Help: internals.dirstate-v2

dirstate-v2 file format

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 'dirstate-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 'dirstate-v2' for new local repositories ------------------------------------------------

When creating a new local repository such as with 'hg init' or 'hg clone', the 'use-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.use-dirstate-v2=1

Checking the format of an existing local repository --------------------------------------------------

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.use-dirstate-v2' configuration is used again.

Example to upgrade:

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

Example to downgrade to 'dirstate-v1':

$ hg debugupgrade --config format.use-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 repository 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 '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/dirstate' 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 process is 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.
"filepath" as the bytes of the ignore file path relative to the root of the repository if inside the repository, or the untouched path as defined in the configuration.

This hash is defined as the SHA-1 of the following line format:

<filepath> <sha1 of the "expanded contents">\n

for each "root" ignore file. (in sorted order)

(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 search to find a child node with a given name in 'O(log(n))' byte sequence comparisons.

The current implementation 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 referred 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: A 'flags' fields that packs some boolean values as bits of a 16-bit integer. Starting from least-significant, bit masks are:
```
WDIR_TRACKED = 1 << 0
P1_TRACKED = 1 << 1
P2_INFO = 1 << 2
MODE_EXEC_PERM = 1 << 3
MODE_IS_SYMLINK = 1 << 4
HAS_FALLBACK_EXEC = 1 << 5
FALLBACK_EXEC = 1 << 6
HAS_FALLBACK_SYMLINK = 1 << 7
FALLBACK_SYMLINK = 1 << 8
EXPECTED_STATE_IS_MODIFIED = 1 << 9
HAS_MODE_AND_SIZE = 1 << 10
HAS_MTIME = 1 << 11
MTIME_SECOND_AMBIGUOUS = 1 << 12
DIRECTORY = 1 << 13
ALL_UNKNOWN_RECORDED = 1 << 14
ALL_IGNORED_RECORDED = 1 << 15
```
The meaning of each bit is described below.

Other bits are unset. They may be assigned meaning if the future, with the limitation that Mercurial versions that pre-date such meaning will always reset those bits to unset when writing nodes. (A new node is written for any mutation in its subtree, leaving the bytes of the old node unreachable until the data file is rewritten entirely.)
Offset 32: A 'size' field described below, as a 32-bit integer. Unlike in dirstate-v1, negative values are not used.
Offset 36: The seconds component of an 'mtime' field described below, as a 32-bit integer. Unlike in dirstate-v1, negative values are not used. When 'mtime' is used, this is number of seconds since the Unix epoch truncated to its lower 31 bits.
Offset 40: The nanoseconds component of an 'mtime' field described below, as a 32-bit integer. When 'mtime' is used, this is the number of nanoseconds since 'mtime.seconds', always strictly 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, …)

When comparing two mtimes and either has this component set to zero, the sub-second precision of both should be ignored. False positives when checking mtime equality due to clock resolution are always possible and the status algorithm needs to deal with them, but having too many false negatives could be harmful too.
(Offset 44: end of this node)

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. - 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'

The nodes contains a "valid" last modification time in the 'mtime' field.

It means the 'mtime' 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).

This means that if 'std::fs::symlink_metadata' later reports the same modification time and ignored patterns haven’t changed, we can assume the node to be unchanged on disk.

The 'mtime' field can then be used to skip more expensive lookup when checking the status of "tracked" nodes.

It can also be set for node where 'DIRECTORY' is set. See 'DIRECTORY' documentation for details.

'DIRECTORY'

When set, this entry will match a directory that exists or existed on the file system.

When 'HAS_MTIME' is set a directory has been seen on the file system and 'mtime' matches its last modification time. However, 'HAS_MTIME' not being set does not indicate the lack of directory on the file system.
When not tracked anywhere, this node does not represent an ignored or unknown file on disk.

If 'HAS_MTIME' is set and 'mtime' matches the last modification time of the directory on disk, the directory is unchanged and we can skip calling 'std::fs::read_dir' again for this directory, and iterate child dirstate nodes instead. (as long as 'ALL_UNKNOWN_RECORDED' and 'ALL_IGNORED_RECORDED' are taken into account)

'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.

Beware that on system without fs support for this information, the value stored in the dirstate might be wrong and should not be relied on.

'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.

Beware that on system without fs support for this information, the value stored in the dirstate might be wrong and should not be relied on.

'EXPECTED_STATE_IS_MODIFIED'

Must be unset for untracked nodes. For: - a file tracked anywhere - that has expected metadata ('HAS_MODE_AND_SIZE' and 'HAS_MTIME') - if that metadata matches metadata found in the working directory with 'stat' This bit indicates the status of the file. If set, the status is modified. If unset, it is clean.

In cases where 'hg status' needs to read the contents of a file because metadata is ambiguous, this bit lets it record the result if the result is modified so that a future run of 'hg status' does not need to do the same again. It is valid to never set this bit, and consider expected metadata ambiguous if it is set.

'ALL_UNKNOWN_RECORDED'

If set, all "unknown" children existing on disk (at the time of the last status) have been recorded and the 'mtime' associated with 'DIRECTORY' can be used for optimization even when "unknown" file are listed.

Note that the amount recorded "unknown" children can still be zero if None where present.

Also note that having this flag unset does not imply that no "unknown" children have been recorded. Some might be present, but there is no guarantee that is will be all of them.

'ALL_IGNORED_RECORDED'

If set, all "ignored" children existing on disk (at the time of the last status) have been recorded and the 'mtime' associated with 'DIRECTORY' can be used for optimization even when "ignored" file are listed.

Note that the amount recorded "ignored" children can still be zero if None where present.

Also note that having this flag unset does not imply that no "ignored" children have been recorded. Some might be present, but there is no guarantee that is will be all of them.

'HAS_FALLBACK_EXEC'

If this flag is set, the entry carries "fallback" information for the executable bit in the 'FALLBACK_EXEC' flag.

Fallback information can be stored in the dirstate to keep track of filesystem attribute tracked by Mercurial when the underlying file system or operating system does not support that property, (e.g. Windows).

'FALLBACK_EXEC'

Should be ignored if 'HAS_FALLBACK_EXEC' is unset. If set the file for this entry should be considered executable if that information cannot be extracted from the file system. If unset it should be considered non-executable instead.

'HAS_FALLBACK_SYMLINK'

If this flag is set, the entry carries "fallback" information for symbolic link status in the 'FALLBACK_SYMLINK' flag.

'FALLBACK_SYMLINK'

Should be ignored if 'HAS_FALLBACK_SYMLINK' is unset. If set the file for this entry should be considered a symlink if that information cannot be extracted from the file system. If unset it should be considered a normal file instead.

'MTIME_SECOND_AMBIGUOUS'

This flag is relevant only when 'HAS_FILE_MTIME' is set. When set, the 'mtime' stored in the entry is only valid for comparison with timestamps that have nanosecond information. If available timestamp does not carries nanosecond information, the 'mtime' should be ignored and no optimization can be applied.

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