[llvm-lit] copy CRT/STL DLLs into the output directory

It turns out that the STL requires you to place the DLLs into the
directory of the executable; else, msvcp140.dll will be taken from
C:\Windows\System32 (which is unsupported).

This was discovered because of the recent constexpr mutex change;
see [microsoft/STL#3824][]. Especially the fuzzer unit tests fail
completely with the mutex changes.

[microsoft/STL#3824]: microsoft/STL#3824

Differential Revision: https://reviews.llvm.org/D158221

llvm/utils/lit/lit/TestRunner.py

@@ -11,6 +11,7 @@
 import shutil
 import tempfile
 import threading
+from pathlib import Path
 
 import io
 
@@ -2066,7 +2067,18 @@ def runOnce(execdir):
         return out, err, exitCode, timeoutInfo, status
 
     # Create the output directory if it does not already exist.
-    lit.util.mkdir_p(os.path.dirname(tmpBase))
+    outputDir = os.path.dirname(tmpBase)
+    lit.util.mkdir_p(outputDir)
+
+    # On Windows, copy the required DLLs from PATH into the test directory
+    # This avoids the loader finding DLLs in C:\Windows\System32
+    if litConfig.isWindows:
+        toolsetDirectory = Path(lit.util.which("cl.exe")).parent
+        for dllToCopy in itertools.chain(\
+                toolsetDirectory.glob('msvcp*.dll'),\
+                toolsetDirectory.glob('vcruntime*.dll'),\
+                toolsetDirectory.glob('ucrtbase*.dll')):
+            shutil.copyfile(dllToCopy, os.path.join(outputDir, dllToCopy.name))
 
     # Re-run failed tests up to test.allowed_retries times.
     execdir = os.path.dirname(test.getExecPath())

llvm/utils/lit/lit/util.py

@@ -241,10 +241,10 @@ def which(command, paths=None):
 
     # Get suffixes to search.
     # On Cygwin, 'PATHEXT' may exist but it should not be used.
-    if os.pathsep == ";":
-        pathext = os.environ.get("PATHEXT", "").split(";")
+    if os.path.splitext(command)[-1] != '' or os.pathsep != ';':
+        pathext = ['']
     else:
-        pathext = [""]
+        pathext = os.environ.get('PATHEXT', '').split(';')
 
     # Search the paths...
     for path in paths.split(os.pathsep):