go/src/internal/coverage/cfile/testsupport.go

// ProcessCoverTestDir is called from
// testmain code when "go test -cover" is in effect. It is not
// intended to be used other than internally by the Go command's
// generated code.
func ProcessCoverTestDir(dir string, cfile string, cm string, cpkg string, w io.Writer, selpkgs []string) error { … }

type tstate …

// processPod reads coverage counter data for a specific pod.
func (ts *tstate) processPod(p pods.Pod, importpaths map[string]struct{ … }

type pkfunc …

func (ts *tstate) readAuxMetaFiles(metafiles string, importpaths map[string]struct{ … }

// Snapshot returns a snapshot of coverage percentage at a moment of
// time within a running test, so as to support the testing.Coverage()
// function. This version doesn't examine coverage meta-data, so the
// result it returns will be less accurate (more "slop") due to the
// fact that we don't look at the meta data to see how many statements
// are associated with each counter.
func Snapshot() float64 { … }