wireprotov2: advertise set of valid values for requestable fields changesetdata, manifestdata, and filedata all allow the caller to specify what data fields to request. Data fields are extensible and may evolve over time. In order to prevent clients from making requests for fields that are not available, the client needs to know what fields are available. This commit teaches the server to declare a set of "valid values" for wire protocol command arguments. That set of values is exposed in the command's capabilities descriptor. The changesetdata, manifestdata, and filedata commands all declare their set of available "fields." Differential Revision: https://phab.mercurial-scm.org/D4619

wireprotov2: expose rich arguments metadata Now that we internally store rich metadata about arguments, it makes sense to make that metadata available to the client. This will allow clients to validate outgoing command requests before they are sent over the wire. Strictly speaking, we should bump the wire protocol version for this change since it is backwards incompatible. But no client-side code touches the arguments map and I don't want to incur the work. Differential Revision: https://phab.mercurial-scm.org/D4618

wireprotov2: advertise recognized path filter prefixes While the wire protocol doesn't yet support it, we'll eventually have commands that accept narrow patterns to specify the set of files relevant to a command. For security and performance reasons, only specific filter types are allowed. This commit teaches the server to advertise the set of allowed filter types. By doing so, clients can e.g. validate user-specified patterns against the server's abilities without having to send a command to retrieve data. Having the data in the capabilities data structure will also serve as a check against unwanted BC. Differential Revision: https://phab.mercurial-scm.org/D4616

wireprotov2: declare command arguments richly Previously, we declared command arguments with an example of their value. After this commit, we declare command arguments as a dict of metadata. This allows us to define the value type, whether the argument is required, and provide default values. This in turn allows us to have nice things, such as less boilerplate code in individual commands for validating input and assigning default values. It should also make command behavior more consistent as a result. Test output changed slightly because I realized that the "fields" argument wasn't being consistently defined as a set. Oops! Other test output changed because of slight differences in code performing type validation. Differential Revision: https://phab.mercurial-scm.org/D4615

chgserver: do not send system() back to client if stdio redirected (issue5992) As the chg client doesn't know server-side stdio redirection, the server shouldn't upcall on "runsystem" request if the stdio streams are redirected. This patch teaches ui to remember the redirection flag, which is updated by the caller right now. Future patches (for default) will add ui methods to manage this flag internally.

chgserver: update comment describing when to fall back to core _runsystem() The condition "output needs to be captured" was moved to the core ui, and removed at fbce78c58f1e "chg: refactor ui.system() to be partly overridden." The next patch will add one more "if" to address the issue 5992.