TestRegisteringHookMultipleTimesShouldBeSafe failure on CPU-constrained systems

[gibmat]

It was reported in a Debian bug that TestRegisteringHookMultipleTimesShouldBeSafe fails when the tests are run on a system with only one or two CPUs, or alternatively on non-amd64 architectures with slower CPUs. The test failure output is at the end of this issue.

The cause appears to be that the metrics server started as a goroutine in httpServePrometheusMetrics() isn't fully ready to serve requests by the time the test code attempts to make a request. As a work around for the time being, I've introduced a quarter second sleep before returning from httpServePrometheusMetrics() (which is probably overkill) that fixes this test failure.

=== RUN   TestRegisteringHookMultipleTimesShouldBeSafe
    promrus_test.go:103: 
        	Error Trace:	/build/golang-github-weaveworks-promrus-1.2.0+git20210208.77c195c/_build/src/github.com/weaveworks/promrus/promrus_test.go:103
        	            				/build/golang-github-weaveworks-promrus-1.2.0+git20210208.77c195c/_build/src/github.com/weaveworks/promrus/promrus_test.go:29
        	Error:      	Expected nil, but got: &url.Error{Op:"Get", URL:"http://localhost:8080/metrics", Err:(*net.OpError)(0xc000025360)}
        	Test:       	TestRegisteringHookMultipleTimesShouldBeSafe
--- FAIL: TestRegisteringHookMultipleTimesShouldBeSafe (0.00s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x40 pc=0x8acf6a]

goroutine 8 [running]:
testing.tRunner.func1.2({0x9157c0, 0xdaaef0})
	/usr/lib/go-1.21/src/testing/testing.go:1545 +0x238
testing.tRunner.func1()
	/usr/lib/go-1.21/src/testing/testing.go:1548 +0x397
panic({0x9157c0?, 0xdaaef0?})
	/usr/lib/go-1.21/src/runtime/panic.go:914 +0x21f
github.com/weaveworks/promrus_test.httpGetMetrics(0x27?)
	/build/golang-github-weaveworks-promrus-1.2.0+git20210208.77c195c/_build/src/github.com/weaveworks/promrus/promrus_test.go:104 +0xca
github.com/weaveworks/promrus_test.TestRegisteringHookMultipleTimesShouldBeSafe(0x0?)
	/build/golang-github-weaveworks-promrus-1.2.0+git20210208.77c195c/_build/src/github.com/weaveworks/promrus/promrus_test.go:29 +0x98
testing.tRunner(0xc000106ea0, 0x9eb450)
	/usr/lib/go-1.21/src/testing/testing.go:1595 +0xff
created by testing.(*T).Run in goroutine 1
	/usr/lib/go-1.21/src/testing/testing.go:1648 +0x3ad
FAIL	github.com/weaveworks/promrus	0.023s