ui: restore behavior to ignore some I/O errors (issue5658) e9646ff34d55 and 1bfb9a63b98e refactored ui methods to no longer silently swallow some IOError instances. This is arguably the correct thing to do. However, it had the unfortunate side-effect of causing StdioError to bubble up to sensitive code like transaction aborts, leading to an uncaught exceptions and failures to e.g. roll back a transaction. This could occur when a remote HTTP or SSH client connection dropped. The new behavior is resulting in semi-frequent "abandonded transaction" errors on multiple high-volume repositories at Mozilla. This commit effectively reverts e9646ff34d55 and 1bfb9a63b98e to restore the old behavior. I agree with the principle that I/O errors shouldn't be ignored. That makes this change... unfortunate. However, our hands are tied for what to do on stable. I think the proper solution is for the ui's behavior to be configurable (possibly via a context manager). During critical sections like transaction rollback and abort, it should be possible to suppress errors. But this feature would not be appropriate on stable.

tests: test behavior of IOError during transactions (issue5658) ui._write(), ui._write_err(), and ui.flush() all trap IOError and re-raise as error.StdioError. If a caller doesn't catch StdioError when writing to stdio, it could bubble all the way to dispatch. This commit adds tests for I/O failures around various transaction operations. The most notable badness is during abort. Here, an uncaught StdioError will result in incomplete transaction rollback, requiring an `hg rollback` to recover. This can result in a client "corrupting" a remote repo via terminated HTTP and SSH socket.

log: mention ui.logtemplate in the help text

hg: avoid relying on errno numbers / descriptions A few tests hardcode errno numbers and/or descriptions in the output, causing test failures on platforms where these values are different. Differential Revision: https://phab.mercurial-scm.org/D362

hg: tolerate long vs. int in test-context.py The file times here can be longs instead of ints on some platforms, which will cause a test failure due to these printing with an L suffix; instead always format with %d which will produce the same output in either case. Differential Revision: https://phab.mercurial-scm.org/D361

setup: fix installing in a mingw environment The addition, in 9a4adc76c88a, of a hack for the MSVC compiler class was overwriting the original class for the Mingw32CCompiler class, leading to an error when the HackedMingw32CCompiler is instantiated. Differential Revision: https://phab.mercurial-scm.org/D329

osx: delay version computation on macOS builds Before this patch, HGVER would be evaluated at the beginning of the make execution, and would be unset because build/mercurial/ doesn't exist yet at that point. Now we compute the version after the `make install` run has completed. This is backported to stable from 8626b44516c1, but that revision had an error in the shell invocation syntax.

branchmap: revert c34532365b38 for Python 2.7 compatibility Old versions of python 2.7 don't like that the second argument to struct.unpack_from is a bytearray, so the change removing the util.buffer around that argument in branchmap broke running on older versions of python 2.7. Differential Revision: https://phab.mercurial-scm.org/D330