bpo-30205: Fix getsockname() for unbound AF_UNIX sockets on Linux (#1370)

* bpo-30205: Fix getsockname() for unbound AF_UNIX sockets on Linux

* Add NEWS entry
diff --git a/Lib/test/test_socket.py b/Lib/test/test_socket.py
index 2155d63..5e37fc6 100644
--- a/Lib/test/test_socket.py
+++ b/Lib/test/test_socket.py
@@ -4682,6 +4682,10 @@
             else:
                 raise
 
+    def testUnbound(self):
+        # Issue #30205
+        self.assertEqual(self.sock.getsockname(), '')
+
     def testStrAddr(self):
         # Test binding to and retrieving a normal string pathname.
         path = os.path.abspath(support.TESTFN)
diff --git a/Misc/NEWS b/Misc/NEWS
index 118483f..c3eaaf2 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -317,6 +317,8 @@
 Library
 -------
 
+- bpo-30205: Fix getsockname() for unbound AF_UNIX sockets on Linux.
+
 - bpo-30228: The seek() and tell() methods of io.FileIO now set the internal
   seekable attribute to avoid one syscall on open() (in buffered or text mode).
 
diff --git a/Modules/socketmodule.c b/Modules/socketmodule.c
index 6d5c256..456c664 100644
--- a/Modules/socketmodule.c
+++ b/Modules/socketmodule.c
@@ -1211,9 +1211,9 @@
     {
         struct sockaddr_un *a = (struct sockaddr_un *) addr;
 #ifdef __linux__
-        if (a->sun_path[0] == 0) {  /* Linux abstract namespace */
-            addrlen -= offsetof(struct sockaddr_un, sun_path);
-            return PyBytes_FromStringAndSize(a->sun_path, addrlen);
+        size_t linuxaddrlen = addrlen - offsetof(struct sockaddr_un, sun_path);
+        if (linuxaddrlen > 0 && a->sun_path[0] == 0) {  /* Linux abstract namespace */
+            return PyBytes_FromStringAndSize(a->sun_path, linuxaddrlen);
         }
         else
 #endif /* linux */