obsolete name is taken to have an empty tree.
- This is used for two purposes:
+* Hash function enablement:
- - On a server, we use this to restrict the propagation of
- new hashes so as to enforce our compatibility intentions.
- Ie, hashes which we are "not ready for" are forbidden.
+ (a) enabled: this hash function is good and available for use
- - Everywhere, we use this to get rid of old hash functions.
- It makes access to old history possible but difficult.
+ (b) deprecated (in favour of H2): this hash function is
+ available for use, but newly created objects will use another
+ hash function instead (specifically, when creating an object,
+ this has function is not considered as a candidate; if as a
+ result there are no candidate hash functions, we use the
+ specified replacement H2). Existing refs referring to objects
+ with this hash, with no ref hint, are treated as having a ref
+ hint specifying H2. If no H2 is specified, the newest hash
+ "best" hash is used.
-* FORGOTTEN: Objects do not have their hashes calculated using this
- hash function. References to objects by all such names return dummy
- objects of the right shape: the empty blob; the empty tree; a root
- commit with an empty tree and dummy metadata.
+ (c) disabled: existing objects using this hash function can be
+ accessed, but no such objects can be created or received.
+ (again, a replacement may be specified). This is used both
+ initially to prevent unintended upgrade, and later to block the
+ introduction of vulnerable data generated by badly configured
+ clients.
- 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.
+Remote protocol
+During the negotation, a receiver needs to specify what hashes it
+understands, and whether it is prepared to see only a partial view.
-Naming in newly-generated objects, queries, etc.
+When the sender is listing its refs, refs naming objects the receiver
+cannot understand are either elided (if the receiver is content with a
+parial view), or cause an error.
-There is a `default' hash function, which is that which HEAD uses.
-(That is, HEAD refers to an object by some name. The default hash
-function is that name's hash function.)
-git tools produce always output object names in the default hash
-function. (Including git-hash-object.)
+Equality testing
-As a consequence, newly generated objects will contain object
-references using the `default' hash function.
+Note that semantically identical trees may (now) have different tree
+objects because those tree objects might use (and be named by)
+different hashes. 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 hash.
-When HEAD is empty, there is a separate record of the default hash
-function. This comes from a configured default in a new tree. In an
-existing tree, using git checkout --orphan remembers the default hash
-function that HEAD had.
-When HEAD is updated to a new commit, the name stored in HEAD uses the
-newer of the previous HEAD hash function and of the hash function used
-in the commit being stored. ("Newer" is a built-in preference order,
-overrideable by configuration.)
+TRANSITION PLAN
-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
-the server accepts it, it's infectious.
+(For brevity I will write `SHA' for hashing with SHA-1, using current
+unqualified object names, and `BLAKE' for hasing with BLAKE2b, using
+H<hex> object names.)
+Y0: Implement all of the above. Test it.
-Naming of refs other than HEAD
+ Default configuration:
+ SHA is enabled
+ BLAKE is disabled in trees without working trees
+ BLAKE is enabled in trees with working trees
+ SHA > BLAKE
-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).
+ Effects:
-git-gc rewrites ref names to the default format.
+ Clients are prepared to process BLAKE data, but it is not
+ generated by default and cannot be pushed to servers.
+ All old git clients still work.
-Remote protocol
+Y4: BLAKE by default for new projects.
+ Conversion enabled for existing projects.
+ Old git software is going to start rotting.
-During the negotation, a client needs to specify what names it
-understands, and which it prefers (its default).
+ Default configuration change:
+ BLAKE > SHA
+ BLAKE enabled (even in trees without working trees)
-When the server is listing its refs, the names are converted to the
-client's preferred format.
+ Suggested bulk hosting site configuration change:
+ Newly created projects should get BLAKE enabled
+ Existing projects should retain BLAKE disabled by default
+ Button should be provided to start conversion (see below)
+ Effects:
-Equality testing
+ When creating a new working tree, it starts using BLAKE.
-All software which tests for equality of git objects by checking
-whether their object names are equal needs to obtain a canonical name
-for both objects.
+ Servers which have been updated will accept BLAKE.
-This is going to be quite annoying.
+ Servers which have not been updated to Y4's git will need a small
+ configuration change (enabling BLAKE) to cope with the new
+ projects that are using BLAKE.
-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.
+ To convert a project, an administrator (or project owner) would
+ set BLAKE to enabled, and SHA to deprecated, on the server. On
+ the next pull the server will provide ref hints naming BLAKE,
+ which will get copied to the user's HEAD. So the user is infected
+ with BLAKE.
+ To convert a project branch-by-branch, the administrator would set
+ BLAKE to enabled but leave SHA enabled. Then each branch retains
+ its own hash. A branch can be converted by pushing a BLAKE commit
+ to it, or by setting a ref hint on the server.
-Transition plan
+Y6: BLAKE by default for all projects
+ Existing projects start being converted infectiously.
+ It is hard for a project to stop this happening if any of
+ their servers are updated.
+ Old git software is firmly stuffed.
-Y0: Implement all of the above. Test it.
+ Default configuration change
+ SHA deprecated in trees without working trees
- Default configuration:
- SHA-1 is ENABLED and is default HEAD hash
+ Effects:
- SHA-512 is FORBIDDEN in bare repos
- SHA-512 is ENABLED in trees with working trees
+ Existing projects are, by default, `converted', as described
+ above.
-Y5: New projects should start using SHA-512.
+Y8: Clients hate SHA
+ Clients insist on trying to convert existing projects
+ It is very hard to stop this happening.
+ Unrepentant servers start being very hard to use.
- Default configuration change:
+ Default configuration change
+ SHA deprecated (even in trees without working trees)
+
+ Effects:
+
+ Clients will generate only BLAKE. Hopefully their server will
+ accept this!
+
+Y10: Stop accepting new SHA
+ No-one can manage to make new SHA commits
+
+ Default configuration change
+ SHA disabled in new trees, except during initial
+ `clone', `mirror' and similar
+
+ Effects:
+
+ Existing SHA history is retained, and copied to new clients and
+ servers. But established clients and servers reject any newly
+ introduced SHA.
- SHA-512 becomes ENABLED in *new* bare repos but remains
- FORBIDDEN in existing ones
-
--