Filesystem Relative Proposal

| Filesystem Relative Draft Proposal

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Introduction
Acknowledgement
Preliminary implementation
Requirements
Issues
Design decisions
Provide separate lexical and operational `relative` functions
Provide separate lexical and operational `proximate` functions
Add lexical functions as `path` member functions
Provide a non-member function `lexically_normal` returning a normal form path
Provide a `weakly_canonical` operational function
Resolve issues in ways that "just work" for users
Specify `lexical relative` in terms of `std::mismatch`
Specify operational `relative` in terms of `weakly_canonical`
Specify operational `relative` in terms of `lexically relative`
Proposed wording
Define normal form
New class path member functions
Synopsis
Specification
New operational functions
Synopsis
Specification

## Introduction

There have been requests for a Filesystem library relative function for at least ten years.

The requested functionality seems simple - given two paths with a common prefix, return the non-common suffix portion of one of the paths such that it is relative to the other path.

In terms of the Filesystem library,

>     path p("/a/b/c");
>     path base("/a/b");
>     path rel = relative(p, base);  // the requested function
>     cout << rel << endl;           // outputs "c"
>     assert(absolute(rel, base) == p);

If that was all there was to it, the Filesystem library would have had a `relative` function years ago.

> Blocking issues: Clashing requirements, symlinks, directory placeholders (dot, dot-dot), user-expectations, corner cases.

### Acknowledgement

A paper by Jamie Allsop, Additions to Filesystem supporting Relative Paths, is what broke my mental logjam. Much of what follows is based directly on Jamie's analysis and proposal. The `weakly_canonical` function and aspects of the semantic specifications are my contributions. Mistakes, of course, are mine.

### Preliminary implementation

A preliminary implementation is available in the feature/relative2 branch of the Boost Filesystem Git repository. See github.com/boostorg/filesystem/tree/feature/relative2

## Requirements

Requirement 1: Some uses require symlinks be followed; i.e. the path must be resolved in the actual file system.

Requirement 2: Some uses require symlinks not be followed; i.e. the path must not be resolved in the actual file system.

Requirement 3: Some uses require removing redundant current directory (dot) or parent directory (dot-dot) placeholders.

Requirement 4: Some uses do not require removing redundant current directory (dot) or parent directory (dot-dot) placeholders since the path is known to be already in normal form.

## Issues

Issue 1: What happens if `p` and `base` are themselves relative?

Issue 2: What happens if there is no common prefix? Is this an error, the whole of `p` is relative to `base`, or something else?

Issue 3: What happens if `p`, `base`, or both are empty?

Issue 4: What happens if `p` and `base` are the same?

Issue 5: How is the "common prefix" determined?

Issue 6: What happens if portions of `p` or `base` exist but the entire path does not exist and yet symlinks need to be followed?

Issue 7: What happens when a symlink in the existing portion of a path is affected by a directory (dot-dot) placeholder in a later non-existent portion of the path?

Issue 8: Overly complex semantics (and thus specifications) in preliminary designs made reasoning about uses difficult.

Issue 9: Some uses never have redundant current directory (dot) or parent directory (dot-dot) placeholders, so a removal operation would be an unnecessary expense although otherwise harmless.

## Design decisions

#### Provide separate lexical and operational `relative` functions

Resolves the conflict between requirement 1 and requirement 2 and ensures both requirements are met.

A purely lexical function is needed by users working with directory hierarchies that do not actually exist.

An operational function that queries the current file system for existence and follows symlinks is needed by users working with actual existing directory hierarchies.

#### Provide separate lexical and operational `proximate` functions

Although not the only possibility, a likely fallback when the relative functions cannot find a relative path is to return the path being made relative. As a convenience, the `proximate` functions do just that.

#### Add lexical functions as `path` member functions

The Filesystem library is unusual in that it has several functions with both lexical (i.e. cheap) and operational (i.e. expensive due to file system access) forms with differing semantics. It is important that users choose the form that meets their application's specific needs. The library has always made the distinction via the convention of lexical functions being members of class `path`, while operational functions are non-member functions. The lexical functions proposed here also use the name prefix `lexically_` to drive home the distinction.

For the contrary argument, see Sutter and Alexandrescu, C++ Coding Standards, 44: "Prefer writing nonmember nonfriend functions", and Meyers, Effective C++ Third Edition, 23: "Prefer non-member non-friend functions to member functions."

#### Provide a non-member function `lexically_normal` returning a normal form path

Enables resolution of requirement 3 and requirement 4 in a way consistent with issue 9. Is a contributor to the resolution of issue 8.

"Normalization" is the process of removing redundant current directory (dot) , parent directory (dot-dot), and directory separator elements.

Normalization is a byproduct the current `canonical` function. But for the path returned by the proposed `weakly_canonical` function, only any leading canonic portion is in canonical form. So any trailing portion of the returned path has not been normalized.

Jamie Allsop has proposed adding a separate normalization function returning a path, and I agree with him.

Boost.filesystem has a deprecated non-const normalization function that modifies the path, but I agree with Jamie that a function returning a path is a better solution.

#### Provide a `weakly_canonical` operational function

Resolves issue 6, issue 7, issue 9, and is a contributor to the resolution of issue 8.

The operational function `weakly_canonical(p)` returns a path composed of `canonical(x)/y`, where `x` is a path composed of the longest leading sequence of elements in `p` that exist, and `y` is a path composed of the remaining trailing non-existent elements of `p` if any. "`weakly`" refers to weakened existence requirements compared to the existing canonical function.

- Having `weakly_canonical` as a separate function, and then specifying the processing of operational `relative` arguments in terms of calls to `weakly_canonical` makes it much easier to specify the operational `relative` function and reason about it. The difficulty of reasoning about operational `relative` semantics before the invention of `weakly_canonical` was what led to its initial development.
- Having `weakly_canonical` as a separate function also allows use in other contexts.
- Specifying the return be in normal form is an engineering trade-off to resolve issue 7 in a way that just works for most use cases.
- Specifying normative encouragement to not perform unneeded normalization is a reasonable resolution for issue 9.

#### Resolve issues in ways that "just work" for users

Resolves issues 1, 2, 3, 4, 6, and 7. Is a contributor to the resolution of issue 8.

The "just works" approach was suggested by Jamie Allsop. It is implemented by specifying a reasonable return value for all of the "What happens if..." corner case issues, rather that treating them as hard errors requiring an exception or error code.

#### Specify `lexically relative` in terms of `std::mismatch`

Resolves issue 5. Is a contributor to the resolution of issue 8.

#### Specify operational `relative` in terms of `weakly_canonical`

Is a contributor to the resolution of issue 8.

- Covers a wide range of uses cases since a single function works for existing, non-existing, and partially existing paths.
- Works correctly for partially existing paths that contain symlinks.

#### Specify operational `relative` in terms of `lexically relative`

Is a contributor to the resolution of issue 5 and issue 8.

If would be confusing to users and difficult to specify correctly if the two functions had differing semantics:

- When either or both paths are empty.
- When all elements of the two paths match exactly.
- Because different matching algorithms were used.
- Because although the same matching algorithm was used, it was applied in different ways.

These problems are avoided by specifying operational `relative` in terms of lexical `relative` after preparatory calls to operational functions.

## Proposed wording

"Overview:" sections below are non-normative experiments attempting to make the normative reference specifications easier to grasp.

### Define normal form

A path is in normal form if it has no redundant current directory (dot) or parent directory (dot-dot) elements. The normal form for an empty path is an empty path. The normal form for a path ending in a directory-separator that is not the root directory is the same path with a current directory (dot) element appended.

The last sentence above is not necessary for POSIX-like or Windows-like operating systems, but supports systems like OpenVMS that use different syntax for directory and regular-file names.

### New class path member functions

#### Synopsis

    path lexically_normal() const;
    path lexically_relative(const path& base) const;
    path lexically_proximate(const path& base) const;

#### Specification

    path lexically_normal() const;

> Overview: Returns `*this` with redundant current directory (dot), parent directory (dot-dot), and directory-separator elements removed.

> Returns: `*this` in normal form.

> Remarks: Uses `operator/=` to compose the returned path.

> [Example:

> `assert(path("foo/./bar/..").lexically_normal() == "foo");
> assert(path("foo/.///bar/../").lexically_normal() == "foo/.");`

> The above assertions will succeed. On Windows, the returned path's directory-separator characters will be backslashes rather than slashes, but that does not affect `path` equality. —end example]

    path lexically_relative(const path& base) const;

> Overview: Returns `*this` made relative to `base`. Treats empty or identical paths as corner cases, not errors. Does not resolve symlinks. Does not first normalize `*this` or `base`.

> Remarks: Uses `std::mismatch(begin(), end(), base.begin(), base.end())`, to determine the first mismatched element of `*this` and `base`. Uses `operator==` to determine if elements match.

> Returns:

> - `path()` if the first mismatched element of `*this` is equal to `begin()` or the first mismatched element of `base` is equal to `base.begin()`, or
> - `path(".")` if the first mismatched element of `*this` is equal to `end()` and the first mismatched element of `base` is equal to `base.end()`, or
> - An object of class `path` composed via application of `operator/= path("..")` for each element in the half-open range [first mismatched element of `base`, `base.end()`), and then application of `operator/=` for each element in the half-open range [first mismatched element of `*this`, `end()`).

> [Example:

> `assert(path("/a/d").lexically_relative("/a/b/c") == "../../d");
> assert(path("/a/b/c").lexically_relative("/a/d") == "../b/c");
> assert(path("a/b/c").lexically_relative("a") == "b/c");
> assert(path("a/b/c").lexically_relative("a/b/c/x/y") == "../..");
> assert(path("a/b/c").lexically_relative("a/b/c") == ".");
> assert(path("a/b").lexically_relative("c/d") == "");`

> The above assertions will succeed. On Windows, the returned path's directory-separators will be backslashes rather than forward slashes, but that does not affect `path` equality. —end example]

> [Note: If symlink following semantics are desired, use the operational function `relative` —end note]

> [Note: If normalization is needed to ensure consistent matching of elements, apply `lexically_normal()` to `*this`, `base`, or both. —end note]

    path lexically_proximate(const path& base) const;

> Returns: If the value of `lexically_relative(base)` is not an empty path, return it. Otherwise return `*this`.

> [Note: If symlink following semantics are desired, use the operational function `proximate` —end note]

> [Note: If normalization is needed to ensure consistent matching of elements, apply `lexically_normal()` to `*this`, `base`, or both. —end note]

### New operational functions

#### Synopsis

    path weakly_canonical(const path& p);
    path weakly_canonical(const path& p, system::error_code& ec);
    path relative(const path& p, system::error_code& ec);
    path relative(const path& p, const path& base=current_path());
    path relative(const path& p, const path& base, system::error_code& ec);
    path proximate(const path& p, system::error_code& ec);
    path proximate(const path& p, const path& base=current_path());
    path proximate(const path& p, const path& base, system::error_code& ec);

#### Specification

    path weakly_canonical(const path& p);
    path weakly_canonical(const path& p, system::error_code& ec);

> Overview: Returns `p` with symlinks resolved and the result normalized.

> Returns: A path composed of the result of calling the `canonical` function on a path composed of the leading elements of `p` that exist, if any, followed by the elements of `p` that do not exist, if any.

> Postcondition: The returned path is in normal form.

> Remarks: Uses `operator/=` to compose the returned path. Uses the `status` function to determine existence.

> Remarks: Implementations are encouraged to avoid unnecessary normalization such as when `canonical` has already been called on the entirety of `p`.

> Throws: As specified in Error reporting.

    path relative(const path& p, system::error_code& ec);

> Returns: `relative(p, current_path(), ec)`.

> Throws: As specified in Error reporting.

    path relative(const path& p, const path& base=current_path());
    path relative(const path& p, const path& base, system::error_code& ec);

> Overview: Returns `p` made relative to `base`. Treats empty or identical paths as corner cases, not errors. Resolves symlinks and normalizes both `p` and `base` before other processing.

> Returns: `weakly_canonical(p).lexically_relative(weakly_canonical(base))`. The second form returns `path()` if an error occurs.

> Throws: As specified in Error reporting.

    path proximate(const path& p, system::error_code& ec);

> Returns: `proximate(p, current_path(), ec)`.

> Throws: As specified in Error reporting.

    path proximate(const path& p, const path& base=current_path());
    path proximate(const path& p, const path& base, system::error_code& ec);

> Returns: `weakly_canonical(p).lexically_proximate(weakly_canonical(base))`. The second form returns `path()` if an error occurs.

> Throws: As specified in Error reporting.

© Copyright Beman Dawes 2015

Distributed under the Boost Software License, Version 1.0. See www.boost.org/LICENSE_1_0.txt

Revised 25 October 2015