cmd/cover: check that the argument of -var is valid

At the moment, the cover tool does not check that the argument of -var
is a valid identifier. Hence, it could generate a file that fails to
compile afterwards.

Updates #25280

Change-Id: I6eb1872736377680900a18a4a28ba002ab5ea8ca
Reviewed-on: https://go-review.googlesource.com/c/120316
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/cover/cover.go

@@ -16,6 +16,7 @@ import (
 	"log"
 	"os"
 	"sort"
+	"unicode"
 
 	"cmd/internal/edit"
 	"cmd/internal/objabi"
@@ -116,6 +117,10 @@ func parseFlags() error {
 		return fmt.Errorf("too many options")
 	}
 
+	if *varVar != "" && !isValidIdentifier(*varVar) {
+		return fmt.Errorf("argument of -var is not a valid identifier: %v", *varVar)
+	}
+
 	if *mode != "" {
 		switch *mode {
 		case "set":
@@ -676,3 +681,14 @@ func (f *File) addVariables(w io.Writer) {
 		fmt.Fprintf(w, "var _ = %s.LoadUint32\n", atomicPackageName)
 	}
 }
+
+func isValidIdentifier(ident string) bool {
+	first := true
+	for _, c := range ident {
+		if !unicode.IsLetter(c) && c != '_' && (first || !unicode.IsDigit(c)) {
+			return false // invalid identifier
+		}
+		first = false
+	}
+	return true
+}

src/cmd/cover/cover_test.go

@@ -145,6 +145,12 @@ func TestCover(t *testing.T) {
 	cmd := exec.Command(testcover, "-mode=count", "-var=thisNameMustBeVeryLongToCauseOverflowOfCounterIncrementStatementOntoNextLineForTest", "-o", coverOutput, coverInput)
 	run(cmd, t)
 
+	cmd = exec.Command(testcover, "-mode=set", "-var=Not_an-identifier", "-o", coverOutput, coverInput)
+	err = cmd.Run()
+	if err == nil {
+		t.Error("Expected cover to fail with an error")
+	}
+
 	// Copy testmain to testTempDir, so that it is in the same directory
 	// as coverOutput.
 	b, err := ioutil.ReadFile(testMain)