Filesystem.php
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Created: Sun, Dec 22, 04:53

Filesystem.php
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	<?php

	/**
	* Simple wrapper class for common filesystem tasks like reading and writing
	* files. When things go wrong, this class throws detailed exceptions with
	* good information about what didn't work.
	*
	* Filesystem will resolve relative paths against PWD from the environment.
	* When Filesystem is unable to complete an operation, it throws a
	* FilesystemException.
	*
	* @task directory Directories
	* @task file Files
	* @task path Paths
	* @task exec Executables
	* @task assert Assertions
	*/
	final class Filesystem extends Phobject {


	/* -( Files )-------------------------------------------------------------- */


	/**
	* Read a file in a manner similar to file_get_contents(), but throw detailed
	* exceptions on failure.
	*
	* @param string File path to read. This file must exist and be readable,
	* or an exception will be thrown.
	* @return string Contents of the specified file.
	*
	* @task file
	*/
	public static function readFile($path) {
	$path = self::resolvePath($path);

	self::assertExists($path);
	self::assertIsFile($path);
	self::assertReadable($path);

	$data = @file_get_contents($path);
	if ($data === false) {
	throw new FilesystemException(
	$path,
	pht("Failed to read file '%s'.", $path));
	}

	return $data;
	}

	/**
	* Make assertions about the state of path in preparation for
	* writeFile() and writeFileIfChanged().
	*/
	private static function assertWritableFile($path) {
	$path = self::resolvePath($path);
	$dir = dirname($path);

	self::assertExists($dir);
	self::assertIsDirectory($dir);

	// File either needs to not exist and have a writable parent, or be
	// writable itself.
	$exists = true;
	try {
	self::assertNotExists($path);
	$exists = false;
	} catch (Exception $ex) {
	self::assertWritable($path);
	}

	if (!$exists) {
	self::assertWritable($dir);
	}
	}

	/**
	* Write a file in a manner similar to file_put_contents(), but throw
	* detailed exceptions on failure. If the file already exists, it will be
	* overwritten.
	*
	* @param string File path to write. This file must be writable and its
	* parent directory must exist.
	* @param string Data to write.
	*
	* @task file
	*/
	public static function writeFile($path, $data) {
	self::assertWritableFile($path);

	if (@file_put_contents($path, $data) === false) {
	throw new FilesystemException(
	$path,
	pht("Failed to write file '%s'.", $path));
	}
	}

	/**
	* Write a file in a manner similar to `file_put_contents()`, but only touch
	* the file if the contents are different, and throw detailed exceptions on
	* failure.
	*
	* As this function is used in build steps to update code, if we write a new
	* file, we do so by writing to a temporary file and moving it into place.
	* This allows a concurrently reading process to see a consistent view of the
	* file without needing locking; any given read of the file is guaranteed to
	* be self-consistent and not see partial file contents.
	*
	* @param string file path to write
	* @param string data to write
	*
	* @return boolean indicating whether the file was changed by this function.
	*/
	public static function writeFileIfChanged($path, $data) {
	if (file_exists($path)) {
	$current = self::readFile($path);
	if ($current === $data) {
	return false;
	}
	}
	self::assertWritableFile($path);

	// Create the temporary file alongside the intended destination,
	// as this ensures that the rename() will be atomic (on the same fs)
	$dir = dirname($path);
	$temp = tempnam($dir, 'GEN');
	if (!$temp) {
	throw new FilesystemException(
	$dir,
	pht('Unable to create temporary file in %s.', $dir));
	}
	try {
	self::writeFile($temp, $data);
	// tempnam will always restrict ownership to us, broaden
	// it so that these files respect the actual umask
	self::changePermissions($temp, 0666 & ~umask());
	// This will appear atomic to concurrent readers
	$ok = rename($temp, $path);
	if (!$ok) {
	throw new FilesystemException(
	$path,
	pht('Unable to move %s to %s.', $temp, $path));
	}
	} catch (Exception $e) {
	// Make best effort to remove temp file
	unlink($temp);
	throw $e;
	}
	return true;
	}


	/**
	* Write data to unique file, without overwriting existing files. This is
	* useful if you want to write a ".bak" file or something similar, but want
	* to make sure you don't overwrite something already on disk.
	*
	* This function will add a number to the filename if the base name already
	* exists, e.g. "example.bak", "example.bak.1", "example.bak.2", etc. (Don't
	* rely on this exact behavior, of course.)
	*
	* @param string Suggested filename, like "example.bak". This name will
	* be used if it does not exist, or some similar name will
	* be chosen if it does.
	* @param string Data to write to the file.
	* @return string Path to a newly created and written file which did not
	* previously exist, like "example.bak.3".
	* @task file
	*/
	public static function writeUniqueFile($base, $data) {
	$full_path = self::resolvePath($base);
	$sequence = 0;
	assert_stringlike($data);
	// Try 'file', 'file.1', 'file.2', etc., until something doesn't exist.

	while (true) {
	$try_path = $full_path;
	if ($sequence) {
	$try_path .= '.'.$sequence;
	}

	$handle = @fopen($try_path, 'x');
	if ($handle) {
	$ok = fwrite($handle, $data);
	if ($ok === false) {
	throw new FilesystemException(
	$try_path,
	pht('Failed to write file data.'));
	}

	$ok = fclose($handle);
	if (!$ok) {
	throw new FilesystemException(
	$try_path,
	pht('Failed to close file handle.'));
	}

	return $try_path;
	}

	$sequence++;
	}
	}


	/**
	* Append to a file without having to deal with file handles, with
	* detailed exceptions on failure.
	*
	* @param string File path to write. This file must be writable or its
	* parent directory must exist and be writable.
	* @param string Data to write.
	*
	* @task file
	*/
	public static function appendFile($path, $data) {
	$path = self::resolvePath($path);

	// Use self::writeFile() if the file doesn't already exist
	try {
	self::assertExists($path);
	} catch (FilesystemException $ex) {
	self::writeFile($path, $data);
	return;
	}

	// File needs to exist or the directory needs to be writable
	$dir = dirname($path);
	self::assertExists($dir);
	self::assertIsDirectory($dir);
	self::assertWritable($dir);
	assert_stringlike($data);

	if (($fh = fopen($path, 'a')) === false) {
	throw new FilesystemException(
	$path,
	pht("Failed to open file '%s'.", $path));
	}
	$dlen = strlen($data);
	if (fwrite($fh, $data) !== $dlen) {
	throw new FilesystemException(
	$path,
	pht("Failed to write %d bytes to '%s'.", $dlen, $path));
	}
	if (!fflush($fh) \|\| !fclose($fh)) {
	throw new FilesystemException(
	$path,
	pht("Failed closing file '%s' after write.", $path));
	}
	}


	/**
	* Copy a file, preserving file attributes (if relevant for the OS).
	*
	* @param string File path to copy from. This file must exist and be
	* readable, or an exception will be thrown.
	* @param string File path to copy to. If a file exists at this path
	* already, it wll be overwritten.
	*
	* @task file
	*/
	public static function copyFile($from, $to) {
	$from = self::resolvePath($from);
	$to = self::resolvePath($to);

	self::assertExists($from);
	self::assertIsFile($from);
	self::assertReadable($from);

	if (phutil_is_windows()) {
	execx('copy /Y %s %s', $from, $to);
	} else {
	execx('cp -p %s %s', $from, $to);
	}
	}


	/**
	* Remove a file or directory.
	*
	* @param string File to a path or directory to remove.
	* @return void
	*
	* @task file
	*/
	public static function remove($path) {
	if (!strlen($path)) {
	// Avoid removing PWD.
	throw new Exception(
	pht(
	'No path provided to %s.',
	__FUNCTION__.'()'));
	}

	$path = self::resolvePath($path);

	if (!file_exists($path)) {
	return;
	}

	self::executeRemovePath($path);
	}

	/**
	* Rename a file or directory.
	*
	* @param string Old path.
	* @param string New path.
	*
	* @task file
	*/
	public static function rename($old, $new) {
	$old = self::resolvePath($old);
	$new = self::resolvePath($new);

	self::assertExists($old);

	$ok = rename($old, $new);
	if (!$ok) {
	throw new FilesystemException(
	$new,
	pht("Failed to rename '%s' to '%s'!", $old, $new));
	}
	}


	/**
	* Internal. Recursively remove a file or an entire directory. Implements
	* the core function of @{method:remove} in a way that works on Windows.
	*
	* @param string File to a path or directory to remove.
	* @return void
	*
	* @task file
	*/
	private static function executeRemovePath($path) {
	if (is_dir($path) && !is_link($path)) {
	foreach (self::listDirectory($path, true) as $child) {
	self::executeRemovePath($path.DIRECTORY_SEPARATOR.$child);
	}
	$ok = rmdir($path);
	if (!$ok) {
	throw new FilesystemException(
	$path,
	pht("Failed to remove directory '%s'!", $path));
	}
	} else {
	$ok = unlink($path);
	if (!$ok) {
	throw new FilesystemException(
	$path,
	pht("Failed to remove file '%s'!", $path));
	}
	}
	}


	/**
	* Change the permissions of a file or directory.
	*
	* @param string Path to the file or directory.
	* @param int Permission umask. Note that umask is in octal, so you
	* should specify it as, e.g., `0777', not `777'.
	* @return void
	*
	* @task file
	*/
	public static function changePermissions($path, $umask) {
	$path = self::resolvePath($path);

	self::assertExists($path);

	if (!@chmod($path, $umask)) {
	$readable_umask = sprintf('%04o', $umask);
	throw new FilesystemException(
	$path,
	pht("Failed to chmod '%s' to '%s'.", $path, $readable_umask));
	}
	}


	/**
	* Get the last modified time of a file
	*
	* @param string Path to file
	* @return int Time last modified
	*
	* @task file
	*/
	public static function getModifiedTime($path) {
	$path = self::resolvePath($path);
	self::assertExists($path);
	self::assertIsFile($path);
	self::assertReadable($path);

	$modified_time = @filemtime($path);

	if ($modified_time === false) {
	throw new FilesystemException(
	$path,
	pht('Failed to read modified time for %s.', $path));
	}

	return $modified_time;
	}


	/**
	* Read random bytes from /dev/urandom or equivalent. See also
	* @{method:readRandomCharacters}.
	*
	* @param int Number of bytes to read.
	* @return string Random bytestring of the provided length.
	*
	* @task file
	*/
	public static function readRandomBytes($number_of_bytes) {
	$number_of_bytes = (int)$number_of_bytes;
	if ($number_of_bytes < 1) {
	throw new Exception(pht('You must generate at least 1 byte of entropy.'));
	}

	// Try to use `openssl_random_pseudo_bytes()` if it's available. This source
	// is the most widely available source, and works on Windows/Linux/OSX/etc.

	if (function_exists('openssl_random_pseudo_bytes')) {
	$strong = true;
	$data = openssl_random_pseudo_bytes($number_of_bytes, $strong);

	if (!$strong) {
	// NOTE: This indicates we're using a weak random source. This is
	// probably OK, but maybe we should be more strict here.
	}

	if ($data === false) {
	throw new Exception(
	pht(
	'%s failed to generate entropy!',
	'openssl_random_pseudo_bytes()'));
	}

	if (strlen($data) != $number_of_bytes) {
	throw new Exception(
	pht(
	'%s returned an unexpected number of bytes (got %s, expected %s)!',
	'openssl_random_pseudo_bytes()',
	new PhutilNumber(strlen($data)),
	new PhutilNumber($number_of_bytes)));
	}

	return $data;
	}


	// Try to use `/dev/urandom` if it's available. This is usually available
	// on non-Windows systems, but some PHP config (open_basedir) and chrooting
	// may limit our access to it.

	$urandom = @fopen('/dev/urandom', 'rb');
	if ($urandom) {
	$data = @fread($urandom, $number_of_bytes);
	@fclose($urandom);
	if (strlen($data) != $number_of_bytes) {
	throw new FilesystemException(
	'/dev/urandom',
	pht('Failed to read random bytes!'));
	}
	return $data;
	}

	// (We might be able to try to generate entropy here from a weaker source
	// if neither of the above sources panned out, see some discussion in
	// T4153.)

	// We've failed to find any valid entropy source. Try to fail in the most
	// useful way we can, based on the platform.

	if (phutil_is_windows()) {
	throw new Exception(
	pht(
	'%s requires the PHP OpenSSL extension to be installed and enabled '.
	'to access an entropy source. On Windows, this extension is usually '.
	'installed but not enabled by default. Enable it in your "s".',
	__METHOD__.'()',
	'php.ini'));
	}

	throw new Exception(
	pht(
	'%s requires the PHP OpenSSL extension or access to "%s". Install or '.
	'enable the OpenSSL extension, or make sure "%s" is accessible.',
	__METHOD__.'()',
	'/dev/urandom',
	'/dev/urandom'));
	}


	/**
	* Read random alphanumeric characters from /dev/urandom or equivalent. This
	* method operates like @{method:readRandomBytes} but produces alphanumeric
	* output (a-z, 0-9) so it's appropriate for use in URIs and other contexts
	* where it needs to be human readable.
	*
	* @param int Number of characters to read.
	* @return string Random character string of the provided length.
	*
	* @task file
	*/
	public static function readRandomCharacters($number_of_characters) {

	// NOTE: To produce the character string, we generate a random byte string
	// of the same length, select the high 5 bits from each byte, and
	// map that to 32 alphanumeric characters. This could be improved (we
	// could improve entropy per character with base-62, and some entropy
	// sources might be less entropic if we discard the low bits) but for
	// reasonable cases where we have a good entropy source and are just
	// generating some kind of human-readable secret this should be more than
	// sufficient and is vastly simpler than trying to do bit fiddling.

	$map = array_merge(range('a', 'z'), range('2', '7'));

	$result = '';
	$bytes = self::readRandomBytes($number_of_characters);
	for ($ii = 0; $ii < $number_of_characters; $ii++) {
	$result .= $map[ord($bytes[$ii]) >> 3];
	}

	return $result;
	}


	/**
	* Identify the MIME type of a file. This returns only the MIME type (like
	* text/plain), not the encoding (like charset=utf-8).
	*
	* @param string Path to the file to examine.
	* @param string Optional default mime type to return if the file's mime
	* type can not be identified.
	* @return string File mime type.
	*
	* @task file
	*
	* @phutil-external-symbol function mime_content_type
	* @phutil-external-symbol function finfo_open
	* @phutil-external-symbol function finfo_file
	*/
	public static function getMimeType(
	$path,
	$default = 'application/octet-stream') {

	$path = self::resolvePath($path);

	self::assertExists($path);
	self::assertIsFile($path);
	self::assertReadable($path);

	$mime_type = null;

	// Fileinfo is the best approach since it doesn't rely on `file`, but
	// it isn't builtin for older versions of PHP.

	if (function_exists('finfo_open')) {
	$finfo = finfo_open(FILEINFO_MIME);
	if ($finfo) {
	$result = finfo_file($finfo, $path);
	if ($result !== false) {
	$mime_type = $result;
	}
	}
	}

	// If we failed Fileinfo, try `file`. This works well but not all systems
	// have the binary.

	if ($mime_type === null) {
	list($err, $stdout) = exec_manual(
	'file --brief --mime %s',
	$path);
	if (!$err) {
	$mime_type = trim($stdout);
	}
	}

	// If we didn't get anywhere, try the deprecated mime_content_type()
	// function.

	if ($mime_type === null) {
	if (function_exists('mime_content_type')) {
	$result = mime_content_type($path);
	if ($result !== false) {
	$mime_type = $result;
	}
	}
	}

	// If we come back with an encoding, strip it off.
	if (strpos($mime_type, ';') !== false) {
	list($type, $encoding) = explode(';', $mime_type, 2);
	$mime_type = $type;
	}

	if ($mime_type === null) {
	$mime_type = $default;
	}

	return $mime_type;
	}


	/* -( Directories )-------------------------------------------------------- */


	/**
	* Create a directory in a manner similar to mkdir(), but throw detailed
	* exceptions on failure.
	*
	* @param string Path to directory. The parent directory must exist and
	* be writable.
	* @param int Permission umask. Note that umask is in octal, so you
	* should specify it as, e.g., `0777', not `777'.
	* @param boolean Recursively create directories. Default to false.
	* @return string Path to the created directory.
	*
	* @task directory
	*/
	public static function createDirectory(
	$path,
	$umask = 0755,
	$recursive = false) {

	$path = self::resolvePath($path);

	if (is_dir($path)) {
	if ($umask) {
	self::changePermissions($path, $umask);
	}
	return $path;
	}

	$dir = dirname($path);
	if ($recursive && !file_exists($dir)) {
	// Note: We could do this with the recursive third parameter of mkdir(),
	// but then we loose the helpful FilesystemExceptions we normally get.
	self::createDirectory($dir, $umask, true);
	}

	self::assertIsDirectory($dir);
	self::assertExists($dir);
	self::assertWritable($dir);
	self::assertNotExists($path);

	if (!mkdir($path, $umask)) {
	throw new FilesystemException(
	$path,
	pht("Failed to create directory '%s'.", $path));
	}

	// Need to change permissions explicitly because mkdir does something
	// slightly different. mkdir(2) man page:
	// 'The parameter mode specifies the permissions to use. It is modified by
	// the process's umask in the usual way: the permissions of the created
	// directory are (mode & ~umask & 0777)."'
	if ($umask) {
	self::changePermissions($path, $umask);
	}

	return $path;
	}


	/**
	* Create a temporary directory and return the path to it. You are
	* responsible for removing it (e.g., with Filesystem::remove())
	* when you are done with it.
	*
	* @param string Optional directory prefix.
	* @param int Permissions to create the directory with. By default,
	* these permissions are very restrictive (0700).
	* @param string Optional root directory. If not provided, the system
	* temporary directory (often "/tmp") will be used.
	* @return string Path to newly created temporary directory.
	*
	* @task directory
	*/
	public static function createTemporaryDirectory(
	$prefix = '',
	$umask = 0700,
	$root_directory = null) {
	$prefix = preg_replace('/[^A-Z0-9._-]+/i', '', $prefix);

	if ($root_directory !== null) {
	$tmp = $root_directory;
	self::assertExists($tmp);
	self::assertIsDirectory($tmp);
	self::assertWritable($tmp);
	} else {
	$tmp = sys_get_temp_dir();
	if (!$tmp) {
	throw new FilesystemException(
	$tmp,
	pht('Unable to determine system temporary directory.'));
	}
	}

	$base = $tmp.DIRECTORY_SEPARATOR.$prefix;

	$tries = 3;
	do {
	$dir = $base.substr(base_convert(md5(mt_rand()), 16, 36), 0, 16);
	try {
	self::createDirectory($dir, $umask);
	break;
	} catch (FilesystemException $ex) {
	// Ignore.
	}
	} while (--$tries);

	if (!$tries) {
	$df = disk_free_space($tmp);
	if ($df !== false && $df < 1024 * 1024) {
	throw new FilesystemException(
	$dir,
	pht('Failed to create a temporary directory: the disk is full.'));
	}

	throw new FilesystemException(
	$dir,
	pht("Failed to create a temporary directory in '%s'.", $tmp));
	}

	return $dir;
	}


	/**
	* List files in a directory.
	*
	* @param string Path, absolute or relative to PWD.
	* @param bool If false, exclude files beginning with a ".".
	*
	* @return array List of files and directories in the specified
	* directory, excluding `.' and `..'.
	*
	* @task directory
	*/
	public static function listDirectory($path, $include_hidden = true) {
	$path = self::resolvePath($path);

	self::assertExists($path);
	self::assertIsDirectory($path);
	self::assertReadable($path);

	$list = @scandir($path);
	if ($list === false) {
	throw new FilesystemException(
	$path,
	pht("Unable to list contents of directory '%s'.", $path));
	}

	foreach ($list as $k => $v) {
	if ($v == '.' \|\| $v == '..' \|\| (!$include_hidden && $v[0] == '.')) {
	unset($list[$k]);
	}
	}

	return array_values($list);
	}


	/**
	* Return all directories between a path and the specified root directory
	* (defaulting to "/"). Iterating over them walks from the path to the root.
	*
	* @param string Path, absolute or relative to PWD.
	* @param string The root directory.
	* @return list<string> List of parent paths, including the provided path.
	* @task directory
	*/
	public static function walkToRoot($path, $root = null) {
	$path = self::resolvePath($path);

	if (is_link($path)) {
	$path = realpath($path);
	}

	// NOTE: On Windows, paths start like "C:\", so "/" does not contain
	// every other path. We could possibly special case "/" to have the same
	// meaning on Windows that it does on Linux, but just special case the
	// common case for now. See PHI817.
	if ($root !== null) {
	$root = self::resolvePath($root);

	if (is_link($root)) {
	$root = realpath($root);
	}

	// NOTE: We don't use `isDescendant()` here because we don't want to
	// reject paths which don't exist on disk.
	$root_list = new FileList(array($root));
	if (!$root_list->contains($path)) {
	return array();
	}
	} else {
	if (phutil_is_windows()) {
	$root = null;
	} else {
	$root = '/';
	}
	}

	$walk = array();
	$parts = explode(DIRECTORY_SEPARATOR, $path);
	foreach ($parts as $k => $part) {
	if (!strlen($part)) {
	unset($parts[$k]);
	}
	}

	while (true) {
	if (phutil_is_windows()) {
	$next = implode(DIRECTORY_SEPARATOR, $parts);
	} else {
	$next = DIRECTORY_SEPARATOR.implode(DIRECTORY_SEPARATOR, $parts);
	}

	$walk[] = $next;
	if ($next == $root) {
	break;
	}

	if (!$parts) {
	break;
	}

	array_pop($parts);
	}

	return $walk;
	}


	/* -( Paths )-------------------------------------------------------------- */


	/**
	* Checks if a path is specified as an absolute path.
	*
	* @param string
	* @return bool
	*/
	public static function isAbsolutePath($path) {
	if (phutil_is_windows()) {
	return (bool)preg_match('/^[A-Za-z]+:/', $path);
	} else {
	return !strncmp($path, DIRECTORY_SEPARATOR, 1);
	}
	}

	/**
	* Canonicalize a path by resolving it relative to some directory (by
	* default PWD), following parent symlinks and removing artifacts. If the
	* path is itself a symlink it is left unresolved.
	*
	* @param string Path, absolute or relative to PWD.
	* @return string Canonical, absolute path.
	*
	* @task path
	*/
	public static function resolvePath($path, $relative_to = null) {
	$is_absolute = self::isAbsolutePath($path);

	if (!$is_absolute) {
	if (!$relative_to) {
	$relative_to = getcwd();
	}
	$path = $relative_to.DIRECTORY_SEPARATOR.$path;
	}

	if (is_link($path)) {
	$parent_realpath = realpath(dirname($path));
	if ($parent_realpath !== false) {
	return $parent_realpath.DIRECTORY_SEPARATOR.basename($path);
	}
	}

	$realpath = realpath($path);
	if ($realpath !== false) {
	return $realpath;
	}


	// This won't work if the file doesn't exist or is on an unreadable mount
	// or something crazy like that. Try to resolve a parent so we at least
	// cover the nonexistent file case.
	$parts = explode(DIRECTORY_SEPARATOR, trim($path, DIRECTORY_SEPARATOR));
	while (end($parts) !== false) {
	array_pop($parts);
	if (phutil_is_windows()) {
	$attempt = implode(DIRECTORY_SEPARATOR, $parts);
	} else {
	$attempt = DIRECTORY_SEPARATOR.implode(DIRECTORY_SEPARATOR, $parts);
	}
	$realpath = realpath($attempt);
	if ($realpath !== false) {
	$path = $realpath.substr($path, strlen($attempt));
	break;
	}
	}

	return $path;
	}

	/**
	* Test whether a path is descendant from some root path after resolving all
	* symlinks and removing artifacts. Both paths must exists for the relation
	* to obtain. A path is always a descendant of itself as long as it exists.
	*
	* @param string Child path, absolute or relative to PWD.
	* @param string Root path, absolute or relative to PWD.
	* @return bool True if resolved child path is in fact a descendant of
	* resolved root path and both exist.
	* @task path
	*/
	public static function isDescendant($path, $root) {
	try {
	self::assertExists($path);
	self::assertExists($root);
	} catch (FilesystemException $e) {
	return false;
	}
	$fs = new FileList(array($root));
	return $fs->contains($path);
	}

	/**
	* Convert a canonical path to its most human-readable format. It is
	* guaranteed that you can use resolvePath() to restore a path to its
	* canonical format.
	*
	* @param string Path, absolute or relative to PWD.
	* @param string Optionally, working directory to make files readable
	* relative to.
	* @return string Human-readable path.
	*
	* @task path
	*/
	public static function readablePath($path, $pwd = null) {
	if ($pwd === null) {
	$pwd = getcwd();
	}

	foreach (array($pwd, self::resolvePath($pwd)) as $parent) {
	$parent = rtrim($parent, DIRECTORY_SEPARATOR).DIRECTORY_SEPARATOR;
	$len = strlen($parent);
	if (!strncmp($parent, $path, $len)) {
	$path = substr($path, $len);
	return $path;
	}
	}

	return $path;
	}

	/**
	* Determine whether or not a path exists in the filesystem. This differs from
	* file_exists() in that it returns true for symlinks. This method does not
	* attempt to resolve paths before testing them.
	*
	* @param string Test for the existence of this path.
	* @return bool True if the path exists in the filesystem.
	* @task path
	*/
	public static function pathExists($path) {
	return file_exists($path) \|\| is_link($path);
	}


	/**
	* Determine if an executable binary (like `git` or `svn`) exists within
	* the configured `$PATH`.
	*
	* @param string Binary name, like `'git'` or `'svn'`.
	* @return bool True if the binary exists and is executable.
	* @task exec
	*/
	public static function binaryExists($binary) {
	return self::resolveBinary($binary) !== null;
	}


	/**
	* Locates the full path that an executable binary (like `git` or `svn`) is at
	* the configured `$PATH`.
	*
	* @param string Binary name, like `'git'` or `'svn'`.
	* @return string The full binary path if it is present, or null.
	* @task exec
	*/
	public static function resolveBinary($binary) {
	if (phutil_is_windows()) {
	list($err, $stdout) = exec_manual('where %s', $binary);
	$stdout = phutil_split_lines($stdout);

	// If `where %s` could not find anything, check for relative binary
	if ($err) {
	$path = self::resolvePath($binary);
	if (self::pathExists($path)) {
	return $path;
	}
	return null;
	}
	$stdout = head($stdout);
	} else {
	list($err, $stdout) = exec_manual('which %s', $binary);
	}

	return $err === 0 ? trim($stdout) : null;
	}


	/**
	* Determine if two paths are equivalent by resolving symlinks. This is
	* different from resolving both paths and comparing them because
	* resolvePath() only resolves symlinks in parent directories, not the
	* path itself.
	*
	* @param string First path to test for equivalence.
	* @param string Second path to test for equivalence.
	* @return bool True if both paths are equivalent, i.e. reference the same
	* entity in the filesystem.
	* @task path
	*/
	public static function pathsAreEquivalent($u, $v) {
	$u = self::resolvePath($u);
	$v = self::resolvePath($v);

	$real_u = realpath($u);
	$real_v = realpath($v);

	if ($real_u) {
	$u = $real_u;
	}
	if ($real_v) {
	$v = $real_v;
	}
	return ($u == $v);
	}


	/* -( Assert )------------------------------------------------------------- */


	/**
	* Assert that something (e.g., a file, directory, or symlink) exists at a
	* specified location.
	*
	* @param string Assert that this path exists.
	* @return void
	*
	* @task assert
	*/
	public static function assertExists($path) {
	if (!self::pathExists($path)) {
	throw new FilesystemException(
	$path,
	pht("File system entity '%s' does not exist.", $path));
	}
	}


	/**
	* Assert that nothing exists at a specified location.
	*
	* @param string Assert that this path does not exist.
	* @return void
	*
	* @task assert
	*/
	public static function assertNotExists($path) {
	if (file_exists($path) \|\| is_link($path)) {
	throw new FilesystemException(
	$path,
	pht("Path '%s' already exists!", $path));
	}
	}


	/**
	* Assert that a path represents a file, strictly (i.e., not a directory).
	*
	* @param string Assert that this path is a file.
	* @return void
	*
	* @task assert
	*/
	public static function assertIsFile($path) {
	if (!is_file($path)) {
	throw new FilesystemException(
	$path,
	pht("Requested path '%s' is not a file.", $path));
	}
	}


	/**
	* Assert that a path represents a directory, strictly (i.e., not a file).
	*
	* @param string Assert that this path is a directory.
	* @return void
	*
	* @task assert
	*/
	public static function assertIsDirectory($path) {
	if (!is_dir($path)) {
	throw new FilesystemException(
	$path,
	pht("Requested path '%s' is not a directory.", $path));
	}
	}


	/**
	* Assert that a file or directory exists and is writable.
	*
	* @param string Assert that this path is writable.
	* @return void
	*
	* @task assert
	*/
	public static function assertWritable($path) {
	if (!is_writable($path)) {
	throw new FilesystemException(
	$path,
	pht("Requested path '%s' is not writable.", $path));
	}
	}


	/**
	* Assert that a file or directory exists and is readable.
	*
	* @param string Assert that this path is readable.
	* @return void
	*
	* @task assert
	*/
	public static function assertReadable($path) {
	if (!is_readable($path)) {
	throw new FilesystemException(
	$path,
	pht("Path '%s' is not readable.", $path));
	}
	}

	}

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