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Most of these are converted to and from local strings in the relevant I/O functions, so that internally the above items are always represented in the local encoding. This restricts UTF-8-aware code to the smallest footprint possible so that the bulk of the code does not need to keep track of what encoding a string is in.

The primary exception to this rule is branch names, which must be preserved as UTF-8 between being read from the dirstate and written to the changelog to avoid transcoding lossage.
These are converted to and from local strings in the relevant I/O functions, so that internally the above items are always represented in the local encoding. This restricts UTF-8-aware code to the smallest footprint possible so that the bulk of the code does not need to keep track of what encoding a string is in.
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== Round-trip conversion ==

Some data, such as branch names, are stored locally as UTF-8, read in for processing, then stored in the repository history as UTF-8 again.

This presents difficulties, as we either need to make sure the dozens of places that handle branch names do so in UTF-8 or we need to avoid conversion loss when converting from the local encoding back to UTF-8. In Mercurial post-1.8, this is facilitated by the `encoding.localstr` class returned by `tolocal` which caches the original UTF-8 version of a string alongside its local encoding. The `fromlocal` function can retrieve this string if it's available, which allows lossless round-trip conversion.

Encoding Strategy

How encoding works in the Mercurial codebase.

/!\ This page is intended for developers.

1. Overview

There are three types of string used in Mercurial:

  • byte string in unknown encoding (tracked data)
  • byte string in local encoding (messages, user input)
  • byte string in UTF-8 encoding (repository metadata)

This page sorts out which type of string can be expected where on disk and in the code and what functions manipulate them.

2. Platform issues

2.1. Linux and Unix

  • kernel and native filesystems are encoding-transparent
  • filesystem APIs are UTF-8-compatible, but accept arbitrary encodings
  • multiple encodings can exist for file names on the same system
  • UTF-8 is the defacto standard console and text file on modern systems, though other encodings are still common
  • common tools like 'make(1)' are designed to assume file encoding and contents match, so UTF-8 filenames in files will be used to find UTF-8 filenames on disk

2.2. Mac OS X

  • kernel is encoding-transparent
  • native HFS+ filesystem encodes filenames in a non-standard normalized form of UTF-8
  • filesystem uses an Unicode-aware case-folding algorithm by default
  • kernel I/O interfaces accept UTF-8 filenames
  • resulting filenames on disk may not match (strcmp) with the names supplied at creation time due to normalization
  • Most tools assume file encoding and contents match
  • UTF-8 is the defacto standard console and text file encoding
  • Legacy tools may use MacRoman

2.3. Windows

  • kernel has several different incompatible 8-bit encoding regimes:
    • default encoding used in the GUI
    • default encoding used in the filesystem
    • default (legacy) encoding used in the console (cmd.exe)
  • kernel has a mix of byte-width and wide character APIs
  • kernel and console environment have basically no support for UTF-8 filename I/O or character display
  • in some circumstances, kernel will accept non-ASCII filenames, list them as different names,
    • and fail to open the file under the original name
  • filesystem uses a highly-obscure Unicode-aware case-folding algorithm by default
  • Many tools attempt to do transcoding of file contents from the local encoding to UTF-16 before passing it off to the filesystem
  • UTF-16 text files are occasionally found
  • Wide character encodings like Shift-JIS cause trouble here because they make "\" byte ambiguous

2.4. Web and XML

  • URLs are encoded in ASCII with %-escaping to ISO-Latin, according to RFC1738
  • translation of URLs to filesystem paths is webserver-dependent
  • HTML defaults to ISO-Latin, but may contain encoding specifiers or Unicode entities
  • XML assumes a subset of UTF-8

2.5. Mercurial assumptions

  • non-ASCII filenames are not reliably portable between systems in general
  • the "makefile issue" (matching of file content and name encoding) means that in general, we must attempt to preserve filename encoding
  • On Windows, we prefer the 8-bit encoding of the GUI environment to that of the console to be compatible with typical editors

3. Unknown byte strings

The following are explicitly treated as binary data in an unknown encoding:

  • file contents
  • file names

These items should be treated as binary data and preserved losslessly wherever possible. Generally speaking, it is impossible to reliably and uniquely identify file type and encoding, thus Mercurial does not attempt to distinguish 'binary' files from 'text' files when storing them and instead aims to always preserve them exactly.

Similarly, for historical reasons, non-ASCII filenames are not necessarily portable from Unix to Windows, and Mercurial does not attempt to 'solve' this problem with transcoding either.

In general, do not attempt to transcode such data to Unicode and back in Mercurial, it will result in data loss.

4. UTF-8 strings

UTF-8 strings are used to store most repository metadata. Unlike repository contents, repository metadata is 'owned and managed' by Mercurial and can be made to conform to its rules. In particular, this includes:

  • commit messages stored in the changelog
  • user names
  • tags
  • branches

The following files are stored in UTF-8:

  • .hgtags
  • .hg/branch
  • .hg/branchheads.cache
  • .hg/tags.cache
  • .hg/bookmarks

These are converted to and from local strings in the relevant I/O functions, so that internally the above items are always represented in the local encoding. This restricts UTF-8-aware code to the smallest footprint possible so that the bulk of the code does not need to keep track of what encoding a string is in.

5. Local strings

Strings not mentioned above are generally assumed to be in the local charset encoding. This includes:

  • command line arguments
  • configuration files like .hgrc

  • prompt input
  • commit message
  • .hg/localtags

All user input in the form of command line arguments, configuration files, etc. are assumed to be in the local encoding.

5.1. Internal messages

All internal messages are written in ASCII, which is assumed to be a subset of the local encoding. Where localized string data is available, these strings are translated to the local encoding via gettext.

6. Mixing output

Mercurial frequently mixes output of all three varieties. For instance, the output of 'hg log -p' will contain internal strings in local encoding to mark fields, UTF-8 metadata, and file contents in an unknown encoding. These are managed as follows:

  • UTF-8 data is converted to local encoding at the earliest opportunity, generally at read time
  • internal ASCII strings are translated to local encoding via gettext() or passed unmodified
  • data in unknown encoding (file contents and filenames) are treated as already being in the local encoding for I/O purposes
  • resulting strings are combined with typical string formatting and I/O operations
  • raw binary output is used with no additional transcoding

Thus, the vast bulk of string operations in Mercurial are done as if they were operating on local strings.

As an example, attempts to view a patch containing UTF-8 characters on a non-UTF-8 terminal may not be entirely human-readable, but the generated patch will be correct in the sense that a standard patch tool will be able to apply it and get the right UTF-8 characters in the result. Similarly, 'hg cat' of a binary file will output an exact copy of the binary file, regardless of the current encoding.

7. Functions

The encoding module defines the following functions:

  • fromlocal(): converts a string from the local encoding to UTF-8 for storage with validation

  • tolocal(): converts string stored as UTF-8 to the local encoding replacing unknown glyphs

  • colwidth(): calculate the width of a local string in terminal columns

Also, encoding.encoding specifies Mercurial's idea of what the current encoding is.

8. Round-trip conversion

Some data, such as branch names, are stored locally as UTF-8, read in for processing, then stored in the repository history as UTF-8 again.

This presents difficulties, as we either need to make sure the dozens of places that handle branch names do so in UTF-8 or we need to avoid conversion loss when converting from the local encoding back to UTF-8. In Mercurial post-1.8, this is facilitated by the encoding.localstr class returned by tolocal which caches the original UTF-8 version of a string alongside its local encoding. The fromlocal function can retrieve this string if it's available, which allows lossless round-trip conversion.

9. Unicode strings

Python Unicode objects are only used in the implementation of the above functions and are carefully avoided elsewhere. Do not pass Unicode objects to any Mercurial APIs. Due to Python's misguided automatic Unicode to byte string conversion, Unicode objects are likely to work in testing, but break as soon as they encounter a non-ASCII character.

10. Historical note

Early versions of Mercurial made no effort to transcode metadata, so the tolocal() function has some fallbacks to allow guessing the encoding of strings that don't appear to be Unicode.


CategoryInternals

EncodingStrategy (last edited 2012-12-03 16:20:41 by mpm)