Simplify syspaths

Here's an attempt at minimising what goes into PATH on startup. Previously, whatever the PATH was set to on the users machine got forwarded into the Rez context, but that's bad as we can never be sure what that PATH actually is. Now we know it includes these three paths, which is what you'll find in a newly installed Windows install. They are added dynamically, based on where `cmd.exe` and `powershell.exe` are first found, which is debatable whether that's desirable. The alternative is simply adding `c:\windows\system32` etc directly, which may be the most secure route.

src/rezplugins/shell/powershell.py

@@ -1,6 +1,5 @@
-"""
-Windows Command Prompt (DOS) shell.
-"""
+"""Windows PowerShell 5+"""
+
 from rez.config import config
 from rez.rex import RexExecutor, OutputStyle, EscapedString
 from rez.shells import Shell
@@ -77,83 +76,19 @@ def get_syspaths(cls):
             cls.syspaths = config.standard_system_paths
             return cls.syspaths
 
-        # detect system paths using registry
-        def gen_expected_regex(parts):
-            whitespace = r"[\s]+"
-            return whitespace.join(parts)
-
-        paths = []
-
-        cmd = [
-            "REG",
-            "QUERY",
-            (
-                "HKLM\\SYSTEM\\CurrentControlSet\\"
-                "Control\\Session Manager\\Environment"
-            ),
-            "/v",
-            "PATH"
+        # Vanilla PATH, before user interference
+        cls.syspaths = [
+            os.path.dirname(which("cmd")),  # e.g. System32/
+            os.path.dirname(which("powershell")),
+            os.path.dirname(which("scrcons")),
         ]
 
-        expected = gen_expected_regex([
-            (
-                "HKEY_LOCAL_MACHINE\\\\SYSTEM\\\\CurrentControlSet\\\\"
-                "Control\\\\Session Manager\\\\Environment"
-            ),
-            "PATH",
-            "REG_(EXPAND_)?SZ",
-            "(.*)"
-        ])
-
-        p = popen(cmd,
-                  stdout=subprocess.PIPE,
-                  stderr=subprocess.PIPE,
-                  universal_newlines=True,
-                  shell=True)
-        out_, _ = p.communicate()
-        out_ = out_.strip()
-
-        if p.returncode == 0:
-            match = re.match(expected, out_)
-            if match:
-                paths.extend(match.group(2).split(os.pathsep))
-
-        cmd = [
-            "REG",
-            "QUERY",
-            "HKCU\\Environment",
-            "/v",
-            "PATH"
+        # And Rez itself, for making rez calls
+        # within the resulting context
+        cls.syspaths += [
+            os.path.dirname(which("rez")),
         ]
 
-        expected = gen_expected_regex([
-            "HKEY_CURRENT_USER\\\\Environment",
-            "PATH",
-            "REG_(EXPAND_)?SZ",
-            "(.*)"
-        ])
-
-        p = popen(cmd,
-                  stdout=subprocess.PIPE,
-                  stderr=subprocess.PIPE,
-                  universal_newlines=True,
-                  shell=True)
-        out_, _ = p.communicate()
-        out_ = out_.strip()
-
-        if p.returncode == 0:
-            match = re.match(expected, out_)
-            if match:
-                paths.extend(match.group(2).split(os.pathsep))
-
-        cls.syspaths = list(set([x for x in paths if x]))
-
-        # add Rez binaries
-        exe = which("rez-env")
-        assert exe, "Could not find rez binary, this is a bug"
-        rez_bin_dir = os.path.dirname(exe)
-        cls.syspaths.insert(0, rez_bin_dir)
-
         return cls.syspaths
 
     def _bind_interactive_rez(self):