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fix(cli-repl): only ever run MongoshRepl.onExit once MONGOSH-1943 (#2300
) Previously, a task in our CI was flaky because it was running `exit\n` as literal input to the shell, expecting it to reliably make the shell exit. `exit` as a command leads to the `MongoshRepl.onExit` function called, which then calls a) `.close()` and, indirectly, `repl.close()` on the Node.js REPL instance as well b) the `CliRepl.exit` function (and through that, indirectly, `process.exit()`). However, closing the Node.js REPL instance makes it flush the history file to disk as a typically fast but asynchronous operation, and, after that, emit `exit` on the REPL; that in turn called `MongoshRepl.onExit` again (i.e. calling `MongoshRepl.onExit` leads to another call to the same function, just asynchronously with a short delay). The flaky test in CI failed because of a combination of issues, including the fact that `process.exit()` did not necessarily actually exit the process because of coverage tooling interfering with it (which in turn happened because it was running with an altered working directory pointing at a temporary directory where `nyc` had not created a subdirectory for storing coverage output). In typical operations, the REPL would flush history quickly, and calling `.onExit()` again would allow the process to exit normally (by ways of calling `CliRepl.exit` and `process.exit()` a second time). However, in the flaky failure case, the history flushing operation would take long enough that the original `process.exit()` call (which failed due to nyc) would set the internal `process._exiting` flag before throwing, which in turn prevents `process.nextTick()` from scheduling callbacks, and which then ultimately prevented the REPL from cleaning itself up, leading to the process timing out instead of exiting. This commit introduces safeguards against calling `.onExit()` twice, always returning the same (never-resolving) `Promise` instance, and explicitly keeps track of whether the REPL was closed or not.
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