posix: move checkexec test file to .hg/cache This avoids unnecessary churn in the working directory. It is not necessarily a fully valid assumption that .hg/cache is on the same filesystem as the working directory, but I think it is an acceptable approximation. It could also be the case that different parts of the working directory is on different mount points so checking in the root folder could also be wrong.

manifest: move manifestctx creation into manifestlog.get() Most manifestctx creation already happened in manifestlog.get(), but there was one spot in the manifestctx class itself that created an instance manually. This patch makes that one instance go through the manifestlog. This means extensions can just wrap manifestlog.get() and it will cover all manifestctx creations. It also means this code path now hits the manifestlog cache.

util: implement zstd compression engine Now that zstd is vendored and being built (in some configurations), we can implement a compression engine for zstd! The zstd engine is a little different from existing engines. Because it may not always be present, we have to defer load the module in case importing it fails. We facilitate this via a cached property that holds a reference to the module or None. The "available" method is implemented to reflect reality. The zstd engine declares its ability to handle bundles using the "zstd" human name and the "ZS" internal name. The latter was chosen because internal names are 2 characters (by only convention I think) and "ZS" seems reasonable. The engine, like others, supports specifying the compression level. However, there are no consumers of this API that yet pass in that argument. I have plans to change that, so stay tuned. Since all we need to do to support bundle generation with a new compression engine is implement and register the compression engine, bundle generation with zstd "just works!" Tests demonstrating this have been added. How does performance of zstd for bundle generation compare? On the mozilla-unified repo, `hg bundle --all -t <engine>-v2` yields the following on my i7-6700K on Linux: engine CPU time bundle size vs orig size throughput none 97.0s 4,054,405,584 100.0% 41.8 MB/s bzip2 (l=9) 393.6s 975,343,098 24.0% 10.3 MB/s gzip (l=6) 184.0s 1,140,533,074 28.1% 22.0 MB/s zstd (l=1) 108.2s 1,119,434,718 27.6% 37.5 MB/s zstd (l=2) 111.3s 1,078,328,002 26.6% 36.4 MB/s zstd (l=3) 113.7s 1,011,823,727 25.0% 35.7 MB/s zstd (l=4) 116.0s 1,008,965,888 24.9% 35.0 MB/s zstd (l=5) 121.0s 977,203,148 24.1% 33.5 MB/s zstd (l=6) 131.7s 927,360,198 22.9% 30.8 MB/s zstd (l=7) 139.0s 912,808,505 22.5% 29.2 MB/s zstd (l=12) 198.1s 854,527,714 21.1% 20.5 MB/s zstd (l=18) 681.6s 789,750,690 19.5% 5.9 MB/s On compression, zstd for bundle generation delivers: * better compression than gzip with significantly less CPU utilization * better than bzip2 compression ratios while still being significantly faster than gzip * ability to aggressively tune compression level to achieve significantly smaller bundles That last point is important. With clone bundles, a server can pre-generate a bundle file, upload it to a static file server, and redirect clients to transparently download it during clone. The server could choose to produce a zstd bundle with the highest compression settings possible. This would take a very long time - a magnitude longer than a typical zstd bundle generation - but the result would be hundreds of megabytes smaller! For the clone volume we do at Mozilla, this could translate to petabytes of bandwidth savings per year and faster clones (due to smaller transfer size). I don't have detailed numbers to report on decompression. However, zstd decompression is fast: >1 GB/s output throughput on this machine, even through the Python bindings. And it can do that regardless of the compression level of the input. By the time you have enough data to worry about overhead of decompression, you have plenty of other things to worry about performance wise. zstd is wins all around. I can't wait to implement support for it on the wire protocol and in revlogs.

hghave: add check for zstd support Not all configurations will support zstd. Add a check so we can conditionalize tests.

exchange: obtain compression engines from the registrar util.compengines has knowledge of all registered compression engines and the metadata that associates them with various bundle types. This patch removes the now redundant declaration of this metadata from exchange.py and obtains it from the new source. The effect of this patch is that once a new compression engine is registered with util.compengines, `hg bundle -t <engine>` will just work.

bundle2: equate 'UN' with no compression An upcoming patch will change the "alg" argument passed to this function from None to "UN" when no compression is wanted. The existing implementation of bundle2 does not set a "Compression" parameter if no compression is used. In theory, setting "Compression=UN" should work. But I haven't audited the code to see if all client versions supporting bundle2 will accept this. Rather than take the risk, avoid the BC breakage and treat "UN" the same as None.