cmd/go: make the default coverage mode -atomic if -race is set

Fixes #7013.

LGTM=adg
R=golang-codereviews, gobot, adg
CC=golang-codereviews
https://golang.org/cl/76370043

src/cmd/go/doc.go

@@ -802,7 +802,8 @@ control the execution of any test:
 
 	-covermode set,count,atomic
 	    Set the mode for coverage analysis for the package[s]
-	    being tested. The default is "set".
+	    being tested. The default is "set" unless -race is enabled,
+	    in which case it is "atomic".
 	    The values:
 		set: bool: does this statement run?
 		count: int: how many times does this statement run?

src/cmd/go/test.bash

@@ -568,6 +568,37 @@ TEST coverage runs
 ./testgo test -short -coverpkg=strings strings regexp || ok=false
 ./testgo test -short -cover strings math regexp || ok=false
 
+# Check that coverage analysis uses set mode.
+TEST coverage uses set mode
+if ./testgo test -short -coverpkg=encoding/binary -coverprofile=testdata/cover.out; then
+	if ! grep -q 'mode: set' testdata/cover.out; then
+		ok=false
+	fi
+else
+	ok=false
+fi
+rm -f testdata/cover.out
+
+TEST coverage uses atomic mode for -race.
+if ./testgo test -short -race -coverpkg=encoding/binary -coverprofile=testdata/cover.out; then
+	if ! grep -q 'mode: atomic' testdata/cover.out; then
+		ok=false
+	fi
+else
+	ok=false
+fi
+rm -f testdata/cover.out
+
+TEST coverage uses actual setting to override even for -race.
+if ./testgo test -short -race -coverpkg=encoding/binary -covermode=count -coverprofile=testdata/cover.out; then
+	if ! grep -q 'mode: count' testdata/cover.out; then
+		ok=false
+	fi
+else
+	ok=false
+fi
+rm -f testdata/cover.out
+
 TEST coverage with cgo
 d=$(TMPDIR=/var/tmp mktemp -d -t testgoXXX)
 ./testgo test -short -cover ./testdata/cgocover >$d/cgo.out 2>&1 || ok=false

src/cmd/go/test.go

@@ -137,7 +137,8 @@ control the execution of any test:
 
 	-covermode set,count,atomic
 	    Set the mode for coverage analysis for the package[s]
-	    being tested. The default is "set".
+	    being tested. The default is "set" unless -race is enabled,
+	    in which case it is "atomic".
 	    The values:
 		set: bool: does this statement run?
 		count: int: how many times does this statement run?

src/cmd/go/testflag.go

@@ -117,7 +117,6 @@ var testFlagDefn = []*testFlagSpec{
 func testFlags(args []string) (packageNames, passToTest []string) {
 	inPkg := false
 	outputDir := ""
-	testCoverMode = "set"
 	for i := 0; i < len(args); i++ {
 		if !strings.HasPrefix(args[i], "-") {
 			if !inPkg && packageNames == nil {
@@ -218,6 +217,14 @@ func testFlags(args []string) (packageNames, passToTest []string) {
 		}
 	}
 
+	if testCoverMode == "" {
+		testCoverMode = "set"
+		if buildRace {
+			// Default coverage mode is atomic when -race is set.
+			testCoverMode = "atomic"
+		}
+	}
+
 	// Tell the test what directory we're running in, so it can write the profiles there.
 	if testProfile && outputDir == "" {
 		dir, err := os.Getwd()