yarn-classic: Adding user-facing docs

This commit introduces user-facing documentation for
Yarn Classic.

Signed-off-by: Alexey Ovchinnikov <[email protected]>

docs/yarn.md

@@ -20,7 +20,9 @@ cache](#building-your-project-using-the-pre-fetched-yarn-dependency-cache)
 ## Cachi2's Yarn support scope
 
 ### Supported Yarn versions
-Cachi2 only currently only supports Yarn in version 3.
+Cachi2 currently supports Yarn versions 1 and 3. Version 1 is referred to as
+"Yarn Classic" and is covered in [yarn_classic.md](yarn_classic.md). This document
+describes Yarn v3 support.
 
 _Note: newer versions of Yarn are likely to be added in future releases._
 

docs/yarn_classic.md

@@ -0,0 +1,160 @@
+# Yarn Classic (Yarn v1)
+
+<https://classic.yarnpkg.com/>
+
+* [Overview]
+  * [Supported dependencies types]
+  * [Dealing with Yarn Zero-Installs]
+ * [Fetching dependencies for Yarn Classic projects]
+ * [Handling of yarn-specific config files]
+ * [Prerequisites for an offline build]
+
+This document outlines the differences between Yarn Classic and Yarn v3 support.
+Please refer to [README] and [Yarn v3 documentation] for
+common aspects of PMs behavior. `Yarn` and `Yarn Classic` will be used interchangeably
+in this document, any other versions of Yarn will be explicitly mentioned.
+
+## Overview
+
+Yarn Classic package manager (PM) relies on Yarn Classic being installed on the system
+where Cachi2 is run. If requested to process a package with Yarn Classic PM it will
+check for yarn version and will refuse to proceed if necessary version is missing.
+Yarn itself is used by Cachi2 under the hood to organize package processing with
+some tweaks to ensure that the packages are prepared to be built in isolation.
+
+The following variables are set for Yarn in the fetch phase:
+
+ * `COREPACK_ENABLE_DOWNLOAD_PROMPT` is set to "0" which prevents
+   Corepack from showing the URL when it needs to download software;
+ * `COREPACK_ENABLE_PROJECT_SPEC` is set to "0" which prevents
+   Corepack from checking if the package manager corresponds to the one
+   defined for the current project;
+ * `YARN_IGNORE_PATH` is set to "true" which ignores any Yarn version specified by a user and
+   uses Corepack's version instead;
+ * `YARN_IGNORE_SCRIPTS`: is set to "true" which prevents execution of any scripts defined in
+   `package.json` or in any dependency;
+ * `YARN_YARN_OFFLINE_MIRROR` is set to point to `deps/yarn-classic` which is relative to
+   output directory and will hold fetched dependencies;
+ * `YARN_YARN_OFFLINE_MIRROR_PRUNING` is set to "false" which prevents Yarn from attempting to
+   ensure that dependencies are up to date.
+
+Once fetch phase is completed Cachi2 will need to generate an
+[environment file][generate this file] with variables pointing to the mirror
+and instructing Yarn not to prune it:
+
+```
+YARN_YARN_OFFLINE_MIRROR=<request output directory>
+YARN_YARN_OFFLINE_MIRROR_PRUNING=false
+```
+
+Sourcing this file will prime Yarn for an offline build
+
+Cachi2 expects to find well-formed `package.json` and `yarn.lock` checked in into a
+repository and will not continue if any of the files are missing.
+
+Prefetching dependencies for Yarn in Cachi2 is be done using Yarn's offline mirror feature.
+The project must be configured to use the offline mirror feature. Refer to
+[Setting up offline mirror] below for details.
+
+
+### Supported dependencies types
+
+Yarn Classic PM is capable of processing the following types of dependencies:
+ * packages from registries;
+ * packages from git repos;
+ * packages from http/https URLs;
+ * packages from local paths;
+ * local workspace packages;
+ * local link packages.
+
+### Dealing with Yarn Zero-Installs
+
+Yarn Classic's [Plug'n'Play] feature is not supported. Any package that uses
+it will be rejected. For further details please refer to [Yarn v3 documentation].
+
+## Fetching dependencies for Yarn Classic projects
+
+The process of fetching dependencies for Yarn Classic is similar to that for any other
+package manager. The name of package manager is `yarn_classic`, and it does not expect
+any additional arguments.
+
+Cachi2 ``fetch-deps`` shell command:
+
+```shell
+cachi2 fetch-deps \
+  --source ./my-repo \
+  --output ./cachi2-output \
+  '<JSON input>'
+```
+
+where JSON input is:
+```jsonc
+{
+  // "yarn_classic" tells Cachi2 to process Yarn packages
+  "type": "yarn_classic",
+  // path to the package (relative to the --source directory)
+  // defaults to "."
+  "path": ".",
+}
+```
+
+or more simply by just invoking:
+``cachi2 fetch-deps yarn_classic``
+
+For complete example of how to pre-fetch dependencies, see [Pre-fetch dependencies].
+
+## Handling of yarn-specific config files
+
+Yarn Classic allows a user to provide additional configuration via [.yarnrc]
+and [.npmrc].  **Cachi2 ignores these settings during prefetch phase**.
+However a `.yarnrc` could be used for setting up an offline mirror
+([Setting up offline mirror]).  These settings will be applied during
+a build phase.
+
+
+### Prerequisites for an offline build
+
+A project that is to be hermetically built must be configured to use an offline
+mirror.  This means that Yarn will store compressed archives on the file system
+in a mirror directory and will install them from there later without network
+access.
+
+The actual build process will use Yarn directly, thus a project must be
+configured to use offline mirror either by providing a `.yarnrc` file or by
+setting up several environment variables. In case when `.yarnrc` is preferred
+it must contain the following lines:
+
+```ini
+yarn-offline-mirror <absolute path to the request output directory>
+yarn-offline-mirror-pruning false
+```
+
+It can be either directly written to or `yarn config` could be used:
+```bash
+$ yarn config set yarn-offline-mirror <absolute path to the request output directory>
+$ yarn config set yarn-offline-mirror-pruning false
+```
+
+In case when environment variables approach is preferred the following
+variables must be set:
+
+```bash
+YARN_YARN_OFFLINE_MIRROR=<absolute path to the request output directory>
+YARN_YARN_OFFLINE_MIRROR_PRUNING=false
+```
+Cachi2 provides a helper that [generates these variables] and places them into a file.
+Sourcing this file is enough to set them.
+
+[README]: ../README.md#yarn
+[Yarn v3 documentation]: yarn.md
+[Pre-fetch dependencies]: usage.md#pre-fetch-dependencies
+[Plug'n'Play]: https://classic.yarnpkg.com/en/docs/pnp
+[.yarnrc]: https://classic.yarnpkg.com/lang/en/docs/yarnrc/
+[.npmrc]: https://classic.yarnpkg.com/en/docs/cli/cache#toc-change-the-cache-path-for-yarn
+[generates these variables]: usage.md#generate-environment-variables
+[Overview]: #overview
+[Supported dependencies types]: #supported-dependencies-types
+[Dealing with Yarn Zero-Installs]: #dealing-with-Yarn-Zero-Installs
+[Fetching dependencies for Yarn Classic projects]: #fetching-dependencies-for-Yarn-Classic-projects
+[Handling of yarn-specific config files]: #handling-of-yarn-specific-config-files
+[Prerequisites for an offline build]: #prerequisites-for-an-offline-build