Add WT_SESSION to WSLENV so it propagates into WSL (#4157)

This makes sure that things running inside WSL can see `WT_SESSION`.

Closes #3948.

src/cascadia/TerminalConnection/ConptyConnection.cpp

@@ -88,6 +88,18 @@ namespace winrt::Microsoft::Terminal::TerminalConnection::implementation
         std::wstring cmdline{ wil::ExpandEnvironmentStringsW<std::wstring>(_commandline.c_str()) }; // mutable copy -- required for CreateProcessW
 
         Utils::EnvironmentVariableMapW environment;
+        auto zeroEnvMap = wil::scope_exit([&] {
+            // Can't zero the keys, but at least we can zero the values.
+            for (auto& [name, value] : environment)
+            {
+                ::SecureZeroMemory(value.data(), value.size() * sizeof(decltype(value.begin())::value_type));
+            }
+
+            environment.clear();
+        });
+
+        // Populate the environment map with the current environment.
+        RETURN_IF_FAILED(Utils::UpdateEnvironmentMapW(environment));
 
         {
             // Convert connection Guid to string and ignore the enclosing '{}'.
@@ -97,7 +109,13 @@ namespace winrt::Microsoft::Terminal::TerminalConnection::implementation
             const auto guidSubStr = std::wstring_view{ wsGuid }.substr(1);
 
             // Ensure every connection has the unique identifier in the environment.
-            environment.emplace(L"WT_SESSION", guidSubStr.data());
+            environment.insert_or_assign(L"WT_SESSION", guidSubStr.data());
+
+            auto wslEnv = environment[L"WSLENV"]; // We always want to load something, even if it's blank.
+            // WSLENV is a colon-delimited list of environment variables (+flags) that should appear inside WSL
+            // https://devblogs.microsoft.com/commandline/share-environment-vars-between-wsl-and-windows/
+            wslEnv = L"WT_SESSION:" + wslEnv; // prepend WT_SESSION to make sure it's visible inside WSL.
+            environment.insert_or_assign(L"WSLENV", wslEnv);
         }
 
         std::vector<wchar_t> newEnvVars;
@@ -106,17 +124,6 @@ namespace winrt::Microsoft::Terminal::TerminalConnection::implementation
                                newEnvVars.size() * sizeof(decltype(newEnvVars.begin())::value_type));
         });
 
-        auto zeroEnvMap = wil::scope_exit([&] {
-            // Can't zero the keys, but at least we can zero the values.
-            for (auto& [name, value] : environment)
-            {
-                ::SecureZeroMemory(value.data(), value.size() * sizeof(decltype(value.begin())::value_type));
-            }
-
-            environment.clear();
-        });
-
-        RETURN_IF_FAILED(Utils::UpdateEnvironmentMapW(environment));
         RETURN_IF_FAILED(Utils::EnvironmentMapToEnvironmentStringsW(environment, newEnvVars));
 
         LPWCH lpEnvironment = newEnvVars.empty() ? nullptr : newEnvVars.data();