fix(pagecache): preserve stale cache object when bgfetch returns 5xx

[lbajsarowicz]

Description

Add return (abandon) when a background fetch receives a 5xx response, preventing a failing backend refresh from replacing a usable stale cache object.

Problem

Magento's VCL templates use grace periods to serve stale content while revalidating in the background. However, if the backend returns a 5xx error during this background fetch, Varnish replaces the cached object with the error response. This defeats the purpose of grace-based serving during backend incidents.

Solution

Add an early check in vcl_backend_response:

if (beresp.status >= 500 && bereq.is_bgfetch) {
    return (abandon);
}

This follows Varnish's documented grace behavior and ensures stale objects survive backend failures during revalidation.

Applied to varnish5, varnish6, and varnish7 templates (bereq.is_bgfetch requires Varnish 5.1+).

Files Changed

• app/code/Magento/PageCache/etc/varnish5.vcl
• app/code/Magento/PageCache/etc/varnish6.vcl
• app/code/Magento/PageCache/etc/varnish7.vcl

Ref #40673

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