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![David Greaves](/assets/img/avatar_default.png)
Whitespace and asciidoc formatting changes only in preparation for content changes. Signed-off-by: David Greaves <david@dgreaves.com> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
463 lines
20 KiB
Plaintext
463 lines
20 KiB
Plaintext
////////////////////////////////////////////////////////////////
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GIT - the stupid content tracker
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////////////////////////////////////////////////////////////////
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"git" can mean anything, depending on your mood.
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- random three-letter combination that is pronounceable, and not
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actually used by any common UNIX command. The fact that it is a
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mispronunciation of "get" may or may not be relevant.
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- stupid. contemptible and despicable. simple. Take your pick from the
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dictionary of slang.
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- "global information tracker": you're in a good mood, and it actually
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works for you. Angels sing, and a light suddenly fills the room.
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- "goddamn idiotic truckload of sh*t": when it breaks
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This is a stupid (but extremely fast) directory content manager. It
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doesn't do a whole lot, but what it _does_ do is track directory
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contents efficiently.
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There are two object abstractions: the "object database", and the
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"current directory cache" aka "index".
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The Object Database
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~~~~~~~~~~~~~~~~~~~
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The object database is literally just a content-addressable collection
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of objects. All objects are named by their content, which is
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approximated by the SHA1 hash of the object itself. Objects may refer
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to other objects (by referencing their SHA1 hash), and so you can
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build up a hierarchy of objects.
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All objects have a statically determined "type" aka "tag", which is
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determined at object creation time, and which identifies the format of
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the object (i.e. how it is used, and how it can refer to other objects).
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There are currently three different object types: "blob", "tree" and
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"commit".
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A "blob" object cannot refer to any other object, and is, like the tag
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implies, a pure storage object containing some user data. It is used to
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actually store the file data, i.e. a blob object is associated with some
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particular version of some file.
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A "tree" object is an object that ties one or more "blob" objects into a
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directory structure. In addition, a tree object can refer to other tree
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objects, thus creating a directory hierarchy.
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Finally, a "commit" object ties such directory hierarchies together into
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a DAG of revisions - each "commit" is associated with exactly one tree
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(the directory hierarchy at the time of the commit). In addition, a
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"commit" refers to one or more "parent" commit objects that describe the
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history of how we arrived at that directory hierarchy.
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As a special case, a commit object with no parents is called the "root"
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object, and is the point of an initial project commit. Each project
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must have at least one root, and while you can tie several different
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root objects together into one project by creating a commit object which
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has two or more separate roots as its ultimate parents, that's probably
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just going to confuse people. So aim for the notion of "one root object
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per project", even if git itself does not enforce that.
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A "tag" object symbolically identifies and can be used to sign other
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objects. It contains the identifier and type of another object, a
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symbolic name (of course!) and, optionally, a signature.
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Regardless of object type, all objects are share the following
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characteristics: they are all in deflated with zlib, and have a header
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that not only specifies their tag, but also size information about the
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data in the object. It's worth noting that the SHA1 hash that is used
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to name the object is the hash of the original data (historical note:
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in the dawn of the age of git this was the sha1 of the _compressed_
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object)
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As a result, the general consistency of an object can always be tested
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independently of the contents or the type of the object: all objects can
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be validated by verifying that (a) their hashes match the content of the
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file and (b) the object successfully inflates to a stream of bytes that
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forms a sequence of <ascii tag without space> + <space> + <ascii decimal
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size> + <byte\0> + <binary object data>.
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The structured objects can further have their structure and
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connectivity to other objects verified. This is generally done with
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the "fsck-cache" program, which generates a full dependency graph of
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all objects, and verifies their internal consistency (in addition to
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just verifying their superficial consistency through the hash).
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The object types in some more detail:
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Blob Object
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~~~~~~~~~~~
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A "blob" object is nothing but a binary blob of data, and doesn't
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refer to anything else. There is no signature or any other
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verification of the data, so while the object is consistent (it _is_
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indexed by its sha1 hash, so the data itself is certainly correct), it
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has absolutely no other attributes. No name associations, no
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permissions. It is purely a blob of data (i.e. normally "file
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contents").
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In particular, since the blob is entirely defined by its data, if two
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files in a directory tree (or in multiple different versions of the
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repository) have the same contents, they will share the same blob
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object. The object is totally independent of it's location in the
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directory tree, and renaming a file does not change the object that
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file is associated with in any way.
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Tree Object
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~~~~~~~~~~~
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The next hierarchical object type is the "tree" object. A tree object
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is a list of mode/name/blob data, sorted by name. Alternatively, the
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mode data may specify a directory mode, in which case instead of
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naming a blob, that name is associated with another TREE object.
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Like the "blob" object, a tree object is uniquely determined by the
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set contents, and so two separate but identical trees will always
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share the exact same object. This is true at all levels, i.e. it's
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true for a "leaf" tree (which does not refer to any other trees, only
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blobs) as well as for a whole subdirectory.
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For that reason a "tree" object is just a pure data abstraction: it
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has no history, no signatures, no verification of validity, except
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that since the contents are again protected by the hash itself, we can
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trust that the tree is immutable and its contents never change.
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So you can trust the contents of a tree to be valid, the same way you
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can trust the contents of a blob, but you don't know where those
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contents _came_ from.
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Side note on trees: since a "tree" object is a sorted list of
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"filename+content", you can create a diff between two trees without
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actually having to unpack two trees. Just ignore all common parts,
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and your diff will look right. In other words, you can effectively
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(and efficiently) tell the difference between any two random trees by
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O(n) where "n" is the size of the difference, rather than the size of
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the tree.
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Side note 2 on trees: since the name of a "blob" depends entirely and
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exclusively on its contents (i.e. there are no names or permissions
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involved), you can see trivial renames or permission changes by
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noticing that the blob stayed the same. However, renames with data
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changes need a smarter "diff" implementation.
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Changeset Object
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~~~~~~~~~~~~~~~~
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The "changeset" object is an object that introduces the notion of
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history into the picture. In contrast to the other objects, it
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doesn't just describe the physical state of a tree, it describes how
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we got there, and why.
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A "changeset" is defined by the tree-object that it results in, the
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parent changesets (zero, one or more) that led up to that point, and a
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comment on what happened. Again, a changeset is not trusted per se:
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the contents are well-defined and "safe" due to the cryptographically
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strong signatures at all levels, but there is no reason to believe
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that the tree is "good" or that the merge information makes sense.
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The parents do not have to actually have any relationship with the
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result, for example.
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Note on changesets: unlike real SCM's, changesets do not contain
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rename information or file mode change information. All of that is
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implicit in the trees involved (the result tree, and the result trees
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of the parents), and describing that makes no sense in this idiotic
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file manager.
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Trust Object
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~~~~~~~~~~~~
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The notion of "trust" is really outside the scope of "git", but it's
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worth noting a few things. First off, since everything is hashed with
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SHA1, you _can_ trust that an object is intact and has not been messed
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with by external sources. So the name of an object uniquely
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identifies a known state - just not a state that you may want to
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trust.
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Furthermore, since the SHA1 signature of a changeset refers to the
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SHA1 signatures of the tree it is associated with and the signatures
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of the parent, a single named changeset specifies uniquely a whole set
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of history, with full contents. You can't later fake any step of the
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way once you have the name of a changeset.
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So to introduce some real trust in the system, the only thing you need
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to do is to digitally sign just _one_ special note, which includes the
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name of a top-level changeset. Your digital signature shows others
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that you trust that changeset, and the immutability of the history of
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changesets tells others that they can trust the whole history.
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In other words, you can easily validate a whole archive by just
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sending out a single email that tells the people the name (SHA1 hash)
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of the top changeset, and digitally sign that email using something
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like GPG/PGP.
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In particular, you can also have a separate archive of "trust points"
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or tags, which document your (and other peoples) trust. You may, of
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course, archive these "certificates of trust" using "git" itself, but
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it's not something "git" does for you.
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Another way of saying the last point: "git" itself only handles
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content integrity, the trust has to come from outside.
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The "index" aka "Current Directory Cache"
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-----------------------------------------
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The index is a simple binary file, which contains an efficient
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representation of a virtual directory content at some random time. It
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does so by a simple array that associates a set of names, dates,
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permissions and content (aka "blob") objects together. The cache is
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always kept ordered by name, and names are unique (with a few very
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specific rules) at any point in time, but the cache has no long-term
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meaning, and can be partially updated at any time.
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In particular, the index certainly does not need to be consistent with
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the current directory contents (in fact, most operations will depend on
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different ways to make the index _not_ be consistent with the directory
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hierarchy), but it has three very important attributes:
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'(a) it can re-generate the full state it caches (not just the
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directory structure: it contains pointers to the "blob" objects so
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that it can regenerate the data too)'
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As a special case, there is a clear and unambiguous one-way mapping
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from a current directory cache to a "tree object", which can be
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efficiently created from just the current directory cache without
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actually looking at any other data. So a directory cache at any one
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time uniquely specifies one and only one "tree" object (but has
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additional data to make it easy to match up that tree object with what
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has happened in the directory)
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'(b) it has efficient methods for finding inconsistencies between that
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cached state ("tree object waiting to be instantiated") and the
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current state.'
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'(c) it can additionally efficiently represent information about merge
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conflicts between different tree objects, allowing each pathname to be
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associated with sufficient information about the trees involved that
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you can create a three-way merge between them.'
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Those are the three ONLY things that the directory cache does. It's a
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cache, and the normal operation is to re-generate it completely from a
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known tree object, or update/compare it with a live tree that is being
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developed. If you blow the directory cache away entirely, you generally
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haven't lost any information as long as you have the name of the tree
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that it described.
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At the same time, the directory index is at the same time also the
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staging area for creating new trees, and creating a new tree always
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involves a controlled modification of the index file. In particular,
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the index file can have the representation of an intermediate tree that
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has not yet been instantiated. So the index can be thought of as a
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write-back cache, which can contain dirty information that has not yet
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been written back to the backing store.
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The Workflow
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------------
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Generally, all "git" operations work on the index file. Some operations
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work *purely* on the index file (showing the current state of the
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index), but most operations move data to and from the index file. Either
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from the database or from the working directory. Thus there are four
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main combinations:
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1) working directory -> index
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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You update the index with information from the working directory with
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the "update-cache" command. You generally update the index
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information by just specifying the filename you want to update, like
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so:
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update-cache filename
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but to avoid common mistakes with filename globbing etc, the command
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will not normally add totally new entries or remove old entries,
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i.e. it will normally just update existing cache entries.
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To tell git that yes, you really do realize that certain files no
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longer exist in the archive, or that new files should be added, you
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should use the "--remove" and "--add" flags respectively.
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NOTE! A "--remove" flag does _not_ mean that subsequent filenames will
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necessarily be removed: if the files still exist in your directory
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structure, the index will be updated with their new status, not
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removed. The only thing "--remove" means is that update-cache will be
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considering a removed file to be a valid thing, and if the file really
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does not exist any more, it will update the index accordingly.
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As a special case, you can also do "update-cache --refresh", which
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will refresh the "stat" information of each index to match the current
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stat information. It will _not_ update the object status itself, and
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it will only update the fields that are used to quickly test whether
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an object still matches its old backing store object.
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2) index -> object database
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~~~~~~~~~~~~~~~~~~~~~~~~~~~
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You write your current index file to a "tree" object with the program
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write-tree
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that doesn't come with any options - it will just write out the
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current index into the set of tree objects that describe that state,
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and it will return the name of the resulting top-level tree. You can
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use that tree to re-generate the index at any time by going in the
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other direction:
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3) object database -> index
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~~~~~~~~~~~~~~~~~~~~~~~~~~~
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You read a "tree" file from the object database, and use that to
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populate (and overwrite - don't do this if your index contains any
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unsaved state that you might want to restore later!) your current
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index. Normal operation is just
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read-tree <sha1 of tree>
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and your index file will now be equivalent to the tree that you saved
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earlier. However, that is only your _index_ file: your working
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directory contents have not been modified.
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4) index -> working directory
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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You update your working directory from the index by "checking out"
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files. This is not a very common operation, since normally you'd just
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keep your files updated, and rather than write to your working
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directory, you'd tell the index files about the changes in your
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working directory (i.e. "update-cache").
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However, if you decide to jump to a new version, or check out somebody
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else's version, or just restore a previous tree, you'd populate your
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index file with read-tree, and then you need to check out the result
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with
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checkout-cache filename
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or, if you want to check out all of the index, use "-a".
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NOTE! checkout-cache normally refuses to overwrite old files, so if
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you have an old version of the tree already checked out, you will need
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to use the "-f" flag (_before_ the "-a" flag or the filename) to
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_force_ the checkout.
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Finally, there are a few odds and ends which are not purely moving
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from one representation to the other:
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5) Tying it all together
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~~~~~~~~~~~~~~~~~~~~~~~~
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To commit a tree you have instantiated with "write-tree", you'd create
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a "commit" object that refers to that tree and the history behind it -
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most notably the "parent" commits that preceded it in history.
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Normally a "commit" has one parent: the previous state of the tree
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before a certain change was made. However, sometimes it can have two
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or more parent commits, in which case we call it a "merge", due to the
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fact that such a commit brings together ("merges") two or more
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previous states represented by other commits.
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In other words, while a "tree" represents a particular directory state
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of a working directory, a "commit" represents that state in "time",
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and explains how we got there.
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You create a commit object by giving it the tree that describes the
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state at the time of the commit, and a list of parents:
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commit-tree <tree> -p <parent> [-p <parent2> ..]
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and then giving the reason for the commit on stdin (either through
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redirection from a pipe or file, or by just typing it at the tty).
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commit-tree will return the name of the object that represents that
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commit, and you should save it away for later use. Normally, you'd
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commit a new "HEAD" state, and while git doesn't care where you save
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the note about that state, in practice we tend to just write the
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result to the file ".git/HEAD", so that we can always see what the
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last committed state was.
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6) Examining the data
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~~~~~~~~~~~~~~~~~~~~~
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You can examine the data represented in the object database and the
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index with various helper tools. For every object, you can use
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"cat-file" to examine details about the object:
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cat-file -t <objectname>
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shows the type of the object, and once you have the type (which is
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usually implicit in where you find the object), you can use
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cat-file blob|tree|commit <objectname>
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to show its contents. NOTE! Trees have binary content, and as a result
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there is a special helper for showing that content, called "ls-tree",
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which turns the binary content into a more easily readable form.
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It's especially instructive to look at "commit" objects, since those
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tend to be small and fairly self-explanatory. In particular, if you
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follow the convention of having the top commit name in ".git/HEAD",
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you can do
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cat-file commit $(cat .git/HEAD)
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to see what the top commit was.
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7) Merging multiple trees
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~~~~~~~~~~~~~~~~~~~~~~~~~
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Git helps you do a three-way merge, which you can expand to n-way by
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repeating the merge procedure arbitrary times until you finally
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"commit" the state. The normal situation is that you'd only do one
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three-way merge (two parents), and commit it, but if you like to, you
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can do multiple parents in one go.
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To do a three-way merge, you need the two sets of "commit" objects
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that you want to merge, use those to find the closest common parent (a
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third "commit" object), and then use those commit objects to find the
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state of the directory ("tree" object) at these points.
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To get the "base" for the merge, you first look up the common parent
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of two commits with
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merge-base <commit1> <commit2>
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which will return you the commit they are both based on. You should
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now look up the "tree" objects of those commits, which you can easily
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do with (for example)
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cat-file commit <commitname> | head -1
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since the tree object information is always the first line in a commit
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object.
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Once you know the three trees you are going to merge (the one
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"original" tree, aka the common case, and the two "result" trees, aka
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the branches you want to merge), you do a "merge" read into the
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index. This will throw away your old index contents, so you should
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make sure that you've committed those - in fact you would normally
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always do a merge against your last commit (which should thus match
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what you have in your current index anyway).
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To do the merge, do
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read-tree -m <origtree> <target1tree> <target2tree>
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which will do all trivial merge operations for you directly in the
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index file, and you can just write the result out with "write-tree".
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NOTE! Because the merge is done in the index file, and not in your
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working directory, your working directory will no longer match your
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index. You can use "checkout-cache -f -a" to make the effect of the
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merge be seen in your working directory.
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NOTE2! Sadly, many merges aren't trivial. If there are files that have
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been added.moved or removed, or if both branches have modified the
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same file, you will be left with an index tree that contains "merge
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entries" in it. Such an index tree can _NOT_ be written out to a tree
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object, and you will have to resolve any such merge clashes using
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other tools before you can write out the result.
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[ fixme: talk about resolving merges here ]
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