go/src/cmd/go/internal/test/cover.go

var coverMerge …

// initCoverProfile initializes the test coverage profile.
// It must be run before any calls to mergeCoverProfile or closeCoverProfile.
// Using this function clears the profile in case it existed from a previous run,
// or in case it doesn't exist and the test is going to fail to create it (or not run).
func initCoverProfile() { … }

// mergeCoverProfile merges file into the profile stored in testCoverProfile.
// It prints any errors it encounters to ew.
func mergeCoverProfile(ew io.Writer, file string) { … }

func closeCoverProfile() { … }