This module contains plain tree indexing code. The index itself is a CACHE: you should only ever use it as an optimisation and never as a primary storage. In practice, this means that when we change index format, the application is expected to throw the old index away and build a fresh index. Please note that tracking index validity is out of scope for this library: this is responsibility of your application. It is advisable that in your validity tracking code, you also check for format validity (see indexFormatValid) and scrap and re-create index when needed.

The index is a binary file that overlays a hashed tree over the working copy. This means that every working file and directory has an entry in the index, that contains its path and hash and validity data. The validity data is a timestamp plus the file size. The file hashes are sha256's of the file's content. It also contains the fileid to track moved files.

There are two entry types, a file entry and a directory entry. Both have a common binary format (see Item). The on-disk format is best described by the section Index format below.

For each file, the index has a copy of the file's last modification timestamp taken at the instant when the hash has been computed. This means that when file size and timestamp of a file in working tree matches those in the index, we assume that the hash stored in the index for given file is valid. These hashes are then exposed in the resulting Tree object, and can be leveraged by eg. diffTrees to compare many files quickly.

You may have noticed that we also keep hashes of directories. These are assumed to be valid whenever the complete subtree has been valid. At any point, as soon as a size or timestamp mismatch is found, the working file in question is opened, its hash (and timestamp and size) is recomputed and updated in-place in the index file (everything lives at a fixed offset and is fixed size, so this isn't an issue). This is also true of directories: when a file in a directory changes hash, this triggers recomputation of all of its parent directory hashes; moreover this is done efficiently -- each directory is updated at most once during an update run.

Endianness

Since version 6 (magic == HSI6), the file format depends on the endianness of the architecture. To account for the (rare) case where darcs executables from different architectures operate on the same repo, we make an additional check in indexFormatValid to detect whether the file's endianness differs from what we expect. If this is detected, the file is considered invalid and will be re-created.

Index format

The index starts with a header consisting of a 4 bytes magic word, followed by a 4 byte word to indicate the endianness of the encoding. This word should, when read directly from the mmapped file, be equal to 1. After the header comes the actual content of the index, which is organised into "lines" where each line describes a single indexed item. It consists of

• size: item size, 8 bytes
• aux: timestamp (for file) or offset to sibling (for dir), 8 bytes
• fileid: inode or fhandle of the item, 8 bytes
• hash: sha256 of content, 32 bytes
• descriptor length: >= 2 due to type and null, 4 bytes
• descriptor:
• type: D or F, one byte
• path: flattened path, variable >= 0
• null: terminating null byte

With directories, the aux holds the offset of the next sibling line in the index, so we can efficiently skip reading the whole subtree starting at a given directory (by just seeking aux bytes forward). The lines are pre-ordered with respect to directory structure -- the directory comes first and after it come all its items. Cf. openIndex'.

For files, the aux field holds a timestamp.

Internally, the item is stored as a pointer to the first field (iBase) from which we directly read off the first three fields (size, aux, fileid), and a ByteString for the rest (iHashAndDescriptor), up to but not including the terminating null byte.

Comments by bf:

The null byte terminator seems useless.

We could as well use just a single ByteString to represent an item; or even a single raw pointer, since finalizers are needed only when we copy hash and path back to the program as ByteStrings.

An alternative representation could be to store the fixed-size fields (i.e everything except the path) as an unboxed array of records (structs). The paths would then be stored in a bidirectional map between item indices and paths.