Auto merge of #12181 - weihanglo:which-channel-backport, r=ehuss

chore: detect the channel a PR wants to merge into

### What does this PR try to resolve?

This detects which channel a pull request wants to merge into.

It is hard because bors runs in a different repo.
Bors knows nothing about branches or tag in this repo.
It only see one base commit and a merge commit AFAIK.

Basically the assumption and idea are

1. bors create a merge commit, so `HEAD~` should find the base branch.
2. Cargo maintains `rust-1.y.0` branch for each Rust minor version releases.
   Therefore, we can use the symbolic name of `origin/rust-1.x.0`
   to determine if it aims for stable, beta, or nightly channel.
3. After we know which channel it is targeted at,
   we can skip jobs that are likely to be broken.

.github/workflows/main.yml

@@ -16,6 +16,20 @@ env:
   CARGO_REGISTRIES_CRATES_IO_PROTOCOL: sparse
 
 jobs:
+  # Determine which channel will be merged into.
+  channel:
+    runs-on: ubuntu-latest
+    outputs:
+      CHANNEL: ${{ steps.channel.outputs.CHANNEL }}
+    env:
+      BASE_SHA: ${{ github.event.pull_request.base.sha }}
+    steps:
+    - uses: actions/checkout@v3
+      with:
+        fetch-depth: 0 # fetch all branches
+    - id: channel
+      run: ci/which-channel.sh
+
   # Check Code style quickly by running `rustfmt` over all code
   rustfmt:
     runs-on: ubuntu-latest
@@ -63,8 +77,42 @@ jobs:
       - run: rustup update stable && rustup default stable
       - run: ci/validate-version-bump.sh
 
+  # Generate strategy matrix for different platforms and channels
+  # (see ci/matrix.json)
+  matrix:
+    runs-on: ubuntu-latest
+    needs:
+    - channel
+    outputs:
+      matrix: ${{ steps.matrix.outputs.matrix }}
+    steps:
+    - uses: actions/checkout@v3
+    - name: Generate strategy matrix
+      id: matrix
+      run: |
+        CHANNEL=${{ needs.channel.outputs.CHANNEL }}
+
+        # This reads ci/matrix.json and then filters the environment we want to
+        # build on, based on the channel this PR want to merge into.
+        #
+        # * For stable, we build only on stable toolchain.
+        # * For beta, we build on stable and beta.
+        # * For nightly, we build on stable, beta, and nightly.
+        MATRIX=$(
+          jq --arg C "$CHANNEL" 'map (. | 
+            if ($C == "beta") then select(.rust | startswith("nightly") | not)
+            elif ($C == "stable") then select(.rust | startswith("stable"))
+            else . end)' ci/matrix.json
+        )
+        echo "$MATRIX"
+
+        # Outputs as Job's outputs for other jobs to reuse.
+        echo "MATRIX={\"include\":$(echo $MATRIX)}" >> "$GITHUB_OUTPUT"
+
   test:
     runs-on: ${{ matrix.os }}
+    needs:
+    - matrix
     env:
       CARGO_PROFILE_DEV_DEBUG: 1
       CARGO_PROFILE_TEST_DEBUG: 1
@@ -73,36 +121,7 @@ jobs:
       # Deny warnings on CI to avoid warnings getting into the codebase.
       RUSTFLAGS: -D warnings
     strategy:
-      matrix:
-        include:
-        - name: Linux x86_64 stable
-          os: ubuntu-latest
-          rust: stable
-          other: i686-unknown-linux-gnu
-        - name: Linux x86_64 beta
-          os: ubuntu-latest
-          rust: beta
-          other: i686-unknown-linux-gnu
-        - name: Linux x86_64 nightly
-          os: ubuntu-latest
-          rust: nightly
-          other: i686-unknown-linux-gnu
-        - name: macOS x86_64 stable
-          os: macos-latest
-          rust: stable
-          other: x86_64-apple-ios
-        - name: macOS x86_64 nightly
-          os: macos-latest
-          rust: nightly
-          other: x86_64-apple-ios
-        - name: Windows x86_64 MSVC stable
-          os: windows-latest
-          rust: stable-msvc
-          other: i686-pc-windows-msvc
-        - name: Windows x86_64 gnu nightly # runs out of space while trying to link the test suite
-          os: windows-latest
-          rust: nightly-gnu
-          other: i686-pc-windows-gnu
+      matrix: ${{ fromJSON(needs.matrix.outputs.MATRIX) }}
     name: Tests ${{ matrix.name }}
     steps:
     - uses: actions/checkout@v3
@@ -180,6 +199,9 @@ jobs:
 
   build_std:
     runs-on: ubuntu-latest
+    needs:
+    - channel
+    if: needs.channel.outputs.CHANNEL == 'master'
     steps:
     - uses: actions/checkout@v3
     - run: rustup update nightly && rustup default nightly
@@ -190,6 +212,8 @@ jobs:
         CARGO_RUN_BUILD_STD_TESTS: 1
   docs:
     runs-on: ubuntu-latest
+    needs:
+    - channel
     steps:
     - uses: actions/checkout@v3
     - run: rustup update nightly && rustup default nightly
@@ -211,22 +235,36 @@ jobs:
     - run: cd src/doc && mdbook build --dest-dir ../../target/doc
     - name: Run linkchecker.sh
       run: |
+        BRANCH=${{ needs.channel.outputs.CHANNEL }}
         cd target
-        curl -sSLO https://raw.githubusercontent.com/rust-lang/rust/master/src/tools/linkchecker/linkcheck.sh
+        curl -sSLO "https://raw.githubusercontent.com/rust-lang/rust/$BRANCH/src/tools/linkchecker/linkcheck.sh"
         sh linkcheck.sh --all --path ../src/doc cargo
 
+  # Jobs that may be skipped if they aren't going to merge into master (nightly).
+  #
+  # This is needed because GitHub Actions treats success() as false if a job is
+  # skipped, and the bors success/failure jobs below need to ignore skipped jobs.
+  z-nightly-jobs:
+    needs:
+    - channel
+    - build_std
+    if: "(needs.channel.outputs.CHANNEL != 'master' && !failure()) || success()"
+    runs-on: ubuntu-latest
+    steps:
+    - run: exit 0
+
   success:
     permissions:
       contents: none
     name: bors build finished
     needs:
-    - build_std
     - docs
     - lockfile
     - resolver
     - rustfmt
     - test
     - test_gitoxide
+    - z-nightly-jobs
     runs-on: ubuntu-latest
     if: "success() && github.event_name == 'push' && github.ref == 'refs/heads/auto-cargo'"
     steps:
@@ -236,13 +274,13 @@ jobs:
       contents: none
     name: bors build finished
     needs:
-    - build_std
     - docs
     - lockfile
     - resolver
     - rustfmt
     - test
     - test_gitoxide
+    - z-nightly-jobs
     runs-on: ubuntu-latest
     if: "!success() && github.event_name == 'push' && github.ref == 'refs/heads/auto-cargo'"
     steps:

ci/matrix.json

@@ -0,0 +1,44 @@
+[
+  {
+    "name": "Linux x86_64 stable",
+    "os": "ubuntu-latest",
+    "rust": "stable",
+    "other": "i686-unknown-linux-gnu"
+  },
+  {
+    "name": "Linux x86_64 beta",
+    "os": "ubuntu-latest",
+    "rust": "beta",
+    "other": "i686-unknown-linux-gnu"
+  },
+  {
+    "name": "Linux x86_64 nightly",
+    "os": "ubuntu-latest",
+    "rust": "nightly",
+    "other": "i686-unknown-linux-gnu"
+  },
+  {
+    "name": "macOS x86_64 stable",
+    "os": "macos-latest",
+    "rust": "stable",
+    "other": "x86_64-apple-ios"
+  },
+  {
+    "name": "macOS x86_64 nightly",
+    "os": "macos-latest",
+    "rust": "nightly",
+    "other": "x86_64-apple-ios"
+  },
+  {
+    "name": "Windows x86_64 MSVC stable",
+    "os": "windows-latest",
+    "rust": "stable-msvc",
+    "other": "i686-pc-windows-msvc"
+  },
+  {
+    "name": "Windows x86_64 gnu nightly",
+    "os": "windows-latest",
+    "rust": "nightly-gnu",
+    "other": "i686-pc-windows-gnu"
+  }
+]

ci/which-channel.sh

@@ -0,0 +1,50 @@
+#!/bin/bash
+# This script outputs the channel where a CI workflow wants to merge into.
+#
+# Inputs:
+#     BASE_SHA    The commit SHA of the branch where the PR wants to merge into.
+#
+# GitHub Action Outputs:
+#     CHANNEL     Target channel where the PR wants to merge into.
+
+set -euo pipefail
+
+# When `BASE_SHA` is missing, we assume it is from bors merge commit,
+# so hope `HEAD~` to find the previous commit on master branch.
+base_sha=$(git rev-parse "${BASE_SHA:-HEAD~1}")
+
+# Get symbolic names for the base_sha.
+# Assumption: Cargo branches are always in the format of `rust-1.*.0`,
+#             otherwise `git name-rev` will return "undefined".
+ref=$(git name-rev --name-only --refs='origin/rust-1.*.0' $base_sha)
+
+# Get the latest `rust-1.*.0` branch from remote origin.
+# Assumption: The latest branch is always beta branch.
+beta=$(
+    git branch --remotes --list 'origin/rust-1.*.0' \
+    | sort --version-sort \
+    | tail -n1 \
+    | tr -d "[:space:]"
+)
+
+master=$(git rev-parse origin/master)
+
+# Backport pull requests always target at a `rust-1.*.0` branch.
+if [[ "$ref" = "undefined" ]] || [[ "$base_sha" = "$master" ]]
+then
+    # Should be nightly but for convenience in CI let's call it master.
+    channel="master"
+elif [[ "$ref" = "$beta" ]]
+then
+    channel="beta"
+else
+    channel="stable"
+fi
+
+echo "Base sha: $base_sha"
+echo "Git Ref:  $ref"
+echo "master:   $master"
+echo "beta:     $beta"
+echo "Channel:  $channel"
+
+echo "CHANNEL=$channel" >> "$GITHUB_OUTPUT"