unix: don't allocate putting common errno values in error interfaces

From main repo: https://go-review.googlesource.com/#/c/6701/3

Fixes golang/go#10266

Change-Id: I141fefe7f317edcea19b588b1569fa8772237f56
Reviewed-on: https://go-review.googlesource.com/8190Reviewed-by: Rob Pike <r@golang.org>

unix/mksyscall.pl

@@ -284,7 +284,7 @@ while(<>) {
 		$text .= "\t}\n";
 	} elsif ($do_errno) {
 		$text .= "\tif e1 != 0 {\n";
-		$text .= "\t\terr = e1\n";
+		$text .= "\t\terr = errnoErr(e1)\n";
 		$text .= "\t}\n";
 	}
 	$text .= "\treturn\n";

unix/syscall_unix.go

@@ -25,6 +25,30 @@ const (
 	netbsd32Bit    = runtime.GOOS == "netbsd" && sizeofPtr == 4
 )
 
+// Do the interface allocations only once for common
+// Errno values.
+var (
+	errEAGAIN error = syscall.EAGAIN
+	errEINVAL error = syscall.EINVAL
+	errENOENT error = syscall.ENOENT
+)
+
+// errnoErr returns common boxed Errno values, to prevent
+// allocations at runtime.
+func errnoErr(e syscall.Errno) error {
+	switch e {
+	case 0:
+		return nil
+	case EAGAIN:
+		return errEAGAIN
+	case EINVAL:
+		return errEINVAL
+	case ENOENT:
+		return errENOENT
+	}
+	return e
+}
+
 func Syscall(trap, a1, a2, a3 uintptr) (r1, r2 uintptr, err syscall.Errno)
 func Syscall6(trap, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2 uintptr, err syscall.Errno)
 func RawSyscall(trap, a1, a2, a3 uintptr) (r1, r2 uintptr, err syscall.Errno)