Don't ever allow ~ as a startingDirectory (#12437)

Basically, some WSL distros ship fragments that replace the `commandline` with the executable for their distro (`ubuntu.exe`, etc.). We didn't expect that when we changed the `startingDirectory` for them all to `~`.

Unfortunately, `~` is really never a valid path for a process on windows, so those distros would now fail with

```
[error 2147942667 (0x8007010b) when launching `ubuntu1804.exe']
Could not access starting directory "~"
```

If we find that we were unable to mangle `~` into the user's WSL `commandline`, then we will re-evaluate that `startingDirectory` as `%USERPROFILE%`, which is at least something sensible, if albeit not what they wanted.

* regressed in #12315
* [x] Closes #12353
* [x] Tested with a (`ubuntu1804.exe`, `~`) profile - launched successfully, where 1.13 in market fails.
* [x] added tests

(cherry picked from commit 1870fee)

src/types/ut_types/UtilsTests.cpp

@@ -479,5 +479,29 @@ void UtilsTests::TestMangleWSLPaths()
         VERIFY_ARE_EQUAL(LR"("wsl" --cd "\\wsl.localhost\Ubuntu\home\user" -d Ubuntu)", commandline);
         VERIFY_ARE_EQUAL(L"", path);
     }
+
+    /// Tests for GH #12353
+
+    const auto expectedUserProfilePath = wil::ExpandEnvironmentStringsW<std::wstring>(L"%USERPROFILE%");
+    {
+        auto [commandline, path] = MangleStartingDirectoryForWSL(LR"(wsl -d Ubuntu)", L"~");
+        VERIFY_ARE_EQUAL(LR"("wsl" --cd "~" -d Ubuntu)", commandline);
+        VERIFY_ARE_EQUAL(L"", path);
+    }
+    {
+        auto [commandline, path] = MangleStartingDirectoryForWSL(LR"(wsl ~ -d Ubuntu)", L"~");
+        VERIFY_ARE_EQUAL(LR"(wsl ~ -d Ubuntu)", commandline);
+        VERIFY_ARE_EQUAL(expectedUserProfilePath, path);
+    }
+    {
+        auto [commandline, path] = MangleStartingDirectoryForWSL(LR"(ubuntu ~ -d Ubuntu)", L"~");
+        VERIFY_ARE_EQUAL(LR"(ubuntu ~ -d Ubuntu)", commandline);
+        VERIFY_ARE_EQUAL(expectedUserProfilePath, path);
+    }
+    {
+        auto [commandline, path] = MangleStartingDirectoryForWSL(LR"(powershell.exe)", L"~");
+        VERIFY_ARE_EQUAL(LR"(powershell.exe)", commandline);
+        VERIFY_ARE_EQUAL(expectedUserProfilePath, path);
+    }
 }
 #endif

src/types/utils.cpp

@@ -670,8 +670,15 @@ std::tuple<std::wstring, std::wstring> Utils::MangleStartingDirectoryForWSL(std:
         }
     } while (false);
 
+    // GH #12353: `~` is never a valid windows path. We can only accept that as
+    // a startingDirectory when the exe is specifically wsl.exe, because that
+    // can override the real startingDirectory. If the user set the
+    // startingDirectory to ~, but the commandline to something like pwsh.exe,
+    // that won't actually work. In that case, mangle the startingDirectory to
+    // %userprofile%, so it's at least something reasonable.
     return {
         std::wstring{ commandLine },
-        std::wstring{ startingDirectory }
+        startingDirectory == L"~" ? wil::ExpandEnvironmentStringsW<std::wstring>(L"%USERPROFILE%") :
+                                    std::wstring{ startingDirectory }
     };
 }