RFR JDK-8167461: jshell tool: Scanner#next() hangs tool

[Jan Lahoda]

Hello,

Bug:
https://bugs.openjdk.java.net/browse/JDK-8167461

The problem is that this sequence of snippets is not working properly 
(won't read the System.in properly/seemingly hangs):
jshell> Scanner input = new Scanner(System.in);
jshell> int x = input.nextInt();

The reason is that the PipeInputStream (which is used to pass data 
received by the remote agent from the socket to System.in) does not 
override the read(byte[], int, int) method. So the default 
implementation will be used, which waits until all the requested data 
are read. Scanner wrap s the System.in with InputStreamReader, and that 
appears to read (up to) 8192 bytes into a buffer, but the user typically 
does not type so many characters. The solution is to override the 
read(byte[], int, int) method, and return only bytes we have buffered 
(except the method will wait for the first byte).

Webrev:
http://cr.openjdk.java.net/~jlahoda/8167461/webrev.00/

Also, more general question related to the System.in handling: unused 
input for one snippet is not passed to following snippets - is that OK, 
or do we need to discuss changing that? I.e.:
jshell> int x = input.nextInt();
1 1
x ==> 1
//the other "1" is thrown away

jshell> int x = input.nextInt();
2
x ==> 2
//the user had to type additional input.

Thanks,
     Jan