cmd/link/internal/ld: don't leave files open in a loop

Noticed by Ingo Oeser in his review of CL 168321.

Change-Id: I2f39db675a7c22b395062a11903657a9d0d1956d
Reviewed-on: https://go-review.googlesource.com/c/go/+/168560
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/link/internal/ld/macho.go

@@ -694,14 +694,20 @@ func Asmbmacho(ctxt *Link) {
 			}
 		}
 	}
-	load, err := hostobjMachoPlatform(hostobj)
+	foundLoad := false
+	for _, h := range hostobj {
+		load, err := hostobjMachoPlatform(&h)
 		if err != nil {
 			Exitf("%v", err)
 		}
 		if load != nil {
 			ml := newMachoLoad(ctxt.Arch, load.cmd.type_, uint32(len(load.cmd.data)))
 			copy(ml.data, load.cmd.data)
-	} else if ctxt.LinkMode == LinkInternal {
+			foundLoad = true
+			break
+		}
+	}
+	if !foundLoad && ctxt.LinkMode == LinkInternal {
 		// For lldb, must say LC_VERSION_MIN_MACOSX or else
 		// it won't know that this Mach-O binary is from OS X
 		// (could be iOS or WatchOS instead).
@@ -1027,9 +1033,8 @@ func Machoemitreloc(ctxt *Link) {
 }
 
 // hostobjMachoPlatform returns the first platform load command found
-// in the host objects, if any.
-func hostobjMachoPlatform(hostobj []Hostobj) (*MachoPlatformLoad, error) {
-	for _, h := range hostobj {
+// in the host object, if any.
+func hostobjMachoPlatform(h *Hostobj) (*MachoPlatformLoad, error) {
 	f, err := os.Open(h.file)
 	if err != nil {
 		return nil, fmt.Errorf("%s: failed to open host object: %v\n", h.file, err)
@@ -1041,15 +1046,7 @@ func hostobjMachoPlatform(hostobj []Hostobj) (*MachoPlatformLoad, error) {
 		// Not a valid Mach-O file.
 		return nil, nil
 	}
-		load, err := peekMachoPlatform(m)
-		if err != nil {
-			return nil, err
-		}
-		if load != nil {
-			return load, nil
-		}
-	}
-	return nil, nil
+	return peekMachoPlatform(m)
 }
 
 // peekMachoPlatform returns the first LC_VERSION_MIN_* or LC_BUILD_VERSION