gofix: be more conservative about rewrite to os.Create

Rewrite only if we understood all the flags we saw.

R=r
CC=golang-dev
https://golang.org/cl/4376046

src/cmd/gofix/osopen.go

diff --git a/src/cmd/gofix/osopen.go b/src/cmd/gofix/osopen.go
index 5d7f5cb..49993d8 100644
--- a/src/cmd/gofix/osopen.go
+++ b/src/cmd/gofix/osopen.go
@@ -70,7 +70,6 @@
 func isCreateFlag(flag ast.Expr) bool {
 	foundCreate := false
 	foundTrunc := false
-	foundAppend := false
 	// OR'ing of flags: is O_CREATE on?  + or | would be fine; we just look for os.O_CREATE
 	// and don't worry about the actual opeator.
 	p := flag.Pos()
@@ -80,14 +79,21 @@
 		if isBinary {
 			lhs = expr.Y
 		}
-		if isPkgDot(lhs, "os", "O_CREATE") {
+		sel, ok := lhs.(*ast.SelectorExpr)
+		if !ok || !isTopName(sel.X, "os") {
+			return false
+		}
+		switch sel.Sel.Name {
+		case "O_CREATE":
 			foundCreate = true
-		}
-		if isPkgDot(lhs, "os", "O_TRUNC") {
+		case "O_TRUNC":
 			foundTrunc = true
-		}
-		if isPkgDot(lhs, "os", "O_APPEND") {
-			foundAppend = true
+		case "O_RDONLY", "O_WRONLY", "O_RDWR":
+			// okay 
+		default:
+			// Unexpected flag, like O_APPEND or O_EXCL.
+			// Be conservative and do not rewrite.
+			return false
 		}
 		if !isBinary {
 			break
@@ -97,9 +103,6 @@
 	if !foundCreate {
 		return false
 	}
-	if foundAppend {
-		return false
-	}
 	if !foundTrunc {
 		warn(p, "rewrote os.Open with O_CREATE but not O_TRUNC to os.Create")
 	}

src/cmd/gofix/osopen_test.go

diff --git a/src/cmd/gofix/osopen_test.go b/src/cmd/gofix/osopen_test.go
index b0a4f63..43ddd1a 100644
--- a/src/cmd/gofix/osopen_test.go
+++ b/src/cmd/gofix/osopen_test.go
@@ -28,6 +28,8 @@
 	os.Open(a, os.O_CREATE|os.O_TRUNC, 0664)
 	os.Open(a, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0666)
 	os.Open(a, os.O_WRONLY|os.O_CREATE|os.O_APPEND, 0666)
+	os.Open(a, os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0666)
+	os.Open(a, os.O_SURPRISE|os.O_CREATE, 0666)
 	_ = os.O_CREAT
 }
 `,
@@ -48,6 +50,8 @@
 	os.Create(a)
 	os.Create(a)
 	os.OpenFile(a, os.O_WRONLY|os.O_CREATE|os.O_APPEND, 0666)
+	os.OpenFile(a, os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0666)
+	os.OpenFile(a, os.O_SURPRISE|os.O_CREATE, 0666)
 	_ = os.O_CREATE
 }
 `,