pythongh-138130: Fix return value of libc_ver() on Emscripten (pythonGH-138132)

Emscripten's libc is a hybrid of musl and llvm libc; but it reports that it is
"glibc". This modifies the return value of `platform.libc_ver()` to return
something that is Emscripten-specific.
(cherry picked from commit 11a5fc8)

Co-authored-by: Hood Chatham <[email protected]>

Author: hoodmane

Lib/platform.py

@@ -173,6 +173,11 @@ def libc_ver(executable=None, lib='', version='', chunksize=16384):
 
     """
     if not executable:
+        if sys.platform == "emscripten":
+            # Emscripten's os.confstr reports that it is glibc, so special case
+            # it.
+            ver = ".".join(str(x) for x in sys._emscripten_info.emscripten_version)
+            return ("emscripten", ver)
         try:
             ver = os.confstr('CS_GNU_LIBC_VERSION')
             # parse 'glibc 2.28' as ('glibc', '2.28')

Lib/test/test_platform.py

@@ -525,8 +525,10 @@ def test_ios_ver(self):
             self.assertEqual(override.model, "Whiz")
             self.assertTrue(override.is_simulator)
 
-    @unittest.skipIf(support.is_emscripten, "Does not apply to Emscripten")
     def test_libc_ver(self):
+        if support.is_emscripten:
+            assert platform.libc_ver() == ("emscripten", "4.0.12")
+            return
         # check that libc_ver(executable) doesn't raise an exception
         if os.path.isdir(sys.executable) and \
            os.path.exists(sys.executable+'.exe'):