sqlitestore: file storage backend using SQLite This commit provides an extension which uses SQLite to store file data (as opposed to revlogs). As the inline documentation describes, there are still several aspects to the extension that are incomplete. But it's a start. The extension does support basic clone, checkout, and commit workflows, which makes it suitable for simple use cases. One notable missing feature is support for "bundlerepos." This is probably responsible for the most test failures when the extension is activated as part of the test suite. All revision data is stored in SQLite. Data is stored as zstd compressed chunks (default if zstd is available), zlib compressed chunks (default if zstd is not available), or raw chunks (if configured or if a compressed delta is not smaller than the raw delta). This makes things very similar to revlogs. Unlike revlogs, the extension doesn't yet enforce a limit on delta chain length. This is an obvious limitation and should be addressed. This is somewhat mitigated by the use of zstd, which is much faster than zlib to decompress. There is a dedicated table for storing deltas. Deltas are stored by the SHA-1 hash of their uncompressed content. The "fileindex" table has columns that reference the delta for each revision and the base delta that delta should be applied against. A recursive SQL query is used to resolve the delta chain along with the delta data. By storing deltas by hash, we are able to de-duplicate delta storage! With revlogs, the same deltas in different revlogs would result in duplicate storage of that delta. In this scheme, inserting the duplicate delta is a no-op and delta chains simply reference the existing delta. When initially implementing this extension, I did not have content-indexed deltas and deltas could be duplicated across files (just like revlogs). When I implemented content-indexed deltas, the size of the SQLite database for a full clone of mozilla-unified dropped: before: 2,554,261,504 bytes after: 2,488,754,176 bytes Surprisingly, this is still larger than the bytes size of revlog files: revlog files: 2,104,861,230 bytes du -b: 2,254,381,614 I would have expected storage to be smaller since we're not limiting delta chain length and since we're using zstd instead of zlib. I suspect the SQLite indexes and per-column overhead account for the bulk of the differences. (Keep in mind that revlog uses a 64-byte packed struct for revision index data and deltas are stored without padding. Aside from the 12 unused bytes in the 32 byte node field, revlogs are pretty efficient.) Another source of overhead is file name storage. With revlogs, file names are stored in the filesystem. But with SQLite, we need to store file names in the database. This is roughly equivalent to the size of the fncache file, which for the mozilla-unified repository is ~34MB. Since the SQLite database isn't append-only and since delta chains can reference any delta, this opens some interesting possibilities. For example, we could store deltas in reverse, such that fulltexts are stored for newer revisions and deltas are applied to reconstruct older revisions. This is likely a more optimal storage strategy for version control, as new data tends to be more frequently accessed than old data. We would obviously need wire protocol support for transferring revision data from newest to oldest. And we would probably need some kind of mechanism for "re-encoding" stores. But it should be doable. This extension is very much experimental quality. There are a handful of features that don't work. It probably isn't suitable for day-to-day use. But it could be used in limited cases (e.g. read-only checkouts like in CI). And it is also a good proving ground for alternate storage backends. As we continue to define interfaces for all things storage, it will be useful to have a viable alternate storage backend to see how things shake out in practice. test-storage.py passes on Python 2 and introduces no new test failures on Python 3. Having the storage-level unit tests has proved to be insanely useful when developing this extension. Those tests caught numerous bugs during development and I'm convinced this style of testing is the way forward for ensuring alternate storage backends work as intended. Of course, test coverage isn't close to what it needs to be. But it is a start. And what coverage we have gives me confidence that basic store functionality is implemented properly. Differential Revision: https://phab.mercurial-scm.org/D4928

storageutil: extract most of peek_censored from revlog This function is super hacky and isn't correct 100% of the time. I'm going to need this functionality on a future non-revlog store. Let's copy things to storageutil so this code only exists once. Differential Revision: https://phab.mercurial-scm.org/D5118

lfs: autoload the extension when cloning from repo with lfs enabled This is based on a patch by Gregory Szorc. I made small adjustments to clean up the messaging when the server has the extension enabled, but the client has it disabled (to prevent autoloading). Additionally, I added a second server capability to distinguish between the server having the extension enabled, and the server having LFS commits. This helps prevent unnecessary requirement propagation- the client shouldn't add a requirement that the server doesn't have, just because the server had the extension loaded. The TODO I had about advertising a capability when the server can natively serve up blobs isn't relevant anymore (we've had 2 releases that support this), so I dropped it. Currently, we lazily add the "lfs" requirement to a repo when we first encounter LFS data. Due to a pretxnchangegroup hook that looks for LFS data, this can happen at the end of clone. Now that we have more control over how repositories are created, we can do better. This commit adds a repo creation option to add the "lfs" requirement. hg.clone() sets this creation option if the remote peer is advertising lfs usage (as opposed to just support needed to push). So, what this change effectively does is have cloned repos automatically inherit the "lfs" requirement. Differential Revision: https://phab.mercurial-scm.org/D5130

testing: switch to inserting deltas As the comment in the test specifies, this was relying on storage backend implementation details. We switch to inserting a raw delta, skipping the regular insert path to ensure we have the desired outcome. This required implementing support for handling deltas in the revlog testing code. Differential Revision: https://phab.mercurial-scm.org/D5116

testing: remove expectation of error on bad node insert addgroup() doesn't necessarily validate the hashes of each incoming revision. This is an optimization that allows delta group application to complete faster. The fact that revlog raises in this particular test is an implementation detail due to the way revlogs are testing multiple deltas. Differential Revision: https://phab.mercurial-scm.org/D5115

storageutil: convert fileid to bytes to avoid cast to %s test-storage.py manages to trigger this on Python 3. Differential Revision: https://phab.mercurial-scm.org/D5117

tests: use byte literals in test-storage.py This fixes a Python 3 breakage due to unknown key due to str/bytes type mismatch. # skip-blame just b'' literals Differential Revision: https://phab.mercurial-scm.org/D5114