cmd/go: fix -covermode=atomic use of sync/atomic in -coverpkg matches

If we're using -covermode=atomic with -coverpkg, to add coverage
to more than just the package being tested, then we need to make sure
to make sync/atomic available to the compiler for every package
being recompiled for coverage.

Fixes #22728.

Change-Id: I27f88f6a62e37d4a7455554cd03c8ca2b21f81a4
Reviewed-on: https://go-review.googlesource.com/81497
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/go_test.go

@@ -2416,6 +2416,14 @@ func TestCoverageUsesAtomicModeForRace(t *testing.T) {
 	checkCoverage(tg, data)
 }
 
+func TestCoverageSyncAtomicImport(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.setenv("GOPATH", filepath.Join(tg.pwd(), "testdata"))
+	tg.run("test", "-short", "-cover", "-covermode=atomic", "-coverpkg=coverdep/p1", "coverdep")
+}
+
 func TestCoverageImportMainLoop(t *testing.T) {
 	tg := testgo(t)
 	defer tg.cleanup()

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

@@ -670,6 +670,9 @@ func runTest(cmd *base.Command, args []string) {
 			coverFiles = append(coverFiles, p.CgoFiles...)
 			coverFiles = append(coverFiles, p.TestGoFiles...)
 			p.Internal.CoverVars = declareCoverVars(p.ImportPath, coverFiles...)
+			if testCover && testCoverMode == "atomic" {
+				ensureImport(p, "sync/atomic")
+			}
 		}
 	}
 

src/cmd/go/testdata/src/coverdep/p.go

+package p
+
+import _ "coverdep/p1"
+
+func F() {
+}

src/cmd/go/testdata/src/coverdep/p1/p1.go

+package p1
+
+import _ "errors"

src/cmd/go/testdata/src/coverdep/p_test.go

+package p
+
+import "testing"
+
+func Test(t *testing.T) {
+	F()
+}