[Ports Overlay] New feature draft spec (Superseded by #6981)

docs/specifications/ports-overlay.md

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+# Ports Overlay (Jun 19, 2019)
+
+
+## 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 `<vcpkg-root>/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 `<vcpkg_root>/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.
+  * `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 `<vcpkg-root>/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/
+  |---- <upstream 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:
+
+  Given that the contents on `preferred-ports.txt` are:
+
+  ```
+  ./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/<Alice's GitHub username>/vcpkg-custom-ports.git
+git push -u origin master
+```
+
+Now her team is able to use: 
+
+```
+git clone https://github.com/<Alice's GitHub username>/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.