Add improved Windows UNC path support in std/os (#20281)

* Add improved Windows UNC path support in std/os

Original issue: `std/os.createDir` tries to create every component of
the given path as a directory. The problem is that `createDir`
interprets every backslash/slash as a path separator. For a UNC path
this is incorrect. E.g. one UNC form is `\\Server\Volume\Path`. It's an
error to create the `\\Server` directory, as well as creating
`\\Server\Volume`.

Add `ntpath.nim` module with `splitDrive` proc. This implements UNC path
parsing as implemented in the Python `ntpath.py` module. The following
UNC forms are supported:

* `\\Server\Volume\Path`
* `\\?\Volume\Path`
* `\\?\UNC\Server\Volume\Path`

Improves support for UNC paths in various procs in `std/os`:
---

* pathnorm.addNormalizePath
  * Issue: This had incomplete support for UNC paths
    * The UNC prefix (first 2 characters of a UNC path) was assumed to
      be exactly `\\`, but it can be `//` and `\/`, etc. as well
    * Also, the UNC prefix must be normalized to the `dirSep` argument
      of `addNormalizePath`
  * Resolution: Changed to account for different UNC prefixes, and
    normalizing the prefixes according to `dirSep`
    * Affected procs that get tests: `relativePath`, `joinPath`
  * Issue: The server/volume part of UNC paths can be stripped when
    normalizing `..` path components
    * This error should be negligable, so ignoring this
* splitPath
  * Now make sure the UNC drive is not split; return the UNC drive as
    `head` if the UNC drive is the only component of the path
  * Consequently fixes `extractFilename`, `lastPathPart`
* parentDir / `/../`
  * Strip away drive before working on the path, prepending the drive
    after all work is done - prevents stripping UNC components
  * Return empty string if drive component is the only component; this
    is the behavior for POSIX paths as well
  * Alternative implementation: Just call something like
    `pathnorm.normalizePath(path & "/..")` for the whole proc - maybe
    too big of a change
* tailDir
  * If drive is present in path, just split that from path and return
    path
* parentDirs iterator
  * Uses `parentDir` for going backwards
  * When going forwards, first `splitDrive`, yield the drive field, and
    then iterate over path field as normal
* splitFile
  * Make sure path parsing stops at end of drive component
* createDir
  * Fixed by skipping drive part before creating directories
  * Alternative implementation: use `parentDirs` iterator instead of
    iterating over characters
    * Consequence is that it will try to create the root directory
* isRootDir
  * Changed to treat UNC drive alone as root (e.g. "//?/c:" is root)
  * This change prevents the empty string being yielded by the
    `parentDirs` iterator with `fromRoot = false`
* Internal `sameRoot`
  * The "root" refers to the drive, so `splitDrive` can be used here

This adds UNC path support to all procs that could use it in std/os. I
don't think any more work has to be done to support UNC paths. For the
future, I believe the path handling code can be refactored due to
duplicate code. There are multiple ways of manipulating paths, such as
manually searching string for path separator and also having a path
normalizer (pathnorm.nim). If all path manipulation used `pathnorm.nim`,
and path component splitting used `parentDirs` iterator, then a lot of
code could be removed.

Tests
---

Added test file for `pathnorm.nim` and `ntpath.nim`.
`pathnorm.normalizePath` has no tests, so I'm adding a few unit tests.
`ntpath.nim` contains tests copied from Python's test suite.

Added integration tests to `tos.nim` that tests UNC paths.

Removed incorrect `relativePath` runnableExamples from being tested on Windows:
---

`relativePath("/Users///me/bar//z.nim", "//Users/", '/') == "me/bar/z.nim"`

This is incorrect on Windows because the `/` and `//` are not the same
root. `/` (or `\`) is expanded to the drive in the current working
directory (e.g. `C:\`). `//` (or `\\`), however, are the first two
characters of a UNC path. The following holds true for normal Windows
installations:

* `dirExists("/Users") != dirExists("//Users")`
* `dirExists("\\Users") != dirExists("\\\\Users")`

Fixes #19103

Questions:
---

* Should the `splitDrive` proc be in `os.nim` instead with copyright
  notice above the proc?
* Is it fine to put most of the new tests into the `runnableExamples`
  section of the procs in std/os?

* [skipci] Apply suggestions from code review

Co-authored-by: Clay Sweetser <Varriount@users.noreply.github.com>

* [skip ci] Update lib/pure/os.nim

Co-authored-by: Clay Sweetser <Varriount@users.noreply.github.com>

* Move runnableExamples tests in os.nim to tos.nim

* tests/topt_no_cursor: Change from using splitFile to splitDrive

`splitFile` can no longer be used in the test, because it generates
different ARC code on Windows and Linux. This replaces `splitFile` with
`splitDrive`, because it generates same ARC code on Windows and Linux,
and returns a tuple. I assume the test wants a proc that returns a
tuple.

* Drop copyright attribute to Python

Co-authored-by: Clay Sweetser <Varriount@users.noreply.github.com>

1 files changed, 61 insertions, 0 deletions

diff --git a/lib/std/private/ntpath.nim b/lib/std/private/ntpath.nim
new file mode 100644
index 000000000..7c8661bb7
--- /dev/null
+++ b/lib/std/private/ntpath.nim
@@ -0,0 +1,61 @@
+# This module is inspired by Python's `ntpath.py` module.
+
+import std/[
+  strutils,
+]
+
+# Adapted `splitdrive` function from the following commits in Python source
+# code:
+# 5a607a3ee5e81bdcef3f886f9d20c1376a533df4 (2009): Initial UNC handling (by Mark Hammond)
+# 2ba0fd5767577954f331ecbd53596cd8035d7186 (2022): Support for "UNC"-device paths (by Barney Gale)
+#
+# FAQ: Why use `strip` below? `\\?\UNC` is the start of a "UNC symbolic link",
+# which is a special UNC form. Running `strip` differentiates `\\?\UNC\` (a UNC
+# symbolic link) from e.g. `\\?\UNCD` (UNCD is the server in the UNC path).
+func splitDrive*(p: string): tuple[drive, path: string] =
+  ## Splits a Windows path into a drive and path part. The drive can be e.g.
+  ## `C:`. It can also be a UNC path (`\\server\drive\path`).
+  ##
+  ## The equivalent `splitDrive` for POSIX systems always returns empty drive.
+  ## Therefore this proc is only necessary on DOS-like file systems (together
+  ## with Nim's `doslikeFileSystem` conditional variable).
+  ##
+  ## This proc's use case is to extract `path` such that it can be manipulated
+  ## like a POSIX path.
+  runnableExamples:
+    doAssert splitDrive("C:") == ("C:", "")
+    doAssert splitDrive(r"C:\") == (r"C:", r"\")
+    doAssert splitDrive(r"\\server\drive\foo\bar") == (r"\\server\drive", r"\foo\bar")
+    doAssert splitDrive(r"\\?\UNC\server\share\dir") == (r"\\?\UNC\server\share", r"\dir")
+
+  result = ("", p)
+  if p.len < 2:
+    return
+  const sep = '\\'
+  let normp = p.replace('/', sep)
+  if p.len > 2 and normp[0] == sep and normp[1] == sep and normp[2] != sep:
+
+    # is a UNC path:
+    # vvvvvvvvvvvvvvvvvvvv drive letter or UNC path
+    # \\machine\mountpoint\directory\etc\...
+    #           directory ^^^^^^^^^^^^^^^
+    let start = block:
+      const unc = "\\\\?\\UNC" # Length is 7
+      let idx = min(8, normp.len)
+      if unc == normp[0..<idx].strip(chars = {sep}, leading = false).toUpperAscii:
+        8
+      else:
+        2
+    let index = normp.find(sep, start)
+    if index == -1:
+      return
+    var index2 = normp.find(sep, index + 1)
+
+    # a UNC path can't have two slashes in a row (after the initial two)
+    if index2 == index + 1:
+      return
+    if index2 == -1:
+      index2 = p.len
+    return (p[0..<index2], p[index2..^1])
+  if p[1] == ':':
+    return (p[0..1], p[2..^1])