build: check potential db migration conflict for open PRs

.github/workflows/check_db_migration_confict.yml

@@ -0,0 +1,59 @@
+name: Check DB migration conflict
+on:
+  push:
+    paths:
+      - "superset/migrations/**"
+
+jobs:
+  check_db_migration_conflict:
+    name: Check DB migration conflict
+    runs-on: ubuntu-20.04
+    steps:
+      - name: "Checkout ${{ github.ref }} ( ${{ github.sha }} )"
+        uses: actions/checkout@v2
+      - name: Check and notify
+        uses: actions/github-script@v3
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+          script: |
+            // API reference: https://octokit.github.io/rest.js
+            const currentBranch = context.ref.replace('refs/heads/', '');
+
+            // Find all pull requests to current branch
+            const opts = github.pulls.list.endpoint.merge({
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              base: context.ref,
+              state: 'open',
+              sort: 'updated',
+              per_page: 100,
+            });
+            const pulls = await github.paginate(opts);
+            if (pulls.length > 0) {
+              console.log(`Found ${pulls.length} open PRs for base branch "${currentBranch}"`)
+            }
+
+            for (const pull of pulls) {
+              const listFilesOpts = await github.pulls.listFiles.endpoint.merge({
+                owner: context.repo.owner,
+                repo: context.repo.repo,
+                pull_number: pull.number,
+              });
+              const files = await github.paginate(listFilesOpts);
+              if (
+                files.some(x => x.contents_url.includes('/contents/superset/migrations'))
+              ) {
+                console.log(`PR #${pull.number} "${pull.title}" also added db migration`)
+                await github.issues.createComment({
+                  issue_number: context.issue.number,
+                  owner: context.repo.owner,
+                  repo: context.repo.repo,
+                  issue_number: pull.number,
+                  body:
+                    `⚠️ @${pull.user.login} Your base branch \`${currentBranch}\` has just ` +
+                    'also updated `superset/migrations`.\n' +
+                    '\n' +
+                    '❗ **Please consider rebasing your branch to avoid db migration conflicts.**',
+                });
+              }
+            }

CONTRIBUTING.md

@@ -1001,7 +1001,7 @@ Submissions will be considered for submission (or removal) on a case-by-case bas
 
 When two DB migrations collide, you'll get an error message like this one:
 
-```
+```text
 alembic.util.exc.CommandError: Multiple head revisions are present for
 given argument 'head'; please specify a specific target
 revision, '<branchname>@head' to narrow to a specific head,
@@ -1016,15 +1016,46 @@ To fix it:
    superset db heads
    ```
 
-   This should list two or more migration hashes.
+   This should list two or more migration hashes. E.g.
+
+   ```bash
+   1412ec1e5a7b (head)
+   67da9ef1ef9c (head)
+   ```
+
+2. Pick one of them as the parent revision, open the script for the other revision
+   and update `Revises` and `down_revision` to the new parent revision. E.g.:
+
+   ```diff
+   --- a/67da9ef1ef9c_add_hide_left_bar_to_tabstate.py
+   +++ b/67da9ef1ef9c_add_hide_left_bar_to_tabstate.py
+   @@ -17,14 +17,14 @@
+   """add hide_left_bar to tabstate
+
+   Revision ID: 67da9ef1ef9c
+   -Revises: c501b7c653a3
+   +Revises: 1412ec1e5a7b
+   Create Date: 2021-02-22 11:22:10.156942
+
+   """
+
+   # revision identifiers, used by Alembic.
+   revision = "67da9ef1ef9c"
+   -down_revision = "c501b7c653a3"
+   +down_revision = "1412ec1e5a7b"
+
+   import sqlalchemy as sa
+   from alembic import op
+   ```
 
-1. Create a new merge migration
+   Alternatively you may also run `superset db merge` to create a migration script
+   just for merging the heads.
 
    ```bash
    superset db merge {HASH1} {HASH2}
    ```
 
-1. Upgrade the DB to the new checkpoint
+3. Upgrade the DB to the new checkpoint
 
    ```bash
    superset db upgrade