cmd/go: ignore coverpkg match on sync/atomic in atomic coverage mode

Otherwise we get into a dependency loop as we try to apply coverage
analysis to sync/atomic when the coverage analysis itself requires
sync/atomic.

Fixes #23694

Change-Id: I3a74ef3881ec5c6197ed348acc7f9e175417f6c7
Reviewed-on: https://go-review.googlesource.com/91875
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/go/go_test.go

@@ -5686,3 +5686,16 @@ func TestCpuprofileTwice(t *testing.T) {
 	tg.run("test", "-o="+bin, "-cpuprofile="+out, "x")
 	tg.mustExist(out)
 }
+
+// Issue 23694.
+func TestAtomicCoverpkgAll(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+
+	tg.tempFile("src/x/x.go", `package x; import _ "sync/atomic"; func F() {}`)
+	tg.tempFile("src/x/x_test.go", `package x; import "testing"; func TestF(t *testing.T) { F() }`)
+	tg.setenv("GOPATH", tg.path("."))
+	tg.run("test", "-coverpkg=all", "-race", "x")
+	tg.run("test", "-coverpkg=all", "-covermode=atomic", "x")
+}

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

@@ -659,6 +659,15 @@ func runTest(cmd *base.Command, args []string) {
 					haveMatch = true
 				}
 			}
+
+			// Silently ignore attempts to run coverage on
+			// sync/atomic when using atomic coverage mode.
+			// Atomic coverage mode uses sync/atomic, so
+			// we can't also do coverage on it.
+			if testCoverMode == "atomic" && p.Standard && p.ImportPath == "sync/atomic" {
+				continue
+			}
+
 			if haveMatch {
 				testCoverPkgs = append(testCoverPkgs, p)
 			}