convertSize(size, fromUnit, toUnit)

Converts a size in one unit to a size in another unit.

This is just a convenience function so that the functionality can be implemented in just one place. Internally, we convert values to bytes and then to the final unit.

• UNIT_BYTES - Bytes
• UNIT_KBYTES - Kilobytes, where 1kB = 1024B
• UNIT_MBYTES - Megabytes, where 1MB = 1024kB
• UNIT_SECTORS - Sectors, where 1 sector = 2048B

Parameters:

size - Size to convert
           (type=Integer or float value in units of fromUnit)

fromUnit - Unit to convert from
           (type=One of the units listed above)

toUnit - Unit to convert to
           (type=One of the units listed above)

Returns:

Number converted to new unit, as a float.

Raises:

ValueError - If one of the units is invalid.

getUidGid(user, group)

Parameters:

user - User name
           (type=User name as a string)

group - Group name
           (type=Group name as a string)

Returns:

Tuple (uid, gid) matching passed-in user and group.

Raises:

ValueError - If the ownership user/group values are invalid

splitCommandLine(commandLine)

Splits a command line string into a list of arguments.

Unfortunately, there is no "standard" way to parse a command line string, and it's actually not an easy problem to solve portably (essentially, we have to emulate the shell argument-processing logic). This code only respects double quotes (") for grouping arguments, not single quotes ('). Make sure you take this into account when building your command line.

Parameters:

commandLine - Command line string
           (type=String, i.e. "cback --verbose stage store")

Returns:

List of arguments, suitable for passing to popen2.Popen4.

executeCommand(command, args, returnOutput=False, ignoreStderr=False)

Executes a shell command, hopefully in a safe way (UNIX-specific).

This function exists to replace direct calls to os.popen() in the Cedar Backup code. It's not safe to call a function such as os.popen() with untrusted arguments, since that can cause problems if the string contains non-safe variables or other constructs (imagine that the argument is $WHATEVER, but $WHATEVER contains something like "; rm -fR ~/; echo" in the current environment).

It's safer to use popen4 (or popen2 or popen3) and pass a list rather than a string for the first argument. When called this way, popen4 will use the list's first item as the command and the remainder of the list's items as arguments to that command.

Under the normal case, this function will return a tuple of (status, None) where the status is the wait-encoded return status of the call per the popen2.Popen4 documentation. If returnOutput is passed in as True, the function will return a tuple of (status, output) where output is a list of strings, one entry per line in the output from the command. Output is always logged to the ouputLogger.info() target, regardless of whether it's returned.

Parameters:

command - Shell command to execute
           (type=List of individual arguments that make up the command)

args - List of arguments to the command
           (type=List of additional arguments to the command)

returnOutput - Indicates whether to return the output of the command
           (type=Boolean True or False)

Returns:

Tuple of (result, output) as described above.

Notes:

• I know that it's a bit confusing that the command and the arguments are both lists. I could have just required the caller to pass in one big list. However, I think it makes some sense to keep the command (the constant part of what we're executing, i.e. "scp -B") separate from its arguments, even if they both end up looking kind of similar.
• You cannot redirect output (i.e. 2>&1, 2>/dev/null, etc.) using this function. The redirection string would be passed to the command just like any other argument. However, you can implement 2>/dev/null by using ignoreStderr=True, as discussed above.

calculateFileAge(file)

Calculates the age (in days) of a file.

The "age" of a file is the amount of time since the file was last used, per the most recent of the file's st_atime and st_mtime values.

Parameters:

file - Path to a file on disk.

Returns:

Age of the file in days.

Raises:

OSError - If the file doesn't exist.

encodePath(path)

Safely encodes a filesystem path.

Many Python filesystem functions, such as os.listdir, behave differently if they are passed unicode arguments versus simple string arguments. For instance, os.listdir generally returns unicode path names if it is passed a unicode argument, and string pathnames if it is passed a string argument.

However, this behavior often isn't as consistent as we might like. As an example, os.listdir "gives up" if it finds a filename that it can't properly encode given the current locale settings. This means that the returned list is a mixed set of unicode and simple string paths. This has consequences later, because other filesystem functions like os.path.join will blow up if they are given one string path and one unicode path.

  The operating system (POSIX) does not have the inherent notion that file
  names are character strings. Instead, in POSIX, file names are primarily
  byte strings. There are some bytes which are interpreted as characters
  (e.g. '.', which is '.', or '/', which is '/'), but apart from
  that, most OS layers think these are just bytes.

  Now, most *people* think that file names are character strings.  To
  interpret a file name as a character string, you need to know what the
  encoding is to interpret the file names (which are byte strings) as
  character strings.

  There is, unfortunately, no operating system API to carry the notion of a
  file system encoding. By convention, the locale settings should be used
  to establish this encoding, in particular the LC_CTYPE facet of the
  locale. This is defined in the environment variables LC_CTYPE, LC_ALL,
  and LANG (searched in this order).

  If LANG is not set, the "C" locale is assumed, which uses ASCII as its
  file system encoding. In this locale, '♪♬' is not a
  valid file name (at least it cannot be interpreted as characters, and
  hence not be converted to Unicode).

  Now, your Python script has requested that all file names *should* be
  returned as character (ie. Unicode) strings, but Python cannot comply,
  since there is no way to find out what this byte string means, in terms
  of characters.

  So we have three options:

  1. Skip this string, only return the ones that can be converted to Unicode. 
     Give the user the impression the file does not exist.
  2. Return the string as a byte string
  3. Refuse to listdir altogether, raising an exception (i.e. return nothing)

  Python has chosen alternative 2, allowing the application to implement 1
  or 3 on top of that if it wants to (or come up with other strategies,
  such as user feedback).

Parameters:

path - Path to encode

Returns:

Path, as a string, encoded appropriately

Raises:

ValueError - If the path cannot be encoded properly.

Note: As a special case, if path is None, then this function will return None.

deviceMounted(devicePath)

Indicates whether a specific filesystem device is currently mounted.

Parameters:

devicePath - Path of device to be checked

Returns:

True if device is mounted, false otherwise.

getFunctionReference(module, function)

Gets a reference to a named function.

  myfunc = getFunctionReference("os.path", "isdir")

Although we won't bomb out directly, behavior is pretty much undefined if you pass in None or "" for either module or function.

The only validation we enforce is that whatever we get back must be callable.

Parameters:

module - Name of module associated with function.
           (type=Something like "os.path" or "CedarBackup2.util")

function - Name of function
           (type=Something like "isdir" or "getUidGid")

Returns:

Reference to function associated with name.

Raises:

ImportError - If the function cannot be found.

ValueError - If the resulting reference is not callable.