Add documentation for vendor mode.

RELNOTES: None
PiperOrigin-RevId: 639001377
Change-Id: Ib914fa8c62999b334ebea552481be5b7ebf52a2e

site/en/_book.yaml

@@ -134,6 +134,8 @@ upper_tabs:
  path: /external/extension
  - title: Module lockfile
  path: /external/lockfile
+ - title: Vendor mode
+ path: /external/vendor
  - title: '`mod` command'
  path: /external/mod-command
  - title: Bzlmod migration guide

site/en/external/vendor.md

@@ -0,0 +1,144 @@
+Project: /_project.yaml
+Book: /_book.yaml
+keywords: product:Bazel,Bzlmod,vendor
+
+{# disableFinding("vendoring") #}
+{# disableFinding("Vendoring") #}
+{# disableFinding("vendored") #}
+{# disableFinding("repo") #}
+
+# Vendor Mode
+
+Vendor mode is a feature of Bzlmod that lets you create a local copy of
+external dependencies. This is useful for offline builds, or when you want to
+control the source of an external dependency.
+
+## Enable vendor mode {:#enable-vendor-mode}
+
+You can enable vendor mode by specifying `--vendor_dir` flag.
+
+For example, by adding it to your `.bazelrc` file:
+
+```none
+# Enable vendor mode with vendor directory under <workspace>/vendor_src
+common --vendor_dir=vendor_src
+```
+
+The vendor directory can be either a relative path to your workspace root or an
+absolute path.
+
+## Vendor a specific external repository {:#vendor-specific-repository}
+
+You can use the `vendor` command with the `--repo` flag to specify which repo
+to vendor, it accepts both [canonical repo
+name](/external/overview#canonical-repo-name) and [apparent repo
+name](/external/overview#apparent-repo-name).
+
+For example, running:
+
+```none
+bazel vendor --vendor_dir=vendor_src --repo=@rules_cc
+```
+
+or
+
+```none
+bazel vendor --vendor_dir=vendor_src --repo=@@rules_cc~
+```
+
+will both get rules_cc to be vendored under
+`<workspace root>/vendor_src/rules_cc~`.
+
+## Vendor external dependencies for given targets {:#vendor-target-dependencies}
+
+To vendor all external dependencies required for building given target patterns,
+you can run `bazel vendor <target patterns>`.
+
+For example
+
+```none
+bazel vendor --vendor_dir=vendor_src //src/main:hello-world //src/test/...
+```
+
+will vendor all repos required for building the `//src/main:hello-world` target
+and all targets under `//src/test/...` with the current configuration.
+
+Under the hood, it's doing a `bazel build --nobuild` command to analyze the
+target patterns, therefore build flags could be applied to this command and
+affect the result.
+
+## Vendor all external dependencies {:#vendor-all-dependencies}
+
+To vendor all repos in your transitive external dependencies graph, you can
+run:
+
+```none
+bazel vendor --vendor_dir=vendor_src
+```
+
+Note that vendoring all dependencies has a few **disadvantages**:
+
+- Fetching all repos, including those introduced transitively, can be time-consuming.
+- The vendor directory can become very large.
+- Some repos may fail to fetch if they are not compatible with the current platform or environment.
+
+Therefore, consider vendoring for specific targets first.
+
+## Configure vendor mode with VENDOR.bazel {:#configure-vendor-mode}
+
+You can control how given repos are handled with the VENDOR.bazel file located
+under the vendor directory.
+
+There are two directives available, both accepting a list of
+[canonical repo names](/external/overview#canonical-repo-name) as arguments:
+
+- `ignore()`: to completely ignore a repository from vendor mode.
+- `pin()`: to pin a repository to its current vendored source as if there is a
+ `--override_repository` flag for this repo. Bazel will NOT update the vendored
+ source for this repo while running the vendor command unless it's unpinned.
+ The user can modify and maintain the vendored source for this repo manually.
+
+For example
+
+```python
+ignore("@@rules_cc~")
+pin("@@bazel_skylib~")
+```
+
+With this configuration
+
+- Both repos will be excluded from subsequent vendor commands.
+- Repo `bazel_skylib` will be overridden to the source located under the
+ vendor directory.
+- The user can safely modify the vendored source of `bazel_skylib`.
+- To re-vendor `bazel_skylib`, the user has to disable the pin statement
+ first.
+
+Note: Repository rules with
+[`local`](/rules/lib/globals/bzl#repository_rule.local) or
+[`configure`](/rules/lib/globals/bzl#repository_rule.configure) set to true are
+always excluded from vendoring.
+
+## Understand how vendor mode works {:#how-vendor-mode-works}
+
+Bazel fetches external dependencies of a project under `$(bazel info
+output_base)/external`. Vendoring external dependencies means copying out
+relevant files and directories to a given vendor directory and use the vendored
+source for later builds.
+
+The content being vendored includes:
+
+- The repo directory
+- The repo marker file
+
+During a build, if the vendored marker file is up-to-date or the repo is
+pinned in the VENDOR.bazel file, then Bazel uses the vendored source by creating
+a symlink to it under `$(bazel info output_base)/external` instead of actually
+running the repository rule. Otherwise, a warning is printed and Bazel will
+fallback to fetching the latest version of the repo.
+
+Note: Bazel assumes the vendored source is not changed by users unless the repo
+is pinned in the VENDOR.bazel file. If a user does change the vendored source
+without pinning the repo, the changed vendored source will be used, but it will
+be overwritten if its existing marker file is
+outdated and the repo is vendored again.