Add notes on musl detection and version extraction

pep-0656.rst

@@ -62,6 +62,49 @@ Tags using the new scheme will take the form::
 
     musllinux_${MUSLMAJOR}_${MUSLMINOR}_${ARCH}
 
+Reading the musl version
+------------------------
+
+The musl version values can be obtains by executing the musl libc
+shared library the Python interpreter is currently running on, and
+parsing the output::
+
+def get_musl_major_minor(ld_so: str) -> Optional[Tuple[int, int]]:
+    """Detect musl runtime version.
+
+    Returns a two-tuple ``(major, minor)`` that indicates musl
+    library's version, or ``None`` if the given value does not output
+    expected information.
+
+    The libc library should output something like this to stderr::
+
+        musl libc (x86_64)
+        Version 1.2.2
+        Dynamic Program Loader
+    """
+    proc = subprocess.run([ld_so], stderr=subprocess.PIPE, text=True)
+    lines = (line.strip() for line in proc.stderr.splitlines())
+    lines = [line for line in lines if line]
+    if len(lines) < 2 or lines[0][:4] != "musl":
+        return None
+    match = re.match(
+        r"Version (\d+)\.(\d+)",
+        lines[1],
+        flags=re.MULTILINE,
+    )
+    if match:
+        return (int(match.group(1)), int(match.group(2)))
+    return None
+
+There are currently two possible ways to find the musl library's
+location that a Python interpreter is running on, either with the
+system ``ldd`` command [ldd]_, or by parsing the ``PT_INTERP``
+section's value from the executable's ELF header [elf]_.
+
+
+Formatting the tag
+------------------
+
 Distributions using the tag make similar promises to those described
 in :pep:`600`, including:
 
@@ -85,6 +128,9 @@ The value can be formatted with the following Python code::
         arch = arch.replace("-", "_")
         return f"musllinux_{musl_version[0]}_{musl_version[1]}_{arch}"
 
+Recommendations to package indexes
+----------------------------------
+
 It is recommended for Python package repositories, including PyPI, to
 accept platform tags matching the following regular expression::
 
@@ -93,7 +139,8 @@ accept platform tags matching the following regular expression::
 Python package repositories may impose additional requirements to
 reject Wheels with known issues, including but not limited to:
 
-* A ``musllinux_1_1`` wheel containing symbols only available in musl 1.2.
+* A ``musllinux_1_1`` wheel containing symbols only available in musl
+  1.2 or later.
 * Wheel that depends on external libraries not considered generally
   available to the intended audience of the package index.
 * A platform tag claiming compatibility to a non-existent musl version
@@ -134,6 +181,10 @@ References
 
 .. [musl-compat-ml] https://mail.python.org/archives/list/distutils-sig@python.org/message/VRXSTNXWHPAVUW253ZCWWMP7WDTBAQDL/
 
+.. [ldd] https://www.unix.com/man-page/posix/1/ldd/
+
+.. [elf] https://refspecs.linuxfoundation.org/elf/elf.pdf
+
 
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 =========