Fix finding pthread_*name_np on vanilla musl libc

newsfragments/2939.bugfix.rst

@@ -0,0 +1 @@
+The pthread functions are now correctly found on systems using vanilla versions of musl libc.

src/trio/_core/_thread_cache.py

@@ -41,10 +41,15 @@
             setname(_to_os_thread_name(name))
 
     # find the pthread library
-    # this will fail on windows
+    # this will fail on windows and musl
     libpthread_path = ctypes.util.find_library("pthread")
     if not libpthread_path:
-        return None
+        # musl includes pthread functions directly in libc.so
+        # (but note that find_library("c") does not work on musl,
+        #  see: https://github.com/python/cpython/issues/65821)
+        # so try that library instead
+        # if it doesn't exist, CDLL() will fail below
+        libpthread_path = "libc.so"
 
     # Sometimes windows can find the path, but gives a permission error when
     # accessing it. Catching a wider exception in case of more esoteric errors.

src/trio/_tests/test_threads.py

@@ -237,9 +237,17 @@
 
     libpthread_path = ctypes.util.find_library("pthread")
     if not libpthread_path:
-        print(f"no pthread on {sys.platform})")
+        # musl includes pthread functions directly in libc.so
+        # (but note that find_library("c") does not work on musl,
+        #  see: https://github.com/python/cpython/issues/65821)
+        # so try that library instead
+        # if it doesn't exist, CDLL() will fail below
+        libpthread_path = "libc.so"
+    try:
+        libpthread = ctypes.CDLL(libpthread_path)
+    except Exception:
+        print(f"no pthread on {sys.platform}")
         return None
-    libpthread = ctypes.CDLL(libpthread_path)
 
     pthread_getname_np = getattr(libpthread, "pthread_getname_np", None)