[lldb] Fix TestSettings.test_pass_host_env_vars on windows

Summary:
A defensive check in ProcessLauncherWindows meant that we would never
attempt to launch a process with a completely empty environment -- the
host environment would be used instead. Instead, I make the function add
an extra null wchar_t at the end of an empty environment. The
documentation on this is a bit fuzzy, but it seems to be what is needed
to make windows accept these kinds of environments.

Reviewers: amccarth, friss

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D76835

lldb/source/Host/windows/ProcessLauncherWindows.cpp

@@ -23,10 +23,9 @@ using namespace lldb_private;
 namespace {
 void CreateEnvironmentBuffer(const Environment &env,
                              std::vector<char> &buffer) {
-  if (env.size() == 0)
-    return;
-
-  // Environment buffer is a null terminated list of null terminated strings
+  // The buffer is a list of null-terminated UTF-16 strings, followed by an
+  // extra L'\0' (two bytes of 0).  An empty environment must have one
+  // empty string, followed by an extra L'\0'.
   for (const auto &KV : env) {
     std::wstring warg;
     if (llvm::ConvertUTF8toWide(Environment::compose(KV), warg)) {
@@ -38,6 +37,9 @@ void CreateEnvironmentBuffer(const Environment &env,
   // One null wchar_t (to end the block) is two null bytes
   buffer.push_back(0);
   buffer.push_back(0);
+  // Insert extra two bytes, just in case the environment was empty.
+  buffer.push_back(0);
+  buffer.push_back(0);
 }
 
 bool GetFlattenedWindowsCommandString(Args args, std::string &command) {
@@ -94,8 +96,7 @@ ProcessLauncherWindows::LaunchProcess(const ProcessLaunchInfo &launch_info,
 
   LPVOID env_block = nullptr;
   ::CreateEnvironmentBuffer(launch_info.GetEnvironment(), environment);
-  if (!environment.empty())
-    env_block = environment.data();
+  env_block = environment.data();
 
   executable = launch_info.GetExecutableFile().GetPath();
   GetFlattenedWindowsCommandString(launch_info.GetArguments(), commandLine);

lldb/test/API/commands/settings/TestSettings.py

@@ -286,7 +286,6 @@ def do_test_run_args_and_env_vars(self, use_launchsimple):
                 "Environment variable 'MY_ENV_VAR' successfully passed."])
 
     @skipIfRemote  # it doesn't make sense to send host env to remote target
-    @skipIf(oslist=["windows"])
     def test_pass_host_env_vars(self):
         """Test that the host env vars are passed to the launched process."""
         self.build()