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290 lines
13 KiB
Markdown
290 lines
13 KiB
Markdown
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# Package Federation: Custom Registries
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**Note: this is the feature as it was initially specified and does not necessarily reflect the current behavior.**
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**Up-to-date documentation is available at [Registries](../users/registries.md).**
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As it is now, vcpkg has over 1400 ports in the default registry (the `/ports` directory).
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For the majority of users, this repository of packages is enough. However, many enterprises
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need to more closely control their dependencies for one reason or another, and this document
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lays out a method which we will build into vcpkg for exactly that reason.
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## Background
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A registry is simply a set of packages. In fact, there is already a registry in vcpkg: the default one.
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Package federation, implemented via custom registries, allows one to add new packages,
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edit existing packages, and have as much or as little control as one likes over the dependencies that one uses.
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It gives the control over dependencies that an enterprise requires.
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### How Does the Current Default Registry Work?
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Of course, the existing vcpkg tool does have packages in the official,
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default registry. The way we describe these packages is in the ports tree –
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at the base of the vcpkg install directory, there is a directory named ports,
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which contains on the order of 1300 directories, one for each package. Then,
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in each package directory, there are at least two files: a CONTROL or
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vcpkg.json file, which contains the name, version, description, and features
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of the package; and a portfile.cmake file which contains the information on
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how to download and build the package. There may be other files in this
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registry, like patches or usage instructions, but only those two files are
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needed.
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### Existing vcpkg Registry-like Features
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There are some existing features in vcpkg that act somewhat like a custom
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registry. The most obvious feature that we have is overlay ports – this
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feature allows you to specify any number of directories as "overlays", which
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either contain a package definition directly, or which contain some number of
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package directories; these overlays will be used instead of the ports tree
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for packages that exist in both places, and are specified exclusively on the
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command line. Additionally, unfortunately, if one installs a package from
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overlay ports that does not exist in the ports tree, one must pass these
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overlays to every vcpkg installation command.
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There is also the less obvious "feature" which works by virtue of the ports
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tree being user-editable: one can always edit the ports tree on their own
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machine, and can even fork vcpkg and publish their own ports tree.
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Unfortunately, this then means that any updates to the source tree require
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merges, as opposed to being able to fast-forward to the newest sources.
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### Why Registries?
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There are many reasons to want custom registries; however, the most important reasons are:
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* Legal requirements – a company like Microsoft or Google
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needs the ability to strictly control the code that goes into their products,
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making certain that they are following the licenses strictly.
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* There have been examples in the past where a library which is licensed under certain terms contains code
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which is not legally allowed to be licensed under those terms (see [this example][legal-example],
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where a person tried to merge Microsoft-owned, Apache-licensed code into the GPL-licensed libstdc++).
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* Technical requirements – a company may wish to run their own tests on the packages they ship,
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such as [fuzzing].
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* Other requirements – an organization may wish to strictly control its dependencies for a myriad of other reasons.
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* Newer versions – vcpkg may not necessarily always be up to date for all libraries in our registry,
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and an organization may require a newer version than we ship;
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they can very easily update this package and have the version that they want.
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* Port modifications – vcpkg has somewhat strict policies on port modifications,
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and an organization may wish to make different modifications than we do.
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It may allow that organization to make certain that the package works on triplets
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that our team does not test as extensively.
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* Testing – just like port modifications, if a team wants to do specific testing on triplets they care about,
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they can do so via their custom registry.
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Then, there is the question of why vcpkg needs a new solution for custom registries,
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beyond the existing overlay ports feature. There are two big reasons –
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the first is to allow a project to define the registries that they use for their dependencies,
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and the second is the clear advantage in the user experience of the vcpkg tool.
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If a project requires specific packages to come from specific registries,
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they can do so without worrying that a user accidentally misses the overlay ports part of a command.
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Additionally, beyond a feature which makes overlay ports easier to use,
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custom registries allow for more complex and useful infrastructure around registries.
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In the initial custom registry implementation, we will allow overlay ports style paths,
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as well as git repositories, which means that people can run and use custom registries
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without writing their own infrastructure around getting people that registry.
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It is the intention of vcpkg to be the most user-friendly package manager for C++,
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and this allows us to fulfill on that intention even further.
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As opposed to having to write `--overlay-ports=path/to/overlay` for every command one runs,
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or adding an environment variable `VCPKG_OVERLAY_PORTS`,
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one can simply write vcpkg install and the registries will be taken care of for you.
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As opposed to having to use git submodules, or custom registry code for every project,
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one can write and run the infrastructure in one place,
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and every project that uses that registry requires only a few lines of JSON.
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[legal-example]: https://gcc.gnu.org/legacy-ml/libstdc++/2019-09/msg00054.html
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[fuzzing]: https://en.wikipedia.org/wiki/Fuzzing
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## Specification
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We will be adding a new file that vcpkg understands - `vcpkg-configuration.json`.
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The way that vcpkg will find this file is different depending on what mode vcpkg is in:
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in classic mode, vcpkg finds this file alongside the vcpkg binary, in the root directory.
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In manifest mode, vcpkg finds this file alongside the manifest. For the initial implementation,
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this is all vcpkg will look for; however, in the future, vcpkg will walk the tree and include
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configuration all along the way: this allows for overriding defaults.
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The specific algorithm for applying this is not yet defined, since currently only one
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`vcpkg-configuration.json` is allowed.
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The only thing allowed in a `vcpkg-configuration.json` is a `<configuration>` object.
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A `<configuration>` is an object:
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* Optionally, `"default-registry"`: A `<registry-implementation>` or `null`
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* Optionally, `"registries"`: An array of `<registry>`s
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Since this is the first RFC that adds anything to this field,
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as of now the only properties that can live in that object will be
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these.
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A `<registry-implementation>` is an object matching one of the following:
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* `<registry-implementation.builtin>`:
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* `"kind"`: The string `"builtin"`
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* `<registry-implementation.directory>`:
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* `"kind"`: The string `"directory"`
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* `"path"`: A path
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* `<registry-implementation.git>`:
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* `"kind"`: The string `"git"`
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* `"repository"`: A URI
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* Optionally, `"path"`: An absolute path into the git repository
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* Optionally, `"ref"`: A git reference
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A `<registry>` is a `<registry-implementation>` object, plus the following properties:
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* Optionally, `"scopes"`: An array of `<package-name>`s
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* Optionally, `"packages"`: An array of `<package-name>`s
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The `"packages"` and `"scopes"` fields of distinct registries must be disjoint,
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and each `<registry>` must have at least one of the `"scopes"` and `"packages"` property,
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since otherwise there's no point.
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As an example, a package which uses a different default registry, and a different registry for boost,
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might look like the following:
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```json
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{
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"default-registry": {
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"kind": "directory",
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"path": "vcpkg-ports"
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},
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"registries": [
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{
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"kind": "git",
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"repository": "https://github.com/boostorg/vcpkg-ports",
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"ref": "v1.73.0",
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"scopes": [ "boost" ]
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},
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{
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"kind": "builtin",
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"packages": [ "cppitertools" ]
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}
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]
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}
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```
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This will install `fmt` from `<directory-of-vcpkg.json>/vcpkg-ports`,
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`cppitertools` from the registry that ships with vcpkg,
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and any `boost` dependencies from `https://github.com/boostorg/vcpkg-ports`.
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Notably, this does not replace behavior up the tree -- only the `vcpkg-configuration.json`s
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for the current invocation do anything.
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### Behavior
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When a vcpkg command requires the installation of dependencies,
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it will generate the initial list of dependencies from the package,
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and then run the following algorithm on each dependency:
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1. Figure out which registry the package should come from by doing the following:
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1. If there is a registry in the registry set which contains the dependency name in the `"packages"` array,
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then use that registry.
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2. For every scope, in order from most specific to least,
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if there is a registry in the registry set which contains that scope in the `"scopes"` array,
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then use that registry.
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(For example, for `"cat.meow.cute"`, check first for `"cat.meow.cute"`, then `"cat.meow"`, then `"cat"`).
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3. If the default registry is not `null`, use that registry.
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4. Else, error.
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2. Then, add that package's dependencies to the list of packages to find, and repeat for the next dependency.
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vcpkg will also rerun this algorithm whenever an install is run with different configuration.
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### How Registries are Laid Out
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There are three kinds of registries, but they only differ in how the registry gets onto one's filesystem.
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Once the registry is there, package lookup runs the same, with each kind having it's own way of defining its
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own root.
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In order to find a port `meow` in a registry with root `R`, vcpkg first sees if `R/meow` exists;
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if it does, then the port root is `R/meow`. Otherwise, see if `R/m-` exists; if it does,
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then the port root is `R/m-/meow`. (note: this algorithm may be extended further in the future).
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For example, given the following port root:
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```
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R/
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abseil/...
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b-/
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boost/...
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boost-build/...
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banana/...
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banana/...
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```
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The port root for `abseil` is `R/abseil`; the port root for `boost` is `R/b-/boost`;
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the port root for `banana` is `R/banana` (although this duplication is not recommended).
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The reason we are making this change to allow more levels in the ports tree is that ~1300
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ports are hard to look through in a tree view, and this allows us to see only the ports we're
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interested in. Additionally, no port name may end in a `-`, so this means that these port subdirectories
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will never intersect with actual ports. Additionally, since we use only ASCII for port names,
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we don't have to worry about graphemes vs. code units vs. code points -- in ASCII, they are equivalent.
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Let's now look at how different registry kinds work:
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#### `<registry.builtin>`
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For a `<registry.builtin>`, there is no configuration required.
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The registry root is simply `<vcpkg-root>/ports`.
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#### `<registry.directory>`
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For a `<registry.directory>`, it is again fairly simple.
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Given `$path` the value of the `"path"` property, the registry root is either:
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* If `$path` is absolute, then the registry root is `$path`.
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* If `$path` is drive-relative (only important on Windows), the registry root is
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`(drive of vcpkg.json)/$path`
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* If `$path` is relative, the registry root is `(directory of vcpkg.json)/$path`
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Note that the path to vcpkg.json is _not_ canonicalized; it is used exactly as it is seen by vcpkg.
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#### `<registry.git>`
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This registry is the most complex. We would like to cache existing registries,
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but we don't want to ignore new updates to the registry.
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It is the opinion of the author that we want to find more updates than not,
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so we will update the registry whenever the `vcpkg.json` or `vcpkg-configuration.json`
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is modified. We will do so by keeping a sha512 of the `vcpkg.json` and `vcpkg-configuration.json`
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inside the `vcpkg-installed` directory.
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We will download the specific ref of the repository to a central location (and update as needed),
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and the root will be either: `<path to repository>`, if the `"path"` property is not defined,
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or else `<path to repository>/<path property>` if it is defined.
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The `"path"` property must be absolute, without a drive, and will be treated as relative to
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the path to the repository. For example:
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```json
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{
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"kind": "git",
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"repository": "https://github.com/microsoft/vcpkg",
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"path": "/ports"
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}
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```
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is the correct way to refer to the registry built in to vcpkg, at the latest version.
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The following are all incorrect:
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```json
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{
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"$reason": "path can't be drive-absolute",
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"kind": "git",
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"repository": "https://github.com/microsoft/vcpkg",
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"path": "F:/ports"
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}
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```
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```json
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{
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"$reason": "path can't be relative",
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"kind": "git",
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"repository": "https://github.com/microsoft/vcpkg",
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"path": "ports"
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}
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```
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```json
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{
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"$reason": "path _really_ can't be relative like that",
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"kind": "git",
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"repository": "https://github.com/microsoft/vcpkg",
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"path": "../../meow/ports"
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}
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```
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