# Ports Overlay (Jun 19, 2019) **Note: this is the feature as it was initially specified and does not necessarily reflect the current behavior.** ## 1. Motivation ### A. Allow users to override ports with alternate versions It's a common scenario for `vcpkg` users to keep specific versions of libraries to use in their own projects. The current recommendation for users is to fork `vcpkg`'s repository and create tags for commits containing the specific versions of the ports they want to use. This proposal adds an alternative to solve this problem. By allowing `vcpkg` users to specify additional locations in their file system containing ports for: * older or newer versions of libraries, * modified libraries, or * libraries not available in `vcpkg`. These locations will be searched when resolving port names during package installation, and override ports in `/ports`. ### B. Allow users to keep unmodified upstream ports Users will be able to keep unmodified versions of the ports shipped with `vcpkg` and update them via `vcpkg update` and `vcpkg upgrade` without having to solve merge conflicts. ## 2. Other design concerns * Allow a set of `vcpkg` commands to optionally accept additional paths to be used when searching for ports. * Additional paths must take precedence when resolving names of ports to install. * Allow users to specify multiple additional paths. * Provide a simple disambiguation mechanism to resolve ambiguous port names. * After resolving a port name, the installation process has to work the same as for ports shipped by `vcpkg`. * This **DOES NOT ENABLE MULTIPLE VERSIONS** of a same library to be **INSTALLED SIDE-BY-SIDE**. ## 3. Proposed solution This document proposes allowing additional locations to search for ports during package installation that will override and complement the set of ports provided by `vcpkg` (ports under the `/ports` directory).` A new option `--overlay-ports` will be added to the `vcpkg install`, `vcpkg update`, `vcpkg upgrade`, `vcpkg export`, and `vcpkg depend-info` commands to specify additional paths containing ports. From a user experience perspective, a user expresses interest in adding additional lookup locations by passing the `--overlay-ports` option followed by a path to: * an individual port (directory containing a `CONTROL` file), * `vcpkg install sqlite3 --overlay-ports="C:\custom-ports\sqlite3"` * a directory containing ports, * `vcpkg install sqlite3 --overlay-ports=\\share\org\custom-ports` * a file listing paths to the former two. > NOTE: Reading paths from a text file is not available in the current implementation, some revisions to this part of the specification are being made and will be implemented in a future date. * `vcpkg install sqlite3 --overlay-ports=..\port-repos.txt` _port-repos.txt_ ``` .\experimental-ports\sqlite3 C:\custom-ports \\share\team\custom-ports \\share\org\custom-ports ``` *Relative paths inside this file are resolved relatively to the file's location. In this case a `experimental-ports` directory should exist at the same level as the `port-repos.txt` file.* _NOTE: It is not the goal of this document to discuss library versioning or project dependency management solutions, which require the ability to install multiple versions of a same library side-by-side._ ### Multiple additional paths Users can provide multiple additional paths by repeating the `--overlay-ports` option. ``` vcpkg install sqlite3 --overlay-ports="..\experimental-ports\sqlite3" --overlay-ports="C:\custom-ports" --overlay-ports="\\share\team\custom-ports ``` ### Overlaying ports Port name resolution follows the order in which additional paths are specified, with the first match being selected for installation, and falling back to `/ports` if the port is not found in any of the additional paths. No effort is made to compare version numbers inside the `CONTROL` files, or to determine which port contains newer or older files. ### Examples Given the following directory structure: ``` team-ports/ |-- sqlite3/ |---- CONTROL |-- rapidjson/ |---- CONTROL |-- curl/ |---- CONTROL my-ports/ |-- sqlite3/ |---- CONTROL |-- rapidjson/ |---- CONTROL vcpkg |-- ports/ |---- |-- vcpkg.exe |-- preferred-ports.txt ``` * #### Example #1: Running: ``` vcpkg/vcpkg.exe install sqlite3 --overlay-ports=my-ports --overlay-ports=team-ports ``` Results in `my-ports/sqlite3` getting installed as that location appears first in the command line arguments. * #### Example #2: A specific version of a port can be given priority by adding its path first in the list of arguments: ``` vcpkg/vcpkg.exe install sqlite3 rapidjson curl --overlay-ports=my-ports/rapidjson --overlay-ports=vcpkg/ports/curl --overlay-ports=team-ports ``` Installs: * `sqlite3` from `team-ports/sqlite3` * `rapidjson` from `my-ports/rapidjson` * `curl` from `vcpkg/ports/curl` * #### Example #3: > NOTE: Reading paths from a text file is not available in the current implementation, some revisions to this part of the specification are being made and will be implemented in a future date. Given the content of `preferred-ports.txt` as: ``` ./ports/curl /my-ports/rapidjson /team-ports ``` A location can be appended or prepended to those included in `preferred-ports.txt` via the command line, like this: ``` vcpkg/vcpkg.exe install sqlite3 curl --overlay-ports=my-ports --overlay-ports=vcpkg/preferred-ports.txt ``` Which results in `my-ports/sqlite3` and `vcpkg/ports/curl` getting installed. ## 4. Proposed User experience ### i. User wants to preserve an older version of a port Developer Alice and her team use `vcpkg` to acquire **OpenCV** and some other packages. She has even contributed many patches to add features to the **OpenCV 3** port in `vcpkg`. But, one day, she notices that a PR to update **OpenCV** to the next major version has been merged. Alice wants to update some packages available in `vcpkg`. Unfortunately, updating her project to use the latest **OpenCV** is not immediately possible. Alice creates a private GitHub repository and checks in the set of ports that she wants to preserve. Then provides her teammates with the link to clone her private ports repository. ``` mkdir vcpkg-custom-ports cd vcpkg-custom-ports git init cp -r %VCPKG_ROOT%/ports/opencv . git add . git commit -m "[opencv] Add OpenCV 3 port" git remote add origin https://github.com//vcpkg-custom-ports.git git push -u origin master ``` Now her team is able to use: ``` git clone https://github.com//vcpkg-custom-ports.git vcpkg update --overlay-ports=./vcpkg-custom-ports vcpkg upgrade --no-dry-run --overlay-ports=./vcpkg-custom-ports ``` to upgrade their packages and preserve the old version of **OpenCV** they require.