Fix #79566: Private SHM is not private on Windows

We map the POSIX semantics of `IPC_PRIVATE` by creating unnamed file
mapping objects on Windows.  While that is not particularly useful for
ext/shmop, which is the only bundled extension which uses `shmget()`,
it may be useful for external extensions.

Author: cmb69

NEWS

@@ -2,6 +2,9 @@ PHP                                                                        NEWS
 |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
 ?? ??? ????, PHP 7.3.19
 
+- Core:
+  . Fixed bug #79566 (Private SHM is not private on Windows). (cmb)
+
 - Opcache:
   . Fixed bug #79535 (PHP crashes with specific opcache.optimization_level).
     (Nikita)

TSRM/tsrm_win32.c

@@ -616,14 +616,16 @@ TSRM_API int shmget(key_t key, size_t size, int flags)
 {/*{{{*/
 	shm_pair *shm;
 	char shm_segment[26], shm_info[29];
-	HANDLE shm_handle, info_handle;
+	HANDLE shm_handle = NULL, info_handle = NULL;
 	BOOL created = FALSE;
 
-	snprintf(shm_segment, sizeof(shm_segment), "TSRM_SHM_SEGMENT:%d", key);
-	snprintf(shm_info, sizeof(shm_info), "TSRM_SHM_DESCRIPTOR:%d", key);
+	if (key != IPC_PRIVATE) {
+		snprintf(shm_segment, sizeof(shm_segment), "TSRM_SHM_SEGMENT:%d", key);
+		snprintf(shm_info, sizeof(shm_info), "TSRM_SHM_DESCRIPTOR:%d", key);
 
-	shm_handle  = OpenFileMapping(FILE_MAP_ALL_ACCESS, FALSE, shm_segment);
-	info_handle = OpenFileMapping(FILE_MAP_ALL_ACCESS, FALSE, shm_info);
+		shm_handle  = OpenFileMapping(FILE_MAP_ALL_ACCESS, FALSE, shm_segment);
+		info_handle = OpenFileMapping(FILE_MAP_ALL_ACCESS, FALSE, shm_info);
+	}
 
 	if (!shm_handle && !info_handle) {
 		if (flags & IPC_CREAT) {
@@ -634,8 +636,8 @@ TSRM_API int shmget(key_t key, size_t size, int flags)
 			DWORD high = 0;
 			DWORD low = size;
 #endif
-			shm_handle	= CreateFileMapping(INVALID_HANDLE_VALUE, NULL, PAGE_READWRITE, high, low, shm_segment);
-			info_handle	= CreateFileMapping(INVALID_HANDLE_VALUE, NULL, PAGE_READWRITE, 0, sizeof(shm->descriptor), shm_info);
+			shm_handle	= CreateFileMapping(INVALID_HANDLE_VALUE, NULL, PAGE_READWRITE, high, low, key == IPC_PRIVATE ? NULL : shm_segment);
+			info_handle	= CreateFileMapping(INVALID_HANDLE_VALUE, NULL, PAGE_READWRITE, 0, sizeof(shm->descriptor), key == IPC_PRIVATE ? NULL : shm_info);
 			created		= TRUE;
 		}
 		if (!shm_handle || !info_handle) {

ext/shmop/tests/shmop_open_private.phpt

@@ -0,0 +1,23 @@
+--TEST--
+shmop_open with IPC_PRIVATE creates private SHM
+--SKIPIF--
+<?php
+if (!extension_loaded('shmop')) die('skip shmop extension not available');
+?>
+--FILE--
+<?php
+$write = 'test';
+
+$shm1 = shmop_open(0, 'c', 0777, 1024);
+shmop_write($shm1, $write, 0);
+
+$shm2 = shmop_open(0, 'c', 0777, 1024);
+$read = shmop_read($shm2, 0, 4);
+
+var_dump(is_string($read) && $read !== $write);
+
+shmop_close($shm1);
+shmop_close($shm2);
+?>
+--EXPECT--
+bool(true)