go/loader: fix fd leak and limit calls to ioutil.ReadDir

This makes 'ulimit -n 128; go test -short -v' pass.
It did not before, and that was breaking the openbsd/386 builder.

For golang/go#11811.

Change-Id: Idfdb2f4007ed06c6084486c0e58a561add552d2c
Reviewed-on: https://go-review.googlesource.com/13695
Reviewed-by: Robert Griesemer <gri@golang.org>

go/loader/cgo.go

@@ -87,9 +87,9 @@ func processCgoFiles(bp *build.Package, fset *token.FileSet, DisplayPath func(pa
 		if err != nil {
 			return nil, err
 		}
-		defer rd.Close()
 		display := filepath.Join(bp.Dir, cgoDisplayFiles[i])
 		f, err := parser.ParseFile(fset, display, rd, mode)
+		rd.Close()
 		if err != nil {
 			return nil, err
 		}

go/loader/loader.go

@@ -449,7 +449,9 @@ func (conf *Config) Load() (*Program, error) {
 		conf.FindPackage = func(ctxt *build.Context, path string) (*build.Package, error) {
 			// TODO(adonovan): cache calls to build.Import
 			// so we don't do it three times per test package.
+			ioLimit <- true
 			bp, err := ctxt.Import(path, conf.Cwd, 0)
+			<-ioLimit
 			if _, ok := err.(*build.NoGoError); ok {
 				return bp, nil // empty directory is not an error
 			}

go/loader/util.go

@@ -19,7 +19,7 @@ import (
 
 // We use a counting semaphore to limit
 // the number of parallel I/O calls per process.
-var sema = make(chan bool, 10)
+var ioLimit = make(chan bool, 10)
 
 // parseFiles parses the Go source files within directory dir and
 // returns the ASTs of the ones that could be at least partially parsed,
@@ -42,10 +42,10 @@ func parseFiles(fset *token.FileSet, ctxt *build.Context, displayPath func(strin
 		}
 		wg.Add(1)
 		go func(i int, file string) {
-			sema <- true // wait
+			ioLimit <- true // wait
 			defer func() {
 				wg.Done()
-				<-sema // signal
+				<-ioLimit // signal
 			}()
 			var rd io.ReadCloser
 			var err error