Quickly invoking PowerShell menu completion multiple times crashes Terminal

[krzysdz]

Windows Terminal version

49e4eea

Windows build number

10.0.19045.4291

Other Software

No response

Steps to reproduce

1. Enable experimental shell completion menu ("experimental.enableShellCompletionMenu": true)
2. Quickly invoke shell completion multiple times, so that Terminal tries to parse JSON with completion data at the same time from multiple threads in one of the following ways:
   • Terminal versions from 432dfcc to 6d0342f:
     1. Set up shell completion as described in How do I test this? section of Add Suggestions UI & experimental shell completions support #14938
     2. Type something e.g. l
     3. Press Ctrl+Space multiple times
     4. Press any character key - because of Output containing OSC sequences (e.g. shell integration) is not rendered (regression) #17204 this will send the completions generated in c. at the same time
   • Terminal versions 0b76c51 and newer or older than 432dfcc:
     1. Set up shell completion as described in How do I test this? section of Add Suggestions UI & experimental shell completions support #14938, but instead of

        $result += $completions.CompletionMatches | ConvertTo-Json -Compress

        append a large JSON string to $result. It doesn't have to be a valid shell completion data, but it should be a valid JSON. 7.6M characters is large enough to reproduce the bug on my computer and probably every other PC without trying to be fast.
     2. Type something e.g. l
     3. Press Ctrl+Space multiple times. Depending on your CPU and the JSON size it may or may not have to be done quickly.

Expected Behavior

Completion menu is shown or nothing happens

Actual Behavior

Terminal crashes or throws an exception or some kind of invalid memory access error, because JSON::CharReader in Command::ParsePowerShellMenuComplete is static and shared between threads:

terminal/src/cascadia/TerminalSettingsModel/Command.cpp

Line 693 in 49e4eea

static std::unique_ptr<Json::CharReader> reader{ Json::CharReaderBuilder{}.newCharReader() };

Reproduction in one of the versions affected by #17204:

2024-05-08.22-04-15.mp4

Reproduction in other Terminal versions and demonstration of a possible fix (no menu is shown, because of the huge JSON, but the Terminal doesn't crash):

2024-05-08.23-51-43.mp4