DEV: Cache database in GitHub actions tests (#15279)

A cached database (and its uploads) will only be used if the current run has exactly the same set of migration files. Otherwise, the database will be migrated from scratch

This saves approximately 75s on the core backend specs and 45s on other runs.

.github/workflows/tests.yml

@@ -24,6 +24,7 @@ jobs:
       PGHOST: postgres
       PGUSER: discourse
       PGPASSWORD: discourse
+      USES_PARALLEL_DATABASES: ${{ matrix.build_type == 'backend' && matrix.target == 'core' }}
 
     strategy:
       fail-fast: false
@@ -102,17 +103,48 @@ jobs:
         if: matrix.target == 'plugins'
         run: bin/rake plugin:install_all_official
 
-      - name: Create database
+      - name: Fetch app state cache
+        uses: actions/cache@v2
+        id: app-cache
+        with:
+          path: tmp/app-cache
+          key: >- # postgres version, hash of migrations, "parallel?"
+            ${{ runner.os }}-
+            ${{ hashFiles('.github/workflows/tests.yml') }}-
+            ${{ matrix.postgres }}-
+            ${{ hashFiles('db/**/*', 'plugins/**/db/**/*') }}-
+            ${{ env.USES_PARALLEL_DATABASES }}
+
+      - name: Restore database from cache
+        if: steps.app-cache.outputs.cache-hit == 'true'
+        run: psql -f tmp/app-cache/cache.sql postgres
+
+      - name: Restore uploads from cache
+        if: steps.app-cache.outputs.cache-hit == 'true'
+        run: rm -rf public/uploads && cp -r tmp/app-cache/uploads public/uploads
+
+      - name: Create and migrate database
+        if: steps.app-cache.outputs.cache-hit != 'true'
         run: |
           bin/rake db:create
           bin/rake db:migrate
 
-      - name: Create parallel databases
-        if: matrix.build_type == 'backend' && matrix.target == 'core'
+      - name: Create and migrate parallel databases
+        if: >-
+          env.USES_PARALLEL_DATABASES == 'true' &&
+          steps.app-cache.outputs.cache-hit != 'true'
         run: |
           bin/rake parallel:create
           bin/rake parallel:migrate
 
+      - name: Dump database for cache
+        if: steps.app-cache.outputs.cache-hit != 'true'
+        run: mkdir -p tmp/app-cache && pg_dumpall > tmp/app-cache/cache.sql
+
+      - name: Dump uploads for cache
+        if: steps.app-cache.outputs.cache-hit != 'true'
+        run: rm -rf tmp/app-cache/uploads && cp -r public/uploads tmp/app-cache/uploads
+
       - name: Core RSpec
         if: matrix.build_type == 'backend' && matrix.target == 'core'
         run: bin/turbo_rspec --verbose