fix(core): Add retry logic in case of transaction deadlocks

Fixes #527.

packages/core/e2e/fixtures/test-plugins/slow-mutation-plugin.ts

@@ -0,0 +1,86 @@
+import { Args, Mutation, Resolver } from '@nestjs/graphql';
+import {
+    Asset,
+    AssetType,
+    Country,
+    Ctx,
+    PluginCommonModule,
+    Product,
+    ProductAsset,
+    RequestContext,
+    TaxCategory,
+    TaxRate,
+    Transaction,
+    TransactionalConnection,
+    VendurePlugin,
+} from '@vendure/core';
+import gql from 'graphql-tag';
+
+@Resolver()
+export class SlowMutationResolver {
+    constructor(private connection: TransactionalConnection) {}
+
+    /**
+     * A mutation which simulates some slow DB operations occurring within a transaction.
+     */
+    @Transaction()
+    @Mutation()
+    async slowMutation(@Ctx() ctx: RequestContext, @Args() args: { delay: number }) {
+        const delay = Math.round(args.delay / 2);
+        const country = await this.connection.getRepository(ctx, Country).findOneOrFail({
+            where: {
+                code: 'AT',
+            },
+        });
+        country.enabled = false;
+        await new Promise(resolve => setTimeout(resolve, delay));
+        await this.connection.getRepository(ctx, Country).save(country);
+        country.enabled = true;
+        await new Promise(resolve => setTimeout(resolve, delay));
+        await this.connection.getRepository(ctx, Country).save(country);
+        return true;
+    }
+
+    /**
+     * This mutation attempts to cause a deadlock
+     */
+    @Transaction()
+    @Mutation()
+    async attemptDeadlock(@Ctx() ctx: RequestContext) {
+        const product = await this.connection.getRepository(ctx, Product).findOneOrFail(1);
+        const asset = await this.connection.getRepository(ctx, Asset).save(
+            new Asset({
+                name: 'test',
+                type: AssetType.BINARY,
+                mimeType: 'test/test',
+                fileSize: 1,
+                source: '',
+                preview: '',
+            }),
+        );
+        await new Promise(resolve => setTimeout(resolve, 100));
+        const productAsset = await this.connection.getRepository(ctx, ProductAsset).save(
+            new ProductAsset({
+                assetId: asset.id,
+                productId: product.id,
+                position: 0,
+            }),
+        );
+        await this.connection.getRepository(ctx, Product).update(product.id, { enabled: false });
+        return true;
+    }
+}
+
+@VendurePlugin({
+    imports: [PluginCommonModule],
+    adminApiExtensions: {
+        resolvers: [SlowMutationResolver],
+        schema: gql`
+            extend type Mutation {
+                slowMutation(delay: Int!): Boolean!
+                attemptDeadlock: Boolean!
+            }
+        `,
+    },
+})
+export class SlowMutationPlugin {}