TestReadFromSquid fails intermittently due to race condition

[dswarbrick]

Describe the bug
The TestReadFromSquid test case fails intermittently (especially on slower hosts, such as low-powered ARM systems) due to race condition in the goroutine which occasionally tries to read from an uninitialized ch.server (net.Conn).

=== RUN   TestReadFromSquid
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x28 pc=0x36ccb4]

goroutine 3 [running]:
github.com/boynux/squid-exporter/collector.TestReadFromSquid.func1()
        /home/daniel/src/squid-exporter/collector/client_test.go:39 +0x44
created by github.com/boynux/squid-exporter/collector.TestReadFromSquid in goroutine 2
        /home/daniel/src/squid-exporter/collector/client_test.go:37 +0x70
FAIL    github.com/boynux/squid-exporter/collector      0.152s

The ch.server is initialized by the coc.readFromSquid call, so any delays between the goroutine starting and coc.readFromSquid being called will trigger the error.

func TestReadFromSquid(t *testing.T) {
    ch := &mockConnectionHandler{}

    go func() {
        b := make([]byte, 256)
        n, _ := ch.server.Read(b)
        ch.buffer = append(ch.buffer, b[:n]...)

        ch.server.Write(b[n:])
        ch.server.Close()
    }() 

    // Here lies raciness ...

    coc := &CacheObjectClient{
        ch, 
        "", 
        []string{},
    }   
    expected := "GET cache_object://localhost/test HTTP/1.0\r\nHost: localhost\r\nUser-Agent: squidclient/3.5.12\r\nAccept: */*\r\n\r\n"
    coc.readFromSquid("test")

    assert.Equal(t, expected, string(ch.buffer))
}

To Reproduce
Add a small delay just before the coc.readFromSquid("test") line. Failure can be reliably reproduced with delays as small as 5 µs, demonstrating the timing-critical nature of it.

Expected behavior
Test should pass reliably on all systems.

OS (please complete the following information):
Irrelevant / any.

[boynux]

Thank you for reporting this and for the analysis. I'll try to get to this as soon as I can. Meanwhile if you already have a fix in mind I'll be happy to review your PR.