cs.sharedfile 20230212

facilities for shared access to files

Facilities for shared access to files.

Latest release 20230212: Remove dependency on cs.logutils.

Function lockfile(path, ext=None, poll_interval=None, timeout=None)

A context manager which takes and holds a lock file.

Parameters:

• path: the base associated with the lock file.
• ext: the extension to the base used to construct the lock file name. Default: ".lock"
• timeout: maximum time to wait before failing, default None (wait forever).
• poll_interval: polling frequency when timeout is not 0.

Class SharedAppendFile

A base class to share a modifiable file between multiple users.

The use case was driven from the shared CSV files used by cs.nodedb.csvdb.Backend_CSVFile, where multiple users can read from a common CSV file, and coordinate updates with a lock file.

This presents the following interfaces:

• __iter__: yields data chunks from the underlying file up to EOF; it blocks no more than reading from the file does. Note that multiple iterators share the same read pointer.

• open: a context manager returning a writable file for writing updates to the file; it blocks reads from this instance (though not, of course, by other users of the file) and arranges that users of __iter__ do not receive their own written data, thus arranging that __iter__ returns only foreign file updates.

Subclasses would normally override __iter__ to parse the received data into their natural records.

Method SharedAppendFile.__init__(self, pathname, read_only=False, write_only=False, binary=False, newline=None, lock_ext=None, lock_timeout=None, poll_interval=None): Initialise this SharedAppendFile.

Parameters:

• pathname: the pathname of the file to open.
• read_only: set to true if we will not write updates.
• write_only: set to true if we will not read updates.
• binary: if the file is to be opened in binary mode, otherwise text mode.
• 'newline: passed to open()`
• lock_ext: lock file extension.
• lock_timeout: maxmimum time to wait for obtaining the lock file.
• poll_interval: poll time when taking a lock file, default DEFAULT_POLL_INTERVAL

Class SharedAppendLines(SharedAppendFile)

A line oriented subclass of SharedAppendFile.

Class SharedCSVFile(SharedAppendLines, SharedAppendFile)

Shared access to a CSV file in UTF-8 encoding.

Class SharedWriteable

Wrapper for a writable file with supported mutex based cooperation.

This is mostly a proxy for the wrapped file exceptthat all .write calls are serialised and when used as a context manager other writers are blocked.

This is to support shared use of an output stream where certain outputs should be contiguous, such as a standard error stream used to maintain a status line or multiline messages.

Release Log

Release 20230212: Remove dependency on cs.logutils.

Release 20211208: Update import.

Release 20201228: New SharedWriteable class to manage concurrent output with a mutexed .write method and a context manager for grouping larger uses.

Release 20190102: Context manager bugfix.

Release 20170608:

• Facilities for shared files, split out from cs.fileutils.
• SharedAppend* classes. lockfile function.