[soil] Implemented 'maybe-merge' task.

This is #745.

Right now it's gated on just two tasks: dummy and dev-minimal.

Notes:

1. I figured out the Github API to fast forward.
2. I generated an API token from the web interface
3. Put that token into Github Actions

.github/workflows/all-builds.yml

@@ -218,14 +218,16 @@ jobs:
     runs-on: ubuntu-18.04
     env:
       REPO_ROOT: ${{ github.workspace }}
-    needs: [dummy, dev-minimal, cpp, ovm-tarball, pea, other-tests, app-tests]
+    #needs: ['dummy', 'dev-minimal', 'cpp', 'ovm-tarball', 'pea', 'other-tests', 'app-tests']
+    # List of tasks to wait on. This is duplicated in soil/maybe-merge.sh
+    needs: ['dummy', 'dev-minimal']
     steps:
       - name: Check out repository code
         uses: actions/checkout@v2
-        with:
-          submodules: recursive
 
       - name: maybe-merge
+        env:
+          SOIL_GITHUB_API_TOKEN: ${{ secrets.SOIL_GITHUB_API_TOKEN }}
         run: |
           cd $REPO_ROOT
           soil/worker.sh run-maybe-merge

soil/maybe-merge.sh

@@ -1,21 +1,138 @@
 #!/usr/bin/env bash
 #
+# Fast forward a green branch to master.
+#
 # Usage:
-#   ./maybe-merge.sh <function name>
+#   soil/maybe-merge.sh <function name>
 
 set -o nounset
 set -o pipefail
 set -o errexit
 
+fast-forward()  {
+  # Generate a token in "Settings" -> Developer Settings
+  # https://docs.github.com/en/authentication/keeping-your-account-and-data-secure/creating-a-personal-access-token
+  # Should be a secret in Github Actions
+  local github_token=$1
+
+  local commit_hash=${2:-}
+  local to_branch=${3:-'master'}
+
+  # local testing
+  if test -z "$github_token"; then
+    # set by YAML
+    github_token=${SOIL_GITHUB_API_TOKEN:-}
+
+    # Local testing
+    if test -z "$github_token"; then
+      github_token=$(cat token.txt)
+    fi
+  fi
+  if test -z "$commit_hash"; then 
+    # $GITHUB_SHA is the commit, set by Github Actions
+    commit_hash=${GITHUB_SHA:-}
+
+    # Local testing
+    if test -z "$commit_hash"; then 
+      commit_hash='ae02c9d6e8ba8e19399de556292a1d93faa220d3'
+    fi
+  fi
+
+  # Adapted from
+  # https://stackoverflow.com/questions/55800253/how-can-i-do-a-fast-forward-merge-using-the-github-api
+
+  local response=_tmp/soil/gh-fast-forward.json
+
+  curl \
+    -o $response \
+    -H "Content-Type: application/json" \
+    -H "Authorization: token ${github_token}" \
+    -X PATCH \
+    https://api.github.com/repos/oilshell/oil/git/refs/heads/$to_branch \
+    -d '{"sha": "'$commit_hash'", "force": false }'
+
+  local error
+  error=$(cat $response | jq '.message')
+
+  if test "$error" = 'null'; then
+    echo "Fast-forwarded branch $to_branch to commit $commit_hash"
+  else
+    echo 'ERROR fast forwarding:'
+  fi
+
+  cat $response
+}
+
+test-fast-forward()  {
+  fast-forward '' '' dev-andy-3
+}
+
+all-status-zero() {
+  ### Do all files contain status 0?
+
+  for path in "$@"; do
+    # There may be a newline on the end, which 'read' stops at.
+    read -r st < $path
+
+    if test "$st" != '0'; then
+      echo "$path = $st"
+      return 1
+    fi
+  done
+
+  return 0
+}
+
 soil-run() {
-  echo 'Hello from maybe-merge.sh'
+  local github_token=${1:-}  # SOIL_GITHUB_API_TOKEN
+  local run_id=${2:-}  # $GITHUB_RUN_ID
+  local jobs=${3:-'dummy dev-minimal'}  # minimal set of jobs to wait for
+  local commit_hash=${4:-}  # GITHUB_SHA
+  local to_branch=${5:-}  # defaults to master
+
+  if test -z "$run_id"; then
+    # GITHUB_RUN_ID is set by Github Actions
+    run_id=${GITHUB_RUN_ID:-}
+
+    # local testing
+    if test -z "$run_id"; then
+      run_id='2526880241'
+    fi
+  fi
 
   local branch=$(git rev-parse --abbrev-ref HEAD)
-  echo "BRANCH = $branch"
+  echo "Should we auto-merge branch $branch to master?"
 
-  if test "$branch" = 'soil-staging'; then
-    echo 'TODO: check all state'
+  if test "$branch" != 'soil-staging'; then
+    echo 'No, must be soil-staging'
+    #return
   fi
+
+  local dir=_tmp/status-api
+  rm -f -v $dir/*
+  mkdir -p $dir
+
+  # These tiny files are written by each Soil task
+  local url_base="http://travis-ci.oilshell.org/status-api/github/$run_id"
+
+  local -a args=()
+  for job in $jobs; do  # relies on word splitting
+
+    # output each URL in a different file
+    args=("${args[@]}" -o $dir/$job $url_base/$job )
+  done
+
+  curl -v ${args[@]}
+
+  if all-status-zero $dir/*; then
+    fast-forward "$github_token" "$commit_hash" "$to_branch"
+  fi
+}
+
+test-soil-run() {
+  # test with non-master branch
+  # other params have testing defaults
+  soil-run '' '' '' '' dev-andy-3
 }
 
 "$@"