@robpike, is cmd/cover compatible with cgo?

It's supposed to. See #6333.

Simple reproducer:

rand.go:

package rand

/*
#include <stdlib.h>
*/
import "C"

func Random() int {
    return int(C.random())
}

func Seed(i int) {
    C.srandom(C.uint(i))
}

rand_test.go:

package rand_test

import (
    "testing"

    "github.com/ALTree/rand"
)

func TestRandom(t *testing.T) {
    x := rand.Random()
    if x < 0 {
        t.Fatalf("%d < 0\n", x)
    }
}

go test -cover gives 0.0% if and only if the tests are in a separate package (in this case, rand_test). If the package declaration in rand_test.go is changed to package rand, -cover reports a correct value; and that is the reason the test introduced in CL 71230049 by rsc didn't catch the problem (that test has package p for both the cgo file and the _test file).

CL https://golang.org/cl/32614 mentions this issue.