put more channel code behind boundary

Author: timfel

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/modules/PosixModuleBuiltins.java

@@ -846,7 +846,7 @@ abstract static class ReadFromChannelNode extends PNodeWithContext {
         ByteSequenceStorage readSeekable(SeekableByteChannel channel, int size) {
             long availableSize;
             try {
-                availableSize = channel.size() - channel.position();
+                availableSize = availableSize(channel);
             } catch (IOException e) {
                 gotException.enter();
                 throw raise(OSError, e.getMessage());
@@ -858,6 +858,11 @@ ByteSequenceStorage readSeekable(SeekableByteChannel channel, int size) {
             return readReadable(channel, sz);
         }
 
+        @TruffleBoundary(transferToInterpreterOnException = false)
+        private static long availableSize(SeekableByteChannel channel) throws IOException {
+            return channel.size() - channel.position();
+        }
+
         @Specialization
         ByteSequenceStorage readReadable(ReadableByteChannel channel, int size) {
             int sz = Math.min(size, ReadNode.MAX_READ);