repair: begin implementation of in-place upgrading Now that all the upgrade planning work is in place, we can start doing the real work: actually upgrading a repository. The main goal of this commit is to get the "framework" for running in-place upgrade actions in place. Rather than get too clever and low-level with regards to in-place upgrades, our strategy is to create a new, temporary repository, copy data to it, then replace the old data with the new. This allows us to reuse a lot of code in localrepo.py around store interaction, which will eventually consume the bulk of the upgrade code. But we have to start small. This patch implements adding new repository requirements. But it still sets up a temporary repository and locks it and the source repo before performing the requirements file swap. This means all the plumbing is in place to implement store copying in subsequent commits.

repair: determine what upgrade will do This commit introduces code for determining what actions/improvements an upgrade should perform. The "upgradefindimprovements" function introduces a mechanism to return a list of improvements that can be made to a repository. Each improvement is effectively an action that an upgrade will perform. Associated with each of these improvements is metadata that will be used to inform users what's wrong and what an upgrade will do. Each "improvement" is categorized as a "deficiency" or an "optimization." TBH, I'm not thrilled about the terminology and am receptive to constructive bikeshedding. The main difference between a "deficiency" and an "optimization" is a deficiency is always corrected (if it deviates from the current config) and an "optimization" is an optional action that goes above and beyond to improve the state of the repository (usually by requiring more CPU during upgrade). Our initial set of improvements identifies missing repository requirements, a single, easily correctable problem with changelog storage, and a set of "optimizations" related to delta recalculation. The main "upgraderepo" function has been expanded to handle improvements. It queries for the list of improvements and determines which of them will run based on the current repository state and user I went through numerous iterations of the output format before settling on a ReST-inspired definition list format. (I used bulleted lists in the first submission of this commit and could not get it to format just right.) Even with the various iterations, I'm still not super thrilled with the format. But, this is a debug* command, so that should mean we can refine the output without BC concerns.

repair: implement requirements checking for upgrades This commit introduces functionality for upgrading a repository in place. The first part that's implemented is testing for upgrade "compatibility." This is done by examining repository requirements. There are 5 functions returning sets of requirements that control upgrading. Why so many functions? Mainly to support extensions. Functions are easier to monkeypatch than module variables. Astute readers will see that we don't support "manifestv2" and "treemanifest" requirements in the upgrade mechanism. I don't have a great answer for why other than this is a complex set of patches and I don't want to deal with the complexity of these experimental features just yet. We can teach the upgrade mechanism about them later, once the basic upgrade mechanism is in place. This commit also introduces the "upgraderepo" function. This will be our main routine for performing an in-place upgrade. Currently, it just implements requirements checking. The structure of some code in this function may look a bit weird (e.g. the inline function that is only called once). But this will make sense after future commits.

debugcommands: stub for debugupgraderepo command Currently, if Mercurial introduces a new repository/store feature or changes behavior of an existing feature, users must perform an `hg clone` to create a new repository with hopefully the correct/optimal settings. Unfortunately, even `hg clone` may not give the correct results. For example, if you do a local `hg clone`, you may get hardlinks to revlog files that inherit the old state. If you `hg clone` from a remote or `hg clone --pull`, changegroup application may bypass some optimization, such as converting to generaldelta. Optimizing a repository is harder than it seems and requires more than a simple `hg` command invocation. This commit starts the process of changing that. We introduce `hg debugupgraderepo`, a command that performs an in-place upgrade of a repository to use new, optimal features. The command is just a stub right now. Features will be added in subsequent commits. This commit does foreshadow some of the behavior of the new command, notably that it doesn't do anything by default and that it takes arguments that influence what actions it performs. These will be explained more in subsequent commits.