vfs: bring open_tests.go generator back into line with the generated tests

In

54deb01f00 vfs: Make OpenFile and friends return EINVAL if O_RDONLY and O_TRUNC

The generated file open_test.go was edited directly without editing
the generator.

This commit brings the generator make_open_tests.go back into line
with that edit. It also makes it so `go generate` can be used to
regenerate the tests.

vfs/make_open_tests.go

@@ -1,6 +1,11 @@
-// This makes the open test suite
+// This makes the open test suite. It tries to open a file (existing
+// or not existing) with all possible file modes and writes a test
+// matrix.
 //
-// Run with go run make_open_tests.go | gofmt > open_test.go
+// The behaviour is as run on Linux, with the small modification that
+// O_TRUNC with O_RDONLY does **not** truncate the file.
+//
+// Run with go generate (defined in vfs.go)
 //
 //+build none
 
@@ -26,6 +31,8 @@ func whichError(err error) string {
 		return "nil"
 	case io.EOF:
 		return "io.EOF"
+	case os.ErrInvalid:
+		return "EINVAL"
 	}
 	s := err.Error()
 	switch {
@@ -40,6 +47,8 @@ func whichError(err error) string {
 	return ""
 }
 
+const accessModeMask = (os.O_RDONLY | os.O_WRONLY | os.O_RDWR)
+
 // test Opening, reading and writing the file handle with the flags given
 func test(fileName string, flags int, mode string) {
 	// first try with file not existing
@@ -123,6 +132,19 @@ func test(fileName string, flags int, mode string) {
 		log.Fatalf("failed to remove: %v", err)
 	}
 
+	// http://pubs.opengroup.org/onlinepubs/7908799/xsh/open.html
+	// The result of using O_TRUNC with O_RDONLY is undefined.
+	// Linux seems to truncate the file, but we prefer to return EINVAL
+	if (flags&accessModeMask) == os.O_RDONLY && flags&os.O_TRUNC != 0 {
+		openNonExistentErr = os.ErrInvalid // EINVAL
+		readNonExistentErr = nil
+		writeNonExistentErr = nil
+		openExistingErr = os.ErrInvalid // EINVAL
+		readExistingErr = nil
+		writeExistingErr = nil
+		contents = "hello"
+	}
+
 	// output the struct
 	fmt.Printf(`{
 	flags: %s,
@@ -147,7 +169,7 @@ func test(fileName string, flags int, mode string) {
 }
 
 func main() {
-	fmt.Printf(`// data generated by go run make_open_tests.go | gofmt > open_test.go
+	fmt.Printf(`// AUTO GENERATED by make_open_tests.go - DO NOT EDIT - use go generate to rebuild
 
 package vfs
 

vfs/open_test.go

@@ -1,4 +1,4 @@
-// data generated by go run make_open_tests.go | gofmt > open_test.go
+// AUTO GENERATED by make_open_tests.go - DO NOT EDIT - use go generate to rebuild
 
 package vfs
 

vfs/vfs.go

@@ -16,6 +16,9 @@
 // error conditions have ocurred.  It may also return general errors
 // it receives.  It tries to use os Error values (eg os.ErrExist)
 // where possible.
+
+//go:generate sh -c "go run make_open_tests.go | gofmt > open_test.go"
+
 package vfs
 
 import (