clonebundles: support for seeding clones from pre-generated bundles Cloning can be an expensive operation for servers because the server generates a bundle from existing repository data at request time. For a large repository like mozilla-central, this consumes 4+ minutes of CPU time on the server. It also results in significant network utilization. Multiplied by hundreds or even thousands of clients and the ensuing load can result in difficulties scaling the Mercurial server. Despite generation of bundles being deterministic until the next changeset is added, the generation of bundles to service a clone request is not cached. Each clone thus performs redundant work. This is wasteful. This patch introduces the "clonebundles" extension and related client-side functionality to help alleviate this deficiency. The client-side feature is behind an experimental flag and is not enabled by default. It works as follows: 1) Server operator generates a bundle and makes it available on a server (likely HTTP). 2) Server operator defines the URL of a bundle file in a .hg/clonebundles.manifest file. 3) Client `hg clone`ing sees the server is advertising bundle URLs. 4) Client fetches and applies the advertised bundle. 5) Client performs equivalent of `hg pull` to fetch changes made since the bundle was created. Essentially, the server performs the expensive work of generating a bundle once and all subsequent clones fetch a static file from somewhere. Scaling static file serving is a much more manageable problem than scaling a Python application like Mercurial. Assuming your repository grows less than 1% per day, the end result is 99+% of CPU and network load from clones is eliminated, allowing Mercurial servers to scale more easily. Serving static files also means data can be transferred to clients as fast as they can consume it, rather than as fast as servers can generate it. This makes clones faster. Mozilla has implemented similar functionality of this patch on hg.mozilla.org using a custom extension. We are hosting bundle files in Amazon S3 and CloudFront (a CDN) and have successfully offloaded >1 TB/day in data transfer from hg.mozilla.org, freeing up significant bandwidth and CPU resources. The positive impact has been stellar and I believe it has proved its value to be included in Mercurial core. I feel it is important for the client-side support to be enabled in core by default because it means that clients will get faster, more reliable clones and will enable server operators to reduce load without requiring any client-side configuration changes (assuming clients are up to date, of course). The scope of this feature is narrowly and specifically tailored to cloning, despite "serve pulls from pre-generated bundles" being a valid and useful feature. I would eventually like for Mercurial servers to support transferring *all* repository data via statically hosted files. You could imagine a server that siphons all pushed data to bundle files and instructs clients to apply a stream of bundles to reconstruct all repository data. This feature, while useful and powerful, is significantly more work to implement because it requires the server component have awareness of discovery and a mapping of which changesets are in which files. Full, clone bundles, by contrast, are much simpler. The wire protocol command is named "clonebundles" instead of something more generic like "staticbundles" to leave the door open for a new, more powerful and more generic server-side component with minimal backwards compatibility implications. The name "bundleclone" is used by Mozilla's extension and would cause problems since there are subtle differences in Mozilla's extension. Mozilla's experience with this idea has taught us that some form of "content negotiation" is required. Not all clients will support all bundle formats or even URLs (advanced TLS requirements, etc). To ensure the highest uptake possible, a server needs to advertise multiple versions of bundles and clients need to be able to choose the most appropriate from that list one. The "attributes" in each server-advertised entry facilitate this filtering and sorting. Their use will become apparent in subsequent patches. Initial inspiration and credit for the idea of cloning from static files belongs to Augie Fackler and his "lookaside clone" extension proof of concept.

sslutil: expose attribute indicating whether SNI is supported This will be used so clone bundles can advertise whether URLs require SNI. This will be explained more in a subsequent patch.

resolve: perform all premerges before performing any file merges (BC) Just like the BC to merge before it, this allows for a maximally consistent state before providing any prompts to the user.

test-resolve.t: add some tests for .orig file contents An upcoming patch will touch some code around this area, and I couldn't find any tests related to this.

test-resolve.t: add some output to show order of operations This basically shows the behavior of resolve with multiple files. An upcoming behavior change will cause this output to also change.

merge.mergestate: perform all premerges before any merges (BC) We perform all that we can non-interactively before prompting the user for input via their merge tool. This allows for a maximally consistent state when the user is first prompted. The test output changes indicate the actual behavior change happening.

merge: introduce a preresolve function The section of code that writes out the version of the file cached in the merge state should only be run at preresolve time. This is so that if the premerge keeps around conflict markers, those don't get overwritten before the main merge.

merge.mergestate._resolve: also return completed status We'll need this for a new 'preresolve' function we're adding.

merge.mergestate: add a wrapper around resolve The resolve function will be broken up into separate pre-resolve and resolve steps.

simplemerge: move conflict warning message to filemerge The current output for a failed merge with conflict markers looks something like: merging foo warning: conflicts during merge. merging foo incomplete! (edit conflicts, then use 'hg resolve --mark') merging bar warning: conflicts during merge. merging bar incomplete! (edit conflicts, then use 'hg resolve --mark') We're going to change the way merges are done to perform all premerges before all merges, so that the output above would look like: merging foo merging bar warning: conflicts during merge. merging foo incomplete! (edit conflicts, then use 'hg resolve --mark') warning: conflicts during merge. merging bar incomplete! (edit conflicts, then use 'hg resolve --mark') The 'warning: conflicts during merge' line has no context, so is pretty confusing. This patch will change the future output to: merging foo merging bar warning: conflicts while merging foo! (edit, then use 'hg resolve --mark') warning: conflicts while merging bar! (edit, then use 'hg resolve --mark') The hint on how to resolve the conflicts makes this a bit unwieldy, but solving that is tricky because we already hint that people run 'hg resolve' to retry unresolved merges. The 'hg resolve --mark' mostly applies to conflict marker based resolution.