The *dirstate* is what Mercurial uses internally to track

the state of files in the working directory,

such as set by commands like `hg add` and `hg rm`.

It also contains some cached data that help make `hg status` faster.

The name refers both to `.hg/dirstate` on the filesystem

and the corresponding data structure in memory while a Mercurial process

is running.

The original file format, retroactively dubbed `dirstate-v1`,

is described at https://www.mercurial-scm.org/wiki/DirState.

It is made of a flat sequence of unordered variable-size entries,

so accessing any information in it requires parsing all of it.

Similarly, saving changes requires rewriting the entire file.

The newer `dirsate-v2` file format is designed to fix these limitations

and make `hg status` faster.

User guide

==========

Compatibility

-------------

The file format is experimental and may still change.

Different versions of Mercurial may not be compatible with each other

when working on a local repository that uses this format.

When using an incompatible version with the experimental format,

anything can happen including data corruption.

Since the dirstate is entirely local and not relevant to the wire protocol,

`dirstate-v2` does not affect compatibility with remote Mercurial versions.

When `share-safe` is enabled, different repositories sharing the same store

can use different dirstate formats.

Enabling `dirsate-v2` for new local repositories

------------------------------------------------

When creating a new local repository such as with `hg init` or `hg clone`,

the `exp-dirstate-v2` boolean in the `format` configuration section

controls whether to use this file format.

This is disabled by default as of this writing.

To enable it for a single repository, run for example::

    $ hg init my-project --config format.exp-dirstate-v2=1

Checking the format of an existing local repsitory

--------------------------------------------------

The `debugformat` commands prints information about

which of multiple optional formats are used in the current repository,

including `dirstate-v2`::

    $ hg debugformat

    format-variant     repo

    fncache:            yes

    dirstate-v2:        yes

    […]

Upgrading or downgrading an existing local repository

-----------------------------------------------------

The `debugupgrade` command does various upgrades or downgrades

on a local repository

based on the current Mercurial version and on configuration.

The same `format.exp-dirstate-v2` configuration is used again.

Example to upgrade::

    $ hg debugupgrade --config format.exp-dirstate-v2=1

Example to downgrade to `dirstate-v1`::

    $ hg debugupgrade --config format.exp-dirstate-v2=0

Both of this commands do nothing but print a list of proposed changes,

which may include changes unrelated to the dirstate.

Those other changes are controlled by their own configuration keys.

Add `--run` to a command to actually apply the proposed changes.

Backups of `.hg/requires` and `.hg/dirstate` are created

in a `.hg/upgradebackup.*` directory.

If something goes wrong, restoring those files should undo the change.

Note that upgrading affects compatibility with older versions of Mercurial

as noted above.

This can be relevant when a repository’s files are on a USB drive

or some other removable media, or shared over the network, etc.

Internal filesystem representation

==================================

Requirements file

-----------------

The `.hg/requires` file indicates which of various optional file formats

are used by a given repository.

Mercurial aborts when seeing a requirement it does not know about,

which avoids older version accidentally messing up a respository

that uses a format that was introduced later.

For versions that do support a format, the presence or absence of

the corresponding requirement indicates whether to use that format.

When the file contains a `exp-dirstate-v2` line,

the `dirstate-v2` format is used.

With no such line `dirstate-v1` is used.

High level description

----------------------

Whereas `dirstate-v1` uses a single `.hg/disrtate` file,

in `dirstate-v2` that file is a "docket" file

that only contains some metadata

and points to separate data file named `.hg/dirstate.{ID}`,

where `{ID}` is a random identifier.

This separation allows making data files append-only

and therefore safer to memory-map.

Creating a new data file (occasionally to clean up unused data)

can be done with a different ID

without disrupting another Mercurial process

that could still be using the previous data file.

Both files have a format designed to reduce the need for parsing,

by using fixed-size binary components as much as possible.

For data that is not fixed-size,

references to other parts of a file can be made by storing "pseudo-pointers":

integers counted in bytes from the start of a file.

For read-only access no data structure is needed,

only a bytes buffer (possibly memory-mapped directly from the filesystem)

with specific parts read on demand.

The data file contains "nodes" organized in a tree.

Each node represents a file or directory inside the working directory

or its parent changeset.

This tree has the same structure as the filesystem,

so a node representing a directory has child nodes representing

the files and subdirectories contained directly in that directory.

The docket file format

----------------------

This is implemented in `rust/hg-core/src/dirstate_tree/on_disk.rs`

and `mercurial/dirstateutils/docket.py`.

Components of the docket file are found at fixed offsets,

counted in bytes from the start of the file:

* Offset 0:

  The 12-bytes marker string "dirstate-v2\n" ending with a newline character.

  This makes it easier to tell a dirstate-v2 file from a dirstate-v1 file,

  although it is not strictly necessary

  since `.hg/requires` determines which format to use.

* Offset 12:

  The changeset node ID on the first parent of the working directory,

  as up to 32 binary bytes.

  If a node ID is shorter (20 bytes for SHA-1),

  it is start-aligned and the rest of the bytes are set to zero.

* Offset 44:

  The changeset node ID on the second parent of the working directory,

  or all zeros if there isn’t one.

  Also 32 binary bytes.

* Offset 76:

  Tree metadata on 44 bytes, described below.

  Its separation in this documentation from the rest of the docket

  reflects a detail of the current implementation.

  Since tree metadata is also made of fields at fixed offsets, those could

  be inlined here by adding 76 bytes to each offset.

* Offset 120:

  The used size of the data file, as a 32-bit big-endian integer.

  The actual size of the data file may be larger

  (if another Mercurial processis in appending to it

  but has not updated the docket yet).

  That extra data must be ignored.

* Offset 124:

  The length of the data file identifier, as a 8-bit integer.

* Offset 125:

  The data file identifier.

* Any additional data is current ignored, and dropped when updating the file.

Tree metadata in the docket file

--------------------------------

Tree metadata is similarly made of components at fixed offsets.

These offsets are counted in bytes from the start of tree metadata,

which is 76 bytes after the start of the docket file.

This metadata can be thought of as the singular root of the tree

formed by nodes in the data file.

* Offset 0:

  Pseudo-pointer to the start of root nodes,

  counted in bytes from the start of the data file,

  as a 32-bit big-endian integer.

  These nodes describe files and directories found directly

  at the root of the working directory.

* Offset 4:

  Number of root nodes, as a 32-bit big-endian integer.

* Offset 8:

  Total number of nodes in the entire tree that "have a dirstate entry",

  as a 32-bit big-endian integer.

  Those nodes represent files that would be present at all in `dirstate-v1`.

  This is typically less than the total number of nodes.

  This counter is used to implement `len(dirstatemap)`.

* Offset 12:

  Number of nodes in the entire tree that have a copy source,

  as a 32-bit big-endian integer.

  At the next commit, these files are recorded

  as having been copied or moved/renamed from that source.

  (A move is recorded as a copy and separate removal of the source.)

  This counter is used to implement `len(dirstatemap.copymap)`.

* Offset 16:

  An estimation of how many bytes of the data file

  (within its used size) are unused, as a 32-bit big-endian integer.

  When appending to an existing data file,

  some existing nodes or paths can be unreachable from the new root

  but they still take up space.

  This counter is used to decide when to write a new data file from scratch

  instead of appending to an existing one,

  in order to get rid of that unreachable data

  and avoid unbounded file size growth.

* Offset 20:

  These four bytes are currently ignored

  and reset to zero when updating a docket file.

  This is an attempt at forward compatibility:

  future Mercurial versions could use this as a bit field

  to indicate that a dirstate has additional data or constraints.

  Finding a dirstate file with the relevant bit unset indicates that

  it was written by a then-older version

  which is not aware of that future change.

* Offset 24:

  Either 20 zero bytes, or a SHA-1 hash as 20 binary bytes.

  When present, the hash is of ignore patterns

  that were used for some previous run of the `status` algorithm.

* (Offset 44: end of tree metadata)

Optional hash of ignore patterns

--------------------------------

The implementation of `status` at `rust/hg-core/src/dirstate_tree/status.rs`

has been optimized such that its run time is dominated by calls

to `stat` for reading the filesystem metadata of a file or directory,

and to `readdir` for listing the contents of a directory.

In some cases the algorithm can skip calls to `readdir`

(saving significant time)

because the dirstate already contains enough of the relevant information

to build the correct `status` results.

The default configuration of `hg status` is to list unknown files

but not ignored files.

In this case, it matters for the `readdir`-skipping optimization

if a given file used to be ignored but became unknown

because `.hgignore` changed.

To detect the possibility of such a change,

the tree metadata contains an optional hash of all ignore patterns.

We define:

* "Root" ignore files as:

  - `.hgignore` at the root of the repository if it exists

  - And all files from `ui.ignore.*` config.

  This set of files is sorted by the string representation of their path.

* The "expanded contents" of an ignore files is the byte string made

  by the concatenation of its contents followed by the "expanded contents"

  of other files included with `include:` or `subinclude:` directives,

  in inclusion order. This definition is recursive, as included files can

  themselves include more files.

This hash is defined as the SHA-1 of the concatenation (in sorted

order) of the "expanded contents" of each "root" ignore file.

(Note that computing this does not require actually concatenating

into a single contiguous byte sequence.

Instead a SHA-1 hasher object can be created

and fed separate chunks one by one.)

The data file format

--------------------

This is implemented in `rust/hg-core/src/dirstate_tree/on_disk.rs`

and `mercurial/dirstateutils/v2.py`.

The data file contains two types of data: paths and nodes.

Paths and nodes can be organized in any order in the file, except that sibling

nodes must be next to each other and sorted by their path.

Contiguity lets the parent refer to them all

by their count and a single pseudo-pointer,

instead of storing one pseudo-pointer per child node.

Sorting allows using binary seach to find a child node with a given name

in `O(log(n))` byte sequence comparisons.

The current implemention writes paths and child node before a given node

for ease of figuring out the value of pseudo-pointers by the time the are to be

written, but this is not an obligation and readers must not rely on it.

A path is stored as a byte string anywhere in the file, without delimiter.

It is refered to by one or more node by a pseudo-pointer to its start, and its

length in bytes. Since there is no delimiter,

when a path is a substring of another the same bytes could be reused,

although the implementation does not exploit this as of this writing.

A node is stored on 43 bytes with components at fixed offsets. Paths and

child nodes relevant to a node are stored externally and referenced though

pseudo-pointers.

All integers are stored in big-endian. All pseudo-pointers are 32-bit integers

counting bytes from the start of the data file. Path lengths and positions

are 16-bit integers, also counted in bytes.

Node components are:

* Offset 0:

  Pseudo-pointer to the full path of this node,

  from the working directory root.

* Offset 4:

  Length of the full path.

* Offset 6:

  Position of the last `/` path separator within the full path,

  in bytes from the start of the full path,

  or zero if there isn’t one.

  The part of the full path after this position is the "base name".

  Since sibling nodes have the same parent, only their base name vary

  and needs to be considered when doing binary search to find a given path.

* Offset 8:

  Pseudo-pointer to the "copy source" path for this node,

  or zero if there is no copy source.

* Offset 12:

  Length of the copy source path, or zero if there isn’t one.

* Offset 14:

  Pseudo-pointer to the start of child nodes.

* Offset 18:

  Number of child nodes, as a 32-bit integer.

  They occupy 43 times this number of bytes

  (not counting space for paths, and further descendants).

* Offset 22:

  Number as a 32-bit integer of descendant nodes in this subtree,

  not including this node itself,

  that "have a dirstate entry".

  Those nodes represent files that would be present at all in `dirstate-v1`.

  This is typically less than the total number of descendants.

  This counter is used to implement `has_dir`.

* Offset 26:

  Number as a 32-bit integer of descendant nodes in this subtree,

  not including this node itself,

  that represent files tracked in the working directory.

  (For example, `hg rm` makes a file untracked.)

  This counter is used to implement `has_tracked_dir`.

* Offset 30:

  Starting from least-significant, bit masks are::

    WDIR_TRACKED = 1 << 0

    P1_TRACKED = 1 << 1

    P2_INFO = 1 << 2

    HAS_MODE_AND_SIZE = 1 << 3

    HAS_MTIME = 1 << 4

    MODE_EXEC_PERM = 1 << 5

    MODE_IS_SYMLINK = 1 << 6

  The meaning of each bit is described below.

  A `size` field described below, as a 32-bit integer.

  Unlike in dirstate-v1, negative values are not used.

  The seconds component of an `mtime` field described below,

  as a 32-bit integer.

  Unlike in dirstate-v1, negative values are not used.

  The nanoseconds component of an `mtime` field described below,

  as a 32-bit integer.

The meaning of the boolean values packed in `flags` is:

`WDIR_TRACKED`

    Set if the working directory contains a tracked file at this node’s path.

    This is typically set and unset by `hg add` and `hg rm`.

`P1_TRACKED`

    Set if the working directory’s first parent changeset

    (whose node identifier is found in tree metadata)

    contains a tracked file at this node’s path.

    This is a cache to reduce manifest lookups.

`P2_INFO`

    Set if the file has been involved in some merge operation.

    Either because it was actually merged,

    or because the version in the second parent p2 version was ahead,

    or because some rename moved it there.

    In either case `hg status` will want it displayed as modified.

Files that would be mentioned at all in the `dirstate-v1` file format

have a node with at least one of the above three bits set in `dirstate-v2`.

Let’s call these files "tracked anywhere",

and "untracked" the nodes with all three of these bits unset.

Untracked nodes are typically for directories:

they hold child nodes and form the tree structure.

Additional untracked nodes may also exist.

Although implementations should strive to clean up nodes

that are entirely unused, other untracked nodes may also exist.

For example, a future version of Mercurial might in some cases

add nodes for untracked files or/and ignored files in the working directory

in order to optimize `hg status`

by enabling it to skip `readdir` in more cases.

`HAS_MODE_AND_SIZE`

    Must be unset for untracked nodes.

    For files tracked anywhere, if this is set:

    - The `size` field is the expected file size,

      in bytes truncated its lower to 31 bits,

      for the file to be clean.

    - The expected execute permission for the file’s owner

      is given by `MODE_EXEC_PERM`

    - The expected file type is given by `MODE_IS_SIMLINK`:

      a symbolic link if set, or a normal file if unset.

    If this is unset the expected size, permission, and file type are unknown.

    The `size` field is unused (set to zero).

`HAS_MTIME`

    If unset, the `mtime` field is unused (set to zero).

    If set, it contains a timestamp represented as

    - the number of seconds since the Unix epoch,

      truncated to its lower 31 bits.

    - and the number of nanoseconds since `mtime.seconds`,

      always stritctly less than one billion.

      This may be zero if more precision is not available.

      (This can happen because of limitations in any of Mercurial, Python,

      libc, the operating system, …)

    If set for a file tracked anywhere,

    `mtime` is the expected modification time for the file to be clean.

    If set for an untracked node, at some point,

    this path in the working directory was observed:

    - To be a directory

    - With the modification time given in `mtime`

    - That time was already strictly in the past when observed,

      meaning that later changes cannot happen in the same clock tick

      and must cause a different modification time

      (unless the system clock jumps back and we get unlucky,

      which is not impossible but deemed unlikely enough).

    - All direct children of this directory

      (as returned by `std::fs::read_dir`)

      either have a corresponding dirstate node,

      or are ignored by ignore patterns whose hash is in tree metadata.

    This means that if `std::fs::symlink_metadata` later reports

    the same modification time

    and ignored patterns haven’t changed,

    a run of status that is not listing ignored files

    can skip calling `std::fs::read_dir` again for this directory,

    and iterate child dirstate nodes instead.

`MODE_EXEC_PERM`

    Must be unset if `HAS_MODE_AND_SIZE` is unset.

    If `HAS_MODE_AND_SIZE` is set,

    this indicates whether the file’s own is expected

    to have execute permission.

`MODE_IS_SYMLINK`

    Must be unset if `HAS_MODE_AND_SIZE` is unset.

    If `HAS_MODE_AND_SIZE` is set,

    this indicates whether the file is expected to be a symlink

    as opposed to a normal file.