add warning if follower is behind

commands/unfollow.js

@@ -19,9 +19,14 @@ function * run (context, heroku) {
 ${cli.color.addon(db.name)} will become writeable and no longer follow ${origin}. This cannot be undone.
 `)
 
+  let response = { leader_xact: 0, follower_xact: 0 }
   yield cli.action(`${cli.color.addon(db.name)} unfollowing`, co(function * () {
-    yield heroku.put(`/client/v11/databases/${db.id}/unfollow`, {host: host(db)})
+    response = yield heroku.put(`/client/v11/databases/${db.id}/unfollow`, {host: host(db)})
   }))
+
+  if (response.leader_xact > response.follower_xact) {
+    cli.warn(`you unfollowed this database while the leader transaction was ${response.leader_xact} and the follower transaction was ${response.follower_xact}. Due to the lag, you may experience read-only mode with the unfollowed database (${db.name}) for a while.`)
+  }
 }
 
 module.exports = {

test/commands/unfollow.js

@@ -41,6 +41,18 @@ describe('pg:unfollow', () => {
     pg.get('/client/v11/databases/1').reply(200, {following: 'postgres://db1'})
     pg.put('/client/v11/databases/1/unfollow').reply(200)
     return cmd.run({app: 'myapp', args: {}, flags: {confirm: 'myapp'}})
-    .then(() => expect(cli.stderr, 'to equal', 'postgres-1 unfollowing... done\n'))
+        .then(() => expect(cli.stderr, 'to equal', 'postgres-1 unfollowing... done\n'))
+  })
+
+  it('warns if there is a lag', () => {
+    api.get('/apps/myapp/config-vars').reply(200, {DATABASE_URL: 'postgres://db1'})
+    pg.get('/client/v11/databases/1').reply(200, {following: 'postgres://db1'})
+    pg.put('/client/v11/databases/1/unfollow').reply(200, { leader_xact: 100, follower_xact: 50 })
+    return cmd.run({app: 'myapp', args: {}, flags: {confirm: 'myapp'}})
+        .then(() => expect(cli.stderr, 'to equal', `postgres-1 unfollowing... done
+ ▸    you unfollowed this database while the leader transaction was 100 and the
+ ▸    follower transaction was 50. Due to the lag, you may experience read-only
+ ▸    mode with the unfollowed database (postgres-1) for a while.
+`))
   })
 })