Make sure a /tmp path exists under linux

CHANGES.md

@@ -13,6 +13,8 @@ The released versions correspond to PyPI releases.
 * Some public constants in `fake_filesystem` that had been moved to `helpers` are
   made accessible from there again (see [#809](../../issues/809)).
 * Add missing fake implementations for `os.getuid` and `os.getgid` (Posix only)
+* Make sure a `/tmp` path exists under linux (`TMPDIR` may point elsewhere)
+  (see [#810](../../issues/810))
 
 
 ## [Version 5.2.1](https://pypi.python.org/pypi/pyfakefs/5.2.1) (2023-04-11)

docs/troubleshooting.rst

@@ -170,8 +170,12 @@ directory named ``/tmp`` when running on Linux or Unix systems,
       # the temp directory is always present at test start
       assert len(os.listdir("/")) == 1
 
-Under macOS, a symlink to the actual temp directory is created additionally as `/tmp`
-in the fake filesystem.
+Under macOS and linux, if the actual temp path is not `/tmp` (which is always the case
+under macOS), a symlink to the actual temp directory is additionally created as `/tmp`
+in the fake filesystem. Note that the file size of this link is ignored while
+calculating the fake filesystem size, so that the used size with an otherwise empty
+fake filesystem can always be assumed to be 0.
+
 
 User rights
 -----------

pyfakefs/fake_filesystem_unittest.py

@@ -895,8 +895,8 @@ def setUp(self, doctester: Any = None) -> None:
         # so we create it here for convenience
         assert self.fs is not None
         self.fs.create_dir(temp_dir)
-        if sys.platform == "darwin" and not self.fs.exists("/tmp"):
-            # under macOS, we also create a link in /tmp for convenience
+        if sys.platform != "win32" and not self.fs.exists("/tmp"):
+            # under Posix, we also create a link in /tmp if the path does not exist
             self.fs.create_symlink("/tmp", temp_dir)
             # reset the used size to 0 to avoid having the link size counted
             # which would make disk size tests more complicated

pyfakefs/tests/fake_filesystem_unittest_test.py

@@ -662,7 +662,7 @@ def testTempDirExists(self):
 
     @unittest.skipIf(sys.platform == "win32", "POSIX only test")
     def testTmpExists(self):
-        # directory under Linux, link under macOS
+        # directory or link under Linux, link under macOS
         self.assertTrue(os.path.exists("/tmp"))