--- --- vfs-fsapi - Tcl-level Virtual Filesystems
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vfs-fsapi(n) 1.0 "Tcl-level Virtual Filesystems"

Name

vfs-fsapi - API for the implementation of a filesystem in Tcl

Description

This document explains the API used by the package vfs to communicate with filesystem implementations written in tcl.

HANDLER OVERVIEW

The package vfs intercepts every filesystem operation which falls within a given mount point, and passes the operation on to the mount point's vfshandler command in the interpreter which registered it.

If the handler takes appropriate action for each of the cases it is called for, a complete, perfect virtual filesystem will be achieved, indistinguishable to Tcl from the native filesystem. (CAVEATS: Right now vfs does not expose to Tcl all the permission-related flags of glob).

vfshandler subcmd root relative actualpath args...

The first argument specifies the operation to perform on behalf of the filesystem code in the tcl core, the remainder specify the file path on which to operate, in different forms, and parts, and any additional arguments which may be required to carry out the action.

To demonstrate the treatment of a path by the generic layer we use "C:/foo/bar/mount.zip/xxx/yyy" as an example and additionally assume that the following conditions are true:

  1. "mount.zip" is a zip archive which has been mounted on top of itself,

  2. said zip archive contains a file with path "xxx/yyy",

  3. the current working directory of the application is inside of directory "xxx",

  4. and the command executed is file exists yyy.

The file separator between root and relative is omitted. Most filesystem operations need only the relative argument for their correct operation, but some actually require the other parts of the path.

subcmd

This argument of the handler can be one of the following access, createdirectory, deletefile, fileattributes, matchindirectory, open, removedirectory, stat, or utime.

The generic layer expects that the subcommands of a handler signal error conditions by calling vfs::filesystem posixerror with the appropriate posix error code instead of throwing a tcl error. If the latter is done nevertheless it will be treated as an unknown posix error.

There are three exceptions to the rule above: If any of open (when an interpreter is given), matchindirectory, and fileattributes (for a set or get operation only) throw a tcl error, this error will be passed up to the caller of the filesystem command which invoked the handler. Note that this does not preclude the ability of these subcommands to use the command vfs::filesystem posixerror to report more regular filesystem errors.

root

Part of the specification of the path to operate upon. It contains the part of the path which lies outside this filesystem's mount point. For example outlined above its value will be "C:/foo/bar/mount.zip".

relative

Part of the specification of the path to operate upon. It contains the part of the path which lies inside this filesystem. For example outlined above its value will be "xxx/yyy".

actualpath

Part of the specification of the path to operate upon. It contains the original (unnormalized) name of the path which was used in the current command wherever it originated (in Tcl or C). For example outlined above its value will be "yyy".

HANDLER METHODS

vfshandler access root relative actualpath mode

Signal a posix error if the specified access mode (an integer number) is not compatible with the file or directory described by the path. The generic layer will ignore any non-empty return value.

The command vfs::accessMode (see section HANDLER ENVIRONMENT) can be used to convert the integer mode into an easier to check string value.

vfshandler createdirectory root relative actualpath

Create a directory with the given name. The command can assume that all sub-directories in the path exist and are valid, and that the actual desired path does not yet exist (Tcl takes care of all of that for us).

vfshandler deletefile root relative actualpath

Delete the given file.

vfshandler fileattributes root relative actualpath ?index? ?value?

The command has to return a list containing the names of all acceptable attributes, if neither index nor value were specified.

The command has to return the value of the index'th attribute if the index is specified, but not the value. The attributes are counted in the same order as their names appear in the list returned by a call where neither index nor value were specified. The first attribute is has the index 0.

The command has to set the value of the index'th attribute to value if both index and value were specified for the call.

vfshandler matchindirectory root relative actualpath pattern types

Return the list of files or directories in the given path which match the glob pattern and are compatible with the specified list of types. The specified path is always the name of an existing directory.

Note: As Tcl generates requests for directory-only matches from the filesystems involved when performing any type of recursive globbing this subcommand absolutely has to handle such (and file-only) requests correctly or bad things (TM) will happen.

The commands vfs::matchDirectories and vfs::matchFiles (see section HANDLER ENVIRONMENT) can aid the implementation greatly in this task.

vfshandler open root relative actualpath mode permissions

Either returns a list describing the successfully opened file, or throws an error describing how the operation failed.

The list returned upon success contains at least one and at most two elements. The first, obligatory, element is always the handle of the channel which was created to allow access to the contents of the file.

If specified the second element will be interpreted as a callback, i.e. a command prefix. This prefix will always be executed as is, i.e. without additional arguments. Any required arguments have to be returned as part of the result of the call to open.

If present the specified callback will be evaluated just before the channel is closed by the generic filesystem layer. The callback itself must not call close.

The channel however is live enough to allow seek and read operations. In addition all available data will have been flushed into it already. This means, for example, that the callback can seek to the beginning of the said channel, read its contents and then store the gathered data elsewhere. In other words, this callback is not only crucial to the cleanup of any resources associated with an opened file, but also for the ability to implement a filesystem which can be written to.

Under normal circumstances return code and any errors thrown by the callback itself are ignored. In that case errors have to be signaled asychronously, for example by calling bgerror. However if, through a call of the subcommand internalerror, an error handling script has been specified for the file system, all errors thrown here will be passed to that script for further action.

mode

can be any of r, w, a, w+, or a+.

permissions

determines the native mode the openend file is created with. Relevant only of the open mode actually requests the creation of a non-existing file, i.e. is not r.

vfshandler removedirectory root relative actualpath recursive

Delete the given directory. Argument recursive is a boolean. If the specified value is true then even if the directory is non-empty, an attempt has to be made to recursively delete it and its contents. If the spcified value is false and the directory is non-empty, a posix error (EEXIST) has to be thrown.

vfshandler stat root relative actualpath

The result has to be a list of keys and values, in a format acceptable to the builtin command array set. It describes the contents of a stat structure. The order of the keys in the list is not important.

Given this the subcommand should use something like

return [list dev 0 type file mtime 1234 ...].

as the last command of its implementation.

The following keys and their values have to be supplied by the filesystem:

dev

A long integer number, the device number of the path stat was called for.

ino

A long integer number, the inode number of the path stat was called for. Each path handled by the filesystem should be uniquely identified by the combination of device and inode number. Violating this principle will cause higher-level algorithms which(have to) keep track of device and inode information to fail in all manners possible.

An example of such an algorithm would be a directory walker using device/inode information to keep itself out of infinite loops generated through symbolic links. Returning non-unique device/inode information will most likely cause such a walker to skip over paths under the wrong assumption of having them seen already.

mode

An integer number, the access mode of the path. It is this mode which is checked by the subcommand access.

nlink

A long integer number, the number of hard links to the specified path.

uid

A long integer number, the id of the user owning the virtual path.

gid

A long integer number, the id of the user group the virtual path belongs to.

size

A long integer number, the true size of the virtual path, in bytes.

atime

A long integer number, the time of the latest access to the path, in seconds since the epoch. Convertible into a readable date/time by the command clock format.

mtime

A long integer number, the time of the latest modification of the path, in seconds since the epoch. Convertible into a readable date/time by the command clock format.

ctime

A long integer number, the time of the path was created, in seconds since the epoch. Convertible into a readable date/time by the command clock format.

type

A string, either directory, or file, describing the type of the given path.

vfshandler utime root relative actualpath actime mtime

Set the access and modification times of the given file (these are read with stat).

HANDLER ENVIRONMENT

The implementation of a filesystem handler can rely on the existence of the following utility commands:

vfs::accessMode mode

This commands converts an access mode given as integer into a string, one of F, X, W, XW, R, RX, and RW.

vfs::matchDirectories types

Checks if the glob types specification ask for the inclusion of directories. Returns a boolean result. true is returned if types does ask for directories, else false.

vfs::matchFiles types

Checks if the glob types specification ask for the inclusion of files. Returns a boolean result. true is returned if types does ask for directories, else false.

vfs::matchCorrectTypes types filelist ?inDir?

Returns that subset of the filelist which are compatible with the types given. The elements of filelist are either absolute paths, or names of files in the directory indir. The latter interpretation is taken if and only if the argument indir is specified.

FILESYSTEM DEBUGGING

To debug a problem in the implementation of a filesystem use code as shown below. This registers the command report as the error handler for the filesystem, which in turn prints out the error stack provided by tcl.

vfs::filesystem internalerror report
proc report {} {
    puts stderr $::errorInfo
}

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