Codec.Archive.Tar.Index

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

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 TarEntryOffsets. 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.