pythongh-131032: Add support.linked_to_musl() function (python#131071)

Skip test_math.test_fma_zero_result() if Python is linked to the musl
C library.

Author: vstinner

Lib/test/support/__init__.py

@@ -3015,3 +3015,22 @@ def is_libssl_fips_mode():
     except ImportError:
         return False  # more of a maybe, unless we add this to the _ssl module.
     return get_fips_mode() != 0
+
+
+def linked_to_musl():
+    """
+    Test if the Python executable is linked to the musl C library.
+    """
+    if sys.platform != 'linux':
+        return False
+
+    import subprocess
+    exe = getattr(sys, '_base_executable', sys.executable)
+    cmd = ['ldd', exe]
+    try:
+        stdout = subprocess.check_output(cmd,
+                                         text=True,
+                                         stderr=subprocess.STDOUT)
+    except (OSError, subprocess.CalledProcessError):
+        return False
+    return ('musl' in stdout)

Lib/test/test_math.py

@@ -2769,7 +2769,8 @@ def test_fma_infinities(self):
     # properly: it doesn't use the right sign when the result is zero.
     @unittest.skipIf(
         sys.platform.startswith(("freebsd", "wasi", "netbsd", "emscripten"))
-        or (sys.platform == "android" and platform.machine() == "x86_64"),
+        or (sys.platform == "android" and platform.machine() == "x86_64")
+        or support.linked_to_musl(),  # gh-131032
         f"this platform doesn't implement IEE 754-2008 properly")
     def test_fma_zero_result(self):
         nonnegative_finites = [0.0, 1e-300, 2.3, 1e300]

Lib/test/test_support.py

@@ -744,6 +744,10 @@ def test_get_signal_name(self):
             self.assertEqual(support.get_signal_name(exitcode), expected,
                              exitcode)
 
+    def test_linked_to_musl(self):
+        linked = support.linked_to_musl()
+        self.assertIsInstance(linked, bool)
+
     # XXX -follows a list of untested API
     # make_legacy_pyc
     # is_resource_enabled