merge: fix race that could cause wrong size in dirstate The problem is that hg merge/update/etc work the following way: 1. figure out what files to update 2. apply the update to disk 3. apply the update to in-memory dirstate 4. write dirstate where step3 looks at the filesystem and assumes it sees the result of step2. If a file is changed between step2 and step3, step3 will record incorrect information in the dirstate. I avoid this by passing the size step3 needs directly from step2, for the common path (not implemented for change/delete conflicts for instance). I didn't fix the same race for the exec bit for now, because it's less likely to be problematic and I had trouble due to the fact that the dirstate stores the permissions differently from the manifest (st_mode vs '' 'l' 'x'), in combination with tests that pretend that symlinks are not supported. However, I moved the lstat from step3 to step2, which should tighten the race window markedly, both for the exec bit and for the mtime. Differential Revision: https://phab.mercurial-scm.org/D6475

worker: support parallelization of functions with return values Currently worker supports running functions that return a progress iterator. Generalize it to handle function that return a progress iterator then a return value. It's unused in this commit, but will be used in the next one. Differential Revision: https://phab.mercurial-scm.org/D6515

tests: show how the dirstate can end up containing wrong information which can result in bad status output. Concretely, this seems to be easily triggered by having a build system watching the filesystem for changes, and rebuilding files that are both tracked and generated while an update is happening. Differential Revision: https://phab.mercurial-scm.org/D6474

rust: new rust options in setup.py The --rust global option turns on usage (and by default compilation) of the rust-cpython based mercurial.rustext. Similarly to what's previously done for zstd, there is a --no-rust option for the build_ext subcommand in order not to build mercurial.rustext, allowing for an OS distribution to prebuild it. The HGWITHRUSTEXT environment variable is still honored, and has the same effect as before, but now it works mostly by making the --rust global option defaulting to True, with some special cases for the direct-ffi case (see more about that below) Coincidentally, the --rust flag can also be passed from the make commands, like actually all global options, in the PURE variable make local PURE=--rust This feels inappropriate, though, and we should follow up with a proper make variable for that case. Although the direct-ffi bindings aren't directly useful any more, we keep them at this stage because - they provide a short prototyping path for experiments in which a C extension module has to call into a Rust extension. The proper way of doing that would be to use capsules, and it's best to wait for our pull request onto rust-cpython for that: https://github.com/dgrunwald/rust-cpython/pull/169 - Build support for capsules defined in Rust will probably need to reuse some of what's currently in use for direct-ffi.

rust: using policy.importrust from Python callers This commit converts all current Python callers of mercurial.rustext to the new policy.importrust system. After this point, going through policy.importrust or policy.importmod (in some more distant future) is mandatory for callers of Rust code outside of Python tests. We felt it to be appropriate to keep Rust-specific tests run inconditionally if the Rust extensions are present.

rust: module policy with importrust We introduce two rust+c module policies and a new `policy.importrust()` that makes use of them. This simple approach provides runtime switching of implementations, which is crucial for the performance measurements such as those Octobus does with ASV. It can also be useful for bug analysis. It also has the advantage of making conditionals in Rust callers more uniform, in particular abstracting over specifics like `demandimport` At this point, the build stays unchanged, with the rust-cpython based `rustext` module being built if HGWITHRUSTEXT=cpython. More transparency for the callers, i.e., just using `policy.importmod` would be a much longer term and riskier effort for the following reasons: 1. It would require to define common module boundaries for the three or four cases (pure, c, rust+ext, cffi) and that is premature with the Rust extension currently under heavy development in areas that are outside the scope of the C extensions. 2. It would imply internal API changes that are not currently wished, as the case of ancestors demonstrates. 3. The lack of data or property-like attributes (tp_member and tp_getset) in current `rust-cpython` makes it impossible to achieve direct transparent replacement of pure Python classes by Rust extension code, meaning that the caller sometimes has to be able to make adjustments or provide additional wrapping.

help: add help entry for internals.mergestate This patch adds an entry for `internals.mergestate` as suggested by @marmoute. Most of the help text is taken from `merge.mergestate`. Differential Revision: https://phab.mercurial-scm.org/D6448 Differential Revision: https://phab.mercurial-scm.org/D6528