GH-127236: `pathname2url()`: generate RFC 1738 URL for absolute POSIX path (#127194)

When handed an absolute Windows path such as `C:\foo` or `//server/share`,
the `urllib.request.pathname2url()` function returns a URL with an
authority section, such as `///C:/foo` or `//server/share` (or before
GH-126205, `////server/share`). Only the `file:` prefix is omitted.

But when handed an absolute POSIX path such as `/etc/hosts`, or a Windows
path of the same form (rooted but lacking a drive), the function returns a
URL without an authority section, such as `/etc/hosts`.

This patch corrects the discrepancy by adding a `//` prefix before
drive-less, rooted paths when generating URLs.

1 files changed, 5 insertions, 3 deletions

diff --git a/Lib/urllib/request.py b/Lib/urllib/request.py
index 9e55543..1fcaa89 100644
--- a/Lib/urllib/request.py
+++ b/Lib/urllib/request.py
@@ -1667,9 +1667,11 @@ else:
     def pathname2url(pathname):
         """OS-specific conversion from a file system path to a relative URL
         of the 'file' scheme; not recommended for general use."""
-        if pathname[:2] == '//':
-            # Add explicitly empty authority to avoid interpreting the path
-            # as authority.
+        if pathname[:1] == '/':
+            # Add explicitly empty authority to absolute path. If the path
+            # starts with exactly one slash then this change is mostly
+            # cosmetic, but if it begins with two or more slashes then this
+            # avoids interpreting the path as a URL authority.
             pathname = '//' + pathname
         encoding = sys.getfilesystemencoding()
         errors = sys.getfilesystemencodeerrors()