[Github][Docs] Add CI Best Practices Docs (#129462)

This is something that Tom and I have discussed briefly for a while now,
a doc that lists out all of the best practices we want to adhere to
surrounding CI things along with their associated motivations/any other
relevant info. This patch adds that doc along with three best practices
surrounding Github Workflows that we try and adhere to (although more
work needs to be done to get 100% adherance).

Author: boomanaiden154

llvm/docs/CIBestPractices.rst

@@ -0,0 +1,94 @@
+======================
+LLVM CI Best Practices
+======================
+
+Overview
+========
+
+This document contains a list of guidelines and best practices to use when
+working on LLVM's CI systems. These are intended to keep our actions reliable,
+consistent, and secure.
+
+Github Actions Best Practices
+=============================
+
+This section contains information on best practices/guidelines when working on
+LLVM's github actions workflows.
+
+Disabling Jobs In Forks
+-----------------------
+
+There are many LLVM forks that exist, and we currently default to preventing
+actions from running outside of the LLVM organization to prevent them from
+running in forks. We default to this as actions running in forks are usually
+not desired and only run by accident. In addition, many of our workflows
+assume that they are operating within the main LLVM repository and break
+otherwise.
+
+Adhering to this best practice looks like adding the following to each of the
+jobs specified within a workflow:
+
+.. code-block:: yaml
+
+  jobs:
+    <job name>:
+      if: github.repository_owner == 'llvm'
+
+We choose to use ``github.repository_owner`` rather than ``github.repository``
+to enable these workflows to run in forks inside the LLVM organization such as
+the ClangIR fork.
+
+There are some exceptions to this rule where ``github.repository`` might be
+used when it makes sense to limit a workflow to only running in the main
+monorepo repository. These include things like the issue subscriber and
+release tasks, which should not run anywhere else.
+
+Hash Pinning Dependencies
+-------------------------
+
+Github Actions allows the use of actions from other repositories as steps in
+jobs. We take advantage of various actions for a variety of different tasks,
+but especially tasks like checking out the repository, and
+downloading/uploading build caches. These actions are typically versioned with
+just a release, which looks like the following:
+
+.. code-block:: yaml
+
+  steps:
+    - name: Checkout LLVM
+      uses: actions/checkout@v4
+
+However, it is best practice to specify an exact commit SHA from which to pull
+the action from, noting the version in a comment:
+
+We plan on revisting this reccomendation once Github's immutable actions have
+been rolled out as GA.
+
+.. code-block:: yaml
+
+  steps:
+    - name: Checkout LLVM
+      uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+
+This is beneficial for two reasons: reliability and security. Specifying an
+exact SHA rather than just a major version ensures we end up running the same
+action originally specified when the workflow as authored and/or updated,
+and that no breaking changes sneak in from new versions of a workflow being
+released. However, this effect could also be achieved by specifying an exact
+dot release. The biggest reason to prefer hash pinned dependencies is security.
+Release assets on Github are mutable, allowing an attacker to change the code
+within a specific version of an action after the fact, potentially stealing
+sensitive tokens and credentials. Hash pinning the dependencies prevents this
+as the hash would change with the code.
+
+Using Versioned Runner Images
+-----------------------------
+
+Github actions allows the use of either specifically versioned runner images
+(e.g., ``ubuntu-22.04``), or just the latest runner image
+(e.g., ``ubuntu-latest``). It is best practice to use explicitly versioned
+runner images. This prevents breakages when Github rolls the latest runner
+image to a new version with potentially breaking changes, instead allowing us
+to explicitly opt-in to using the new image when we have done sufficient
+testing to ensure that our existing workflows work as expected in the new
+environment.

llvm/docs/Reference.rst

@@ -14,6 +14,7 @@ LLVM and API reference documentation.
    BlockFrequencyTerminology
    BranchWeightMetadata
    Bugpoint
+   CIBestPractices
    CommandGuide/index
    ConvergenceAndUniformity
    ConvergentOperations