hidden: extract the code generating "filtered rev" error for wrapping The goal is to help experimentation in extensions (ie: evolve) around more advance messages.

serve: add support for Mercurial subrepositories I've been using `hg serve --web-conf ...` with a simple '/=projects/**' [paths] configuration for awhile without issue. Let's ditch the need for the manual configuration in this case, and limit the repos served to the actual subrepos. This doesn't attempt to handle the case where a new subrepo appears while the server is running. That could probably be handled with a hook if somebody wants it. But it's such a rare case, it probably doesn't matter for the temporary serves. The main repo is served at '/', just like a repository without subrepos. I'm not sure why the duplicate 'adding ...' lines appear on Linux. They don't appear on Windows (see 594dd384803c), so they are optional. Subrepositories that are configured with '../path' or absolute paths are not cloneable from the server. (They aren't cloneable locally either, unless they also exist at their configured source, perhaps via the share extension.) They are still served, so that they can be browsed, or cloned individually. If we care about that cloning someday, we can probably just add the extra entries to the webconf dictionary. Even if the entries use '../' to escape the root, only the related subrepositories would end up in the dictionary.

hgwebdir: allow a repository to be hosted at "/" This can be useful in general, but will also be useful for hosting subrepos, with the main repo at /.

httppeer: eliminate decompressresponse() proxy Now that the response instance itself is wrapped with error handling, we no longer need this code. This code became dead with the previous patch because the added code catches HTTPException and re-raises as something else.

httppeer: wrap HTTPResponse.read() globally There were a handful of places in the code where HTTPResponse.read() was called with no explicit error handling or with inconsistent error handling. In order to eliminate this class of bug, we globally swap out HTTPResponse.read() with a unified error handler. I initially attempted to fix all call sites. However, after going down that rabbit hole, I figured it was best to just change read() to do what we want. This appears to be a worthwhile change, as the tests demonstrate many of our uncaught exceptions go away. To better represent this class of failure, we introduce a new error type. The main benefit over IOError is it can hold a hint. I'm receptive to tweaking its name or inheritance.

tests: add tests for poorly behaving HTTP server I've spent several hours over the past few weeks investigating networking failures involving hg.mozilla.org. As part of this, it has become clear that the Mercurial client's error handling when it encounters network failures is far from robust. To prove this is true, I've devised a battery of tests simulating various network failures, notably premature connection closes. To achieve this, I've implemented an extension that monkeypatches the built-in HTTP server and hooks in at the socket level and allows various events to occur based on config options. For example, you can refuse to accept() a client socket or you can close() the socket after N bytes have been sent or received. The latter effectively simulates an unexpected connection drop (and these occur all the time in the real world). The new test file launches servers exhibiting various "bad" behaviors and points a client at them. As the many TODO comments in the test call attention to, Mercurial often displays unhelpful errors when network-related failures occur. This makes it difficult for users to understand what's going on and difficult for server administrators to pinpoint root causes without packet tracing. Upcoming patches will attempt to fix these error handling deficiencies.

phases: emit phases to pushkey protocol in deterministic order An upcoming test will report exact bytes sent over the wire protocol. Without this change, the ordering of phases listkey data is non-deterministic.

keepalive: send HTTP request headers in a deterministic order An upcoming patch will add low-level testing of the bytes being sent over the wire. As part of developing that test, I discovered that the order of headers in HTTP requests wasn't deterministic. This patch makes the order deterministic to make things easier to test.