dispatch: handle IOError when writing to stderr Previously, attempts to write to stderr in dispatch.run() may lead to an exception being thrown. This would likely be handled by Python's default exception handler, which would print the exception and exit 1. Code in this function is already catching IOError for stdout failures and converting to exit code 255 (-1 & 255 == 255). Why we weren't doing the same for stderr for the sake of consistency, I don't know. I do know that chg and hg diverged in behavior here (as the changed test-basic.t shows). After this commit, we catch I/O failure on stderr and change the exit code to 255. chg and hg now behave consistently. As a bonus, Rust hg also now passes this test. I'm skeptical at changing the exit code due to failures this late in the process. I think we should consider preserving the current exit code - assuming it is non-0. And, we may want to preserve the exit code completely if the I/O error is EPIPE (and potentially other special error classes). There's definitely room to tweak behavior. But for now, let's at least prevent the uncaught exception. Differential Revision: https://phab.mercurial-scm.org/D1860