revlog: determine sidedata support based on the revlog version Revlog version "2" support sidedata, previous version does not. So lets make the initialization simpler. Differential Revision: https://phab.mercurial-scm.org/D10613

requirements: no longer drop `generaldelta` requirement with revlogv2 A repository could use a mix of revlogv1 and revlogv2, making the requirements still necessary. Overall we should move away from the "requirements" file being used a way to configure the repository and stick to it "what do you need to access this repository". However this is a wider work for another time. In addition the logic we just dropped was confusing the `hg debugformat` command, breaking the upgrade code and inconsistent (eg: `sparse-revlog` is also implied by `revlogv2`). Finally, multiple other config option would imply the use of the `revlogv2` requirements, without drop the `generaldelta` one, leading to more inconsistency. Differential Revision: https://phab.mercurial-scm.org/D10612

config: drop the `format.exp-revlogv2.2` option It is oddly named and not used at all. The one used by the code is still `experimental.revlogv2`. So we drop that one option for consistency. We move associate documentation to the actual option. Differential Revision: https://phab.mercurial-scm.org/D10611

revlog: only use the `_indexfp` method for read operation This will avoid "other" code to not overlook `_writing` usage. We introduces private method dedicated to writing to make use the right option are always used. Differential Revision: https://phab.mercurial-scm.org/D10610

revlog: use `_writing` in `rewrite_sidedata` Special cases aren't special enough to break the rules. Differential Revision: https://phab.mercurial-scm.org/D10609

revlog: open files in 'r+' instead of 'a+' The code doing actual writing is already doing the necessary seeking, so we could safely use 'r+'. This make the file objecs usable in more situation, like updating the sidedata information during pull. revlog: forcibly move the file cursor at the right location before writing This is a paranoid change in case the changelog computation moved the cursors under our feets. This is not known to happens right now. Differential Revision: https://phab.mercurial-scm.org/D10608

revlog: pass a transaction object to `rewrite_sidedata` The `_writing` context need one, so we update the function signature before considering using `_writing` in rewrite_sidedata. Differential Revision: https://phab.mercurial-scm.org/D10607

revlog: rename variable in `rewrite_sidedata` to match other code Let's call the index file object and `ifh` and the data file object `dfh` as the rest of the revlog code. This will make future change clearer. Differential Revision: https://phab.mercurial-scm.org/D10606

revlog: introduce a mandatory `_writing` context to update revlog content Before this change, various revlog methods where managing the opening and closing of the revlog files manually and passing the file descriptor alors the call path. To simplify the tracking of the write operation by a future docket, we need something more organised. As a result, we introduce a `revlog._writing` context manager that will wrap each revlog update operation. The file descriptor are kept in the existing `revlog._writinghandles` parameter that was already used by the `addgroup` logic. All this change is internal to the revlog only, the "public" interface is not affected. The `addrevision` and `addgroup` logic are still responsible for setup up this context. However this new context give us multiple benefits: * all writer use a same, unified, logic, * this context is programmatically enforced, * each write "session" as a clearly identified start and end. The post-pull sidedata update logic is still doing writing by end and will be adjusted in a later changesets. This change affect the concurrency checker test, because register the state of the file in the transaction sooner in `addrevision` (about as early as what `addgroup` would do), so the abort is rollbacking the other commit. I don't want to weaken the current main logic. Differential Revision: https://phab.mercurial-scm.org/D10605

revlog: preindent some code in _enforceinlinesize Indenting this beforehand will make a future changeset much simpler. Differential Revision: https://phab.mercurial-scm.org/D10604