'fncache' Repository Format
fncache is a repository layout (or format) for Mercurial that reorganized the revlog data file names (and directory names) inside the store to work around some nasty file name limitations of Windows (long path names, names reserved by Windows).
Contents
1. Details and usage
The fncache layout was first released with Mercurial 1.1 and bugfixed in Mercurial 1.1.1 (see WhatsNew).
The "format" is not really new, since the contents of the files in the store did not change. We just changed where we store the bits - not how we store them.
Since this was a backwards-incompatible change in the way the files in the store are named, we introduced an new format specifier ('fncache') in the requires file, thus telling old versions of Mercurial that it should keep its fingers off from new 'fncache' repositories (since we know those old versions of Mercurial won't know how to find the files in the store).
With this change, all newly created repositories on all platforms will be fncache repositories. You don't have to do anything (besides using a version of Mercurial containing this change).
The new layout does not affect the wire (or bundle) protocol(s) in any way. So you can push/pull/clone over the wire to/from any repo being in any layout using any Mercurial version on both ends.
New repositories are for example created by non-hardlink cloning of existing repos or if you clone over the wire (http, ssh).
For example, if you have a (pre 1.1) non-fncache repo and you do a local clone --pull you will end up with an fncache repo. If you do a plain local clone (without --pull) of a non-fncache repo, you will get a non-fncache repo with hardlinks to the existing repo.
In short, use clone --pull to convert repos (in case you want to convert repos to the fncache repo format, which will almost never be needed).
Of course old versions of Mercurial will not be able to read fncache repos. If you try to access an fncache repo with a version of Mercurial prior to 1.1 it will abort with:
abort: requirement 'fncache' not supported!
which tells you that the repo at hand requires knowledge of the fncache repo format in Mercurial.
(BTW, if, for whatever reason, the fnache file in the repo becomes corrupted, you can do a clone --pull to rebuild it. The fncache file contains a list of all revlog files in the repo).
Existing non-fncache repositories, that is, repositories created with Mercurial 1.0 (or older), will remain as they are, as Mercurial will still be able to read and write non-fncache repositories.
The fncache repo format can be disabled with
[format] usefncache = False
in the hgrc (see http://www.selenic.com/mercurial/hgrc.5.html#format) or with --config format.usefncache=0 on the command line. For example, the command
hg --config format.usefncache=0 clone --pull A B
converts the local fncache repo A to non-fncache repo B.
2. New entry 'fncache' in the requires file
Mercurial writes a file named 'requires' in the .hg directory when creating a new repository (see RequiresFile). For an fncache repository, the requires file contains:
revlogv1 store fncache
In a pre-fncache repository, the entry 'fncache' in the requires file is missing.
3. Encoding of Windows reserved names
Path elements consisting of Windows reserved names are now encoded using ~xx where xx is the two digit ASCII hex code of the third character of that reserved name. For example "aux" is encoded as "au~78".
Windows reserved names are: 'con', 'prn', 'aux', 'nul', 'com1'..'com9' and 'lpt1'..'lpt9'.
For example the path
data/aux.bla/bla.aux/prn/PRN/lpt/com3/nul/coma/foo.NUL/normal.c.i
is encoded as
data/au~78.bla/bla.aux/pr~6e/_p_r_n/lpt/co~6d3/nu~6c/coma/foo._n_u_l/normal.c.i
Note that 'aux.bla' needs to be encoded, but not 'bla.aux'.
4. Hashing of long paths
Paths inside the store that would be longer than 120 chars are now hash encoded.
For the encoding used see the function mercurial.store.hybridencode.
Some encoding examples for paths that are hashed (A1→B1, A2→B2, ...):
(A1) data/AUX/SECOND/X.PRN/FOURTH/FI:FTH/SIXTH/SEVENTH/EIGHTH/NINETH/TENTH/ELEVENTH/LOREMIPSUM.TXT.i (B1) dh/au~78/second/x.prn/fourth/fi~3afth/sixth/seventh/eighth/nineth/tenth/loremia20419e358ddff1bf8751e38288aff1d7c32ec05.i (A2) data/enterprise/openesbaddons/contrib-imola/corba-bc/netbeansplugin/wsdlExtension/src/main/java/META-INF/services/org.netbeans.modules.xml.wsdl.bindingsupport.spi.ExtensibilityElementTemplateProvider.i (B2) dh/enterpri/openesba/contrib-/corba-bc/netbeans/wsdlexte/src/main/java/org.net7018f27961fdf338a598a40c4683429e7ffb9743.i (A3) data/AUX.THE-QUICK-BROWN-FOX-JU:MPS-OVER-THE-LAZY-DOG-THE-QUICK-BROWN-FOX-JUMPS-OVER-THE-LAZY-DOG.TXT.i (B3) dh/au~78.the-quick-brown-fox-ju~3amps-over-the-lazy-dog-the-quick-brown-fox-jud4dcadd033000ab2b26eb66bae1906bcb15d4a70.i
All paths that are hashed are stored in the directory 'dh' inside '.hg/store'. Non-hashed paths are stored inside '.hg/store/data'.
The hashing used is the sha1 digest (40 characters) of the direncoded path below '.hg/store', as pre-encoded by mercurial.filelog.encodedir.
For the hashencoded path, the first eight characters of the first n directory levels are taken (converted to lowercase), where n is adapted slightly to use more levels if space allows (see store.hybridencode). If space allows, the filename before the hash value is filled up with to lowercase converted chars from the filename of the input path.
As you can see, the path encoding done may fold multiple files originating from different input path directories into the same encoded path directory. The sha1 digest part of the filename ensures that the filenames are distinct and no name clashes occur.
5. The fncache file
For the fncache repository format Mercurial maintains a new file 'fncache' (thus the name of the format) inside '.hg/store'. The fncache file contains the paths of all filelog files in the store as encoded by mercurial.filelog.encodedir. The paths are separated by '\n' (LF).
The fncache file is used to enumerate all filelog files in the store, for example when doing a clone --uncompressed. The fncache file may contain duplicates or inexistent entries (this can happen when using the strip or rollback commmands).
During a clone --uncompressed or a hg verify the fncache file is read and rewritten if duplicates or entries with missing filelog files are detected, so even operations that don't modify the history of the repository may lead to modifying the fncache file (this was a deliberate design decision as discussed with mpm).
The fncache file is not read by a hg clone --pull, so that command may be used to resurrect a damaged fncache file, since hg clone --pull rewrites the fncache file from the information found in all manifest revisions. That's also the reason why it is basically cached information.
The verify command checks the fncache file and removes non-existent or duplicate entries. If a filelog file referenced in a manifest revision is not found in the fncache file, hg verify reports an error.