Flaky test cache filter

[szuecs]

time="2026-06-17T18:07:08Z" level=warning msg="cache: background revalidation fetch failed" error="no fetch stub set" url="http://localhost:9090/spaces/abc/entries/swr"
panic: close of closed channel
	
	runtime/debug.Stack()
		/opt/hostedtoolcache/go/1.26.4/x64/src/runtime/debug/stack.go:26 +0x68
	golang.org/x/sync/singleflight.newPanicError({0x1018640, 0x11676d0})
		/home/runner/go/pkg/mod/golang.org/x/sync@v0.21.0/singleflight/singleflight.go:44 +0x2b
	golang.org/x/sync/singleflight.(*Group).doCall.func2.1()
		/home/runner/go/pkg/mod/golang.org/x/sync@v0.21.0/singleflight/singleflight.go:193 +0x51
	panic({0x1018640?, 0x11676d0?})
		/opt/hostedtoolcache/go/1.26.4/x64/src/runtime/panic.go:860 +0x13a
	github.com/zalando/skipper/filters/cache.TestCacheFilter_ColdMissCoalescing.func1(0x0?)
		/home/runner/work/skipper/skipper/filters/cache/filter_test.go:386 +0x6f
	github.com/zalando/skipper/filters/cache.(*cacheFilter).coalesce.func1()
		/home/runner/work/skipper/skipper/filters/cache/filter.go:337 +0xba
	golang.org/x/sync/singleflight.(*Group).doCall.func2(0xc0000a6f07, 0xc00003a0a0, 0xc0004b1ce0)
		/home/runner/go/pkg/mod/golang.org/x/sync@v0.21.0/singleflight/singleflight.go:198 +0xaa
	golang.org/x/sync/singleflight.(*Group).doCall(0xc000136560, 0xc00003a0a0, {0xc00015efc0, 0x40}, 0xc0004b1ce0)
		/home/runner/go/pkg/mod/golang.org/x/sync@v0.21.0/singleflight/singleflight.go:200 +0x132
	created by golang.org/x/sync/singleflight.(*Group).DoChan in goroutine 136
		/home/runner/go/pkg/mod/golang.org/x/sync@v0.21.0/singleflight/singleflight.go:138 +0x55d
	

goroutine 169 [running]:
golang.org/x/sync/singleflight.(*Group).doCall.func1.gowrap2()
	/home/runner/go/pkg/mod/golang.org/x/sync@v0.21.0/singleflight/singleflight.go:167 +0x39
created by golang.org/x/sync/singleflight.(*Group).doCall.func1 in goroutine 168
	/home/runner/go/pkg/mod/golang.org/x/sync@v0.21.0/singleflight/singleflight.go:167 +0x4b3
FAIL	github.com/zalando/skipper/filters/cache	0.082s

[AshrafAhmed9]

Hi @szuecs, I'd like to work on this flaky test if it's available. From the trace, the panic is a close of closed channel in TestCacheFilter_ColdMissCoalescing (filter_test.go:386), happening during coalesced revalidation when the fetch stub isn't set. It looks like a race where the channel can be closed more than once under concurrent singleflight calls. I'd start by adding a guard (e.g. sync.Once or a select-based close) and ensuring the test sets up the fetch stub before revalidation runs. Would you be open to assigning this to me? Thanks!

[szuecs]

There is active work ongoing on cache filters, so ignore these for now