Codec.Archive.Tar.Index

tar-0.4.2.1: Reading, writing and manipulating ".tar" archive files.

Copyright | (c) 2010-2015 Duncan Coutts
License | BSD3
Maintainer | duncan@community.haskell.org
Portability | portable
Safe Haskell | None
Language | Haskell98

Codec.Archive.Tar.Index

Contents

- Index type
- Index lookup
  - I/O operations
- Index construction
  - Incremental construction
- Serialising indexes
- Lower level operations with offsets and I/O on tar files

Description

Random access to the content of a `.tar` archive.

This module uses common names and so is designed to be imported qualified:

    import qualified Codec.Archive.Tar.Index as Tar

Synopsis

- data TarIndex
- lookup :: TarIndex -> FilePath -> Maybe TarIndexEntry
- data TarIndexEntry
  - = TarFileEntry !TarEntryOffset
  - | TarDir [(FilePath, TarIndexEntry)]
- type TarEntryOffset = Word32
- hReadEntry :: Handle -> TarEntryOffset -> IO Entry
- hReadEntryHeader :: Handle -> TarEntryOffset -> IO Entry
- build :: Entries e -> Either e TarIndex
- data IndexBuilder
- emptyIndex :: IndexBuilder
- addNextEntry :: Entry -> IndexBuilder -> IndexBuilder
- skipNextEntry :: Entry -> IndexBuilder -> IndexBuilder
- finaliseIndex :: IndexBuilder -> TarIndex
- serialise :: TarIndex -> Builder
- deserialise :: ByteString -> Maybe (TarIndex, ByteString)
- hReadEntryHeaderOrEof :: Handle -> TarEntryOffset -> IO (Maybe (Entry, TarEntryOffset))
- hSeekEntryOffset :: Handle -> TarEntryOffset -> IO ()
- hSeekEntryContentOffset :: Handle -> TarEntryOffset -> IO ()
- hSeekEndEntryOffset :: Handle -> Maybe TarIndex -> IO TarEntryOffset
- nextEntryOffset :: Entry -> TarEntryOffset -> TarEntryOffset
- indexEndEntryOffset :: TarIndex -> TarEntryOffset
- indexNextEntryOffset :: IndexBuilder -> TarEntryOffset

# Documentation

The `tar` format does not contain an index of files within the archive. Normally, `tar` file have to be processed linearly. It is sometimes useful however to be able to get random access to files within the archive.

This module provides an index of a `tar` file. A linear pass of the `tar` file is needed to `build` the `TarIndex`, but thereafter you can `lookup` paths in the `tar` file, and then use `hReadEntry` to seek to the right part of the file and read the entry.

# Index type

data TarIndex

An index of the entries in a tar file.

This index type is designed to be quite compact and suitable to store either on disk or in memory.

Instances

Eq TarIndex |
Show TarIndex |

# Index lookup

lookup :: TarIndex -> FilePath -> Maybe TarIndexEntry

Look up a given filepath in the `TarIndex`. It may return a `TarFileEntry` containing the `TarEntryOffset` of the file within the tar file, or if the filepath identifies a directory then it returns a `TarDir` containing the list of files within that directory.

Given the `TarEntryOffset` you can then use one of the I/O operations:

- `hReadEntry` to read the whole entry;
- `hReadEntryHeader` to read just the file metadata (e.g. its length);

data TarIndexEntry

The result of `lookup` in a `TarIndex`. It can either be a file directly, or a directory entry containing further entries (and all subdirectories recursively). Note that the subtrees are constructed lazily, so it's cheaper if you don't look at them.

Constructors

TarFileEntry !TarEntryOffset |
TarDir [(FilePath, TarIndexEntry)] |

Instances

Show TarIndexEntry |

## I/O operations

type TarEntryOffset = Word32

An offset within a tar file. Use `hReadEntry`, `hReadEntryHeader` or `hSeekEntryOffset`.

This is actually a tar "record" number, not a byte offset.

hReadEntry :: Handle -> TarEntryOffset -> IO Entry

Reads an entire `Entry` at the given `TarEntryOffset` in the tar file. The `Handle` must be open for reading and be seekable.

This reads the whole entry into memory strictly, not incrementally. For more control, use `hReadEntryHeader` and then read the entry content manually.

hReadEntryHeader :: Handle -> TarEntryOffset -> IO Entry

Read the header for a `Entry` at the given `TarEntryOffset` in the tar file. The `entryContent` will contain the correct metadata but an empty file content. The `Handle` must be open for reading and be seekable.

The `Handle` position is advanced to the beginning of the entry content (if any). You must check the `entryContent` to see if the entry is of type `NormalFile`. If it is, the `NormalFile` gives the content length and you are free to read this much data from the `Handle`.

    entry <- Tar.hReadEntryHeader hnd
    case Tar.entryContent entry of
      Tar.NormalFile _ size -> do content <- BS.hGet hnd size
                                  ...

Of course you don't have to read it all in one go (as `hReadEntry` does), you can use any appropriate method to read it incrementally.

In addition to I/O errors, this can throw a `FormatError` if the offset is wrong, or if the file is not valid tar format.

There is also the lower level operation `hSeekEntryOffset`.

# Index construction

build :: Entries e -> Either e TarIndex

Build a `TarIndex` from a sequence of tar `Entries`. The `Entries` are assumed to start at offset `0` within a file.

## Incremental construction

If you need more control than `build` then you can construct the index in an acumulator style using the `IndexBuilder` and operations.

Start with the `emptyIndex` and use `addNextEntry` (or `skipNextEntry`) for each `Entry` in the tar file in order. Every entry must added or skipped in order, otherwise the resulting `TarIndex` will report the wrong `TarEntryOffset`s. At the end use `finaliseIndex` to get the `TarIndex`.

For example, `build` is simply:

    build = go emptyIndex
      where
        go !builder (Next e es) = go (addNextEntry e builder) es
        go !builder  Done       = Right $! finaliseIndex builder
        go !_       (Fail err)  = Left err

data IndexBuilder

The intermediate type used for incremental construction of a `TarIndex`.

emptyIndex :: IndexBuilder

The initial empty `IndexBuilder`.

addNextEntry :: Entry -> IndexBuilder -> IndexBuilder

Add the next `Entry` into the `IndexBuilder`.

skipNextEntry :: Entry -> IndexBuilder -> IndexBuilder

Use this function if you want to skip some entries and not add them to the final `TarIndex`.

finaliseIndex :: IndexBuilder -> TarIndex

Finish accumulating `Entry` information and build the compact `TarIndex` lookup structure.

# Serialising indexes

serialise :: TarIndex -> Builder

The `TarIndex` is compact in memory, and it has a similarly compact external representation.

deserialise :: ByteString -> Maybe (TarIndex, ByteString)

Read the external representation back into a `TarIndex`.

# Lower level operations with offsets and I/O on tar files

hReadEntryHeaderOrEof :: Handle -> TarEntryOffset -> IO (Maybe (Entry, TarEntryOffset))

This is a low level variant on `hReadEntryHeader`, that can be used to iterate through a tar file, entry by entry.

It has a few differences compared to `hReadEntryHeader`:

- It returns an indication when the end of the tar file is reached.
- It does not move the `Handle` position to the beginning of the entry content.
- It returns the `TarEntryOffset` of the next entry.

After this action, the `Handle` position is not in any useful place. If you want to skip to the next entry, take the `TarEntryOffset` returned and use `hReadEntryHeaderOrEof` again. Or if having inspected the `Entry` header you want to read the entry content (if it has one) then use `hSeekEntryContentOffset` on the original input `TarEntryOffset`.

hSeekEntryOffset :: Handle -> TarEntryOffset -> IO ()

Set the `Handle` position to the position corresponding to the given `TarEntryOffset`.

This position is where the entry metadata can be read. If you already know the entry has a body (and perhaps know it's length), you may wish to seek to the body content directly using `hSeekEntryContentOffset`.

hSeekEntryContentOffset :: Handle -> TarEntryOffset -> IO ()

Set the `Handle` position to the entry content position corresponding to the given `TarEntryOffset`.

This position is where the entry content can be read using ordinary I/O operations (though you have to know in advance how big the entry content is). This is only valid if you already know the entry has a body (i.e. is a normal file).

hSeekEndEntryOffset :: Handle -> Maybe TarIndex -> IO TarEntryOffset

Seek to the end of a tar file, to the position where new entries can be appended, and return that `TarEntryOffset`.

If you have a valid `TarIndex` for this tar file then you should supply it because it allows seeking directly to the correct location.

If you do not have an index, then this becomes an expensive linear operation because we have to read each tar entry header from the beginning to find the location immediately after the last entry (this is because tar files have a variable length trailer and we cannot reliably find that by starting at the end). In this mode, it will fail with an exception if the file is not in fact in the tar format.

nextEntryOffset :: Entry -> TarEntryOffset -> TarEntryOffset

Calculate the `TarEntryOffset` of the next entry, given the size and offset of the current entry.

This is much like using `skipNextEntry` and `indexNextEntryOffset`, but without using an `IndexBuilder`.

indexEndEntryOffset :: TarIndex -> TarEntryOffset

This is the offset immediately following the last entry in the tar file. This can be useful to append further entries into the tar file. Use with `hSeekEntryOffset`, or just use `hSeekEndEntryOffset` directly.

indexNextEntryOffset :: IndexBuilder -> TarEntryOffset

This is the offset immediately following the entry most recently added to the `IndexBuilder`. You might use this if you need to know the offsets but don't want to use the `TarIndex` lookup structure. Use with `hSeekEntryOffset`. See also `nextEntryOffset`.