bpo-25920: Remove socket.getaddrinfo() lock on macOS (GH-20177)

On macOS, socket.getaddrinfo() no longer uses an internal lock to
prevent race conditions when calling getaddrinfo(). getaddrinfo is
thread-safe is macOS 10.5, whereas Python 3.9 requires macOS 10.6 or
newer.

The lock was also used on FreeBSD older than 5.3, OpenBSD older than
201311 and NetBSD older than 4.

Author: vstinner

Misc/NEWS.d/next/Library/2020-05-18-15-38-25.bpo-25920.PxrLY8.rst

@@ -0,0 +1,7 @@
+On macOS, when building Python for macOS 10.4 and older, which wasn't the case
+for python.org macOS installer, :func:`socket.getaddrinfo` no longer uses an
+internal lock to prevent race conditions when calling ``getaddrinfo()`` which
+is thread-safe since macOS 10.5. Python 3.9 requires macOS 10.6 or newer. The
+internal lock caused random hang on fork when another thread was calling
+:func:`socket.getaddrinfo`. The lock was also used on FreeBSD older than 5.3,
+OpenBSD older than 201311 and NetBSD older than 4.

Modules/socketmodule.c

@@ -197,43 +197,6 @@ if_indextoname(index) -- return the corresponding interface name\n\
 # define USE_GETHOSTBYNAME_LOCK
 #endif
 
-/* To use __FreeBSD_version, __OpenBSD__, and __NetBSD_Version__ */
-#ifdef HAVE_SYS_PARAM_H
-#include <sys/param.h>
-#endif
-/* On systems on which getaddrinfo() is believed to not be thread-safe,
-   (this includes the getaddrinfo emulation) protect access with a lock.
-
-   getaddrinfo is thread-safe on Mac OS X 10.5 and later. Originally it was
-   a mix of code including an unsafe implementation from an old BSD's
-   libresolv. In 10.5 Apple reimplemented it as a safe IPC call to the
-   mDNSResponder process. 10.5 is the first be UNIX '03 certified, which
-   includes the requirement that getaddrinfo be thread-safe. See issue #25924.
-
-   It's thread-safe in OpenBSD starting with 5.4, released Nov 2013:
-   http://www.openbsd.org/plus54.html
-
-   It's thread-safe in NetBSD starting with 4.0, released Dec 2007:
-
-http://cvsweb.netbsd.org/bsdweb.cgi/src/lib/libc/net/getaddrinfo.c.diff?r1=1.82&r2=1.83
- */
-#if ((defined(__APPLE__) && \
-        MAC_OS_X_VERSION_MIN_REQUIRED < MAC_OS_X_VERSION_10_5) || \
-    (defined(__FreeBSD__) && __FreeBSD_version+0 < 503000) || \
-    (defined(__OpenBSD__) && OpenBSD+0 < 201311) || \
-    (defined(__NetBSD__) && __NetBSD_Version__+0 < 400000000) || \
-    !defined(HAVE_GETADDRINFO))
-#define USE_GETADDRINFO_LOCK
-#endif
-
-#ifdef USE_GETADDRINFO_LOCK
-#define ACQUIRE_GETADDRINFO_LOCK PyThread_acquire_lock(netdb_lock, 1);
-#define RELEASE_GETADDRINFO_LOCK PyThread_release_lock(netdb_lock);
-#else
-#define ACQUIRE_GETADDRINFO_LOCK
-#define RELEASE_GETADDRINFO_LOCK
-#endif
-
 #if defined(__APPLE__) || defined(__CYGWIN__) || defined(__NetBSD__)
 #  include <sys/ioctl.h>
 #endif
@@ -1061,7 +1024,7 @@ new_sockobject(SOCKET_T fd, int family, int type, int proto)
 
 /* Lock to allow python interpreter to continue, but only allow one
    thread to be in gethostbyname or getaddrinfo */
-#if defined(USE_GETHOSTBYNAME_LOCK) || defined(USE_GETADDRINFO_LOCK)
+#if defined(USE_GETHOSTBYNAME_LOCK)
 static PyThread_type_lock netdb_lock;
 #endif
 
@@ -1086,14 +1049,12 @@ setipaddr(const char *name, struct sockaddr *addr_ret, size_t addr_ret_size, int
         hints.ai_socktype = SOCK_DGRAM;         /*dummy*/
         hints.ai_flags = AI_PASSIVE;
         Py_BEGIN_ALLOW_THREADS
-        ACQUIRE_GETADDRINFO_LOCK
         error = getaddrinfo(NULL, "0", &hints, &res);
         Py_END_ALLOW_THREADS
         /* We assume that those thread-unsafe getaddrinfo() versions
            *are* safe regarding their return value, ie. that a
            subsequent call to getaddrinfo() does not destroy the
            outcome of the first call. */
-        RELEASE_GETADDRINFO_LOCK
         if (error) {
             set_gaierror(error);
             return -1;
@@ -1194,7 +1155,6 @@ setipaddr(const char *name, struct sockaddr *addr_ret, size_t addr_ret_size, int
     memset(&hints, 0, sizeof(hints));
     hints.ai_family = af;
     Py_BEGIN_ALLOW_THREADS
-    ACQUIRE_GETADDRINFO_LOCK
     error = getaddrinfo(name, NULL, &hints, &res);
 #if defined(__digital__) && defined(__unix__)
     if (error == EAI_NONAME && af == AF_UNSPEC) {
@@ -1205,7 +1165,6 @@ setipaddr(const char *name, struct sockaddr *addr_ret, size_t addr_ret_size, int
     }
 #endif
     Py_END_ALLOW_THREADS
-    RELEASE_GETADDRINFO_LOCK  /* see comment in setipaddr() */
     if (error) {
         set_gaierror(error);
         return -1;
@@ -6563,10 +6522,8 @@ socket_getaddrinfo(PyObject *self, PyObject *args, PyObject* kwargs)
     hints.ai_protocol = protocol;
     hints.ai_flags = flags;
     Py_BEGIN_ALLOW_THREADS
-    ACQUIRE_GETADDRINFO_LOCK
     error = getaddrinfo(hptr, pptr, &hints, &res0);
     Py_END_ALLOW_THREADS
-    RELEASE_GETADDRINFO_LOCK  /* see comment in setipaddr() */
     if (error) {
         set_gaierror(error);
         goto err;
@@ -6659,10 +6616,8 @@ socket_getnameinfo(PyObject *self, PyObject *args)
     hints.ai_socktype = SOCK_DGRAM;     /* make numeric port happy */
     hints.ai_flags = AI_NUMERICHOST;    /* don't do any name resolution */
     Py_BEGIN_ALLOW_THREADS
-    ACQUIRE_GETADDRINFO_LOCK
     error = getaddrinfo(hostp, pbuf, &hints, &res);
     Py_END_ALLOW_THREADS
-    RELEASE_GETADDRINFO_LOCK  /* see comment in setipaddr() */
     if (error) {
         set_gaierror(error);
         goto fail;
@@ -8422,7 +8377,7 @@ PyInit__socket(void)
 #endif /* _MSTCPIP_ */
 
     /* Initialize gethostbyname lock */
-#if defined(USE_GETHOSTBYNAME_LOCK) || defined(USE_GETADDRINFO_LOCK)
+#if defined(USE_GETHOSTBYNAME_LOCK)
     netdb_lock = PyThread_allocate_lock();
 #endif