RFR: JDK-8154144 Tests in com/sun/jdi fails intermittently with "jdb input stream closed prematurely

[Daniel D. Daugherty]

 > When JDB is done, as part of the shutdown it calls System.exit()
 > at Env.java:92 in a different thread 'event-handler'.

Based on the above line from below, I'm even more convinced that
this bug is hit because the debuggee VM is allowed to run off the
end of main() and that creates a race between debuggee VM's attempt
to exit and the debugger trying to control the test execution.

While this fix makes this bug "go away", I don't think this is the
right solution. You're basically trying to harden our ability to
handle this race condition instead of closing the race condition.

I'll repeat part of the comment that I added to the bug on 04.13:

 > One possible solution is to add a breakpoint after main() returns and 
change
 > the debuggee <-> debugger protocol to always expect one final breakpoint.
 > Positive enforcement that the test reaches that point and it should 
catch any
 > accidental "run of the end of main()" issues.

Dan


On 4/28/16 2:25 AM, Sharath Ballal wrote:
>
> Hi Staffan,
>
> The root cause of this problem is that BufferedReader.readLine() is 
> intermittently returning 'null' during System.exit(0).
>
> In TTY.java:751 we are always blocking on readLine(). Whenever a user 
> enters a command in JDB, the readLine() returns the command, which 
> gets executed. This is running in the 'main' thread.
>
> When JDB is done, as part of the shutdown it calls System.exit() at 
> Env.java:92 in a different thread 'event-handler'.
>
> Usually (in the passing cases) calling System.exit() is the end of the 
> process and readLine() doesn't return 'null', but in the failing case 
> readLine() at TTY.java:751 returns 'null'. This causes the string 
> "Input stream closed." to be printed. The jtreg testcase looks for 
> this string to decide that the testcase failed.
>
> The fix I have done is avoiding the print because we know that we are 
> shutting down and hence can discard the ‘null’ returned by readLine().
>
> Regarding the testing, I have run few of the failing jtreg testcases 
> about 200 times and not seen the problem (earlier I was able to 
> recreate without the fix).  I have to do other tests, but sent for 
> review to do it in parallel.
>
> jdk/src/jdk.jdi/share/classes/com/sun/tools/example/debug/tty/TTY.java
>
> 708 BufferedReader in =
> 709 new BufferedReader(new InputStreamReader(System.in));
> .......
> .......
> 748 // Process interactive commands.
> 749 MessageOutput.printPrompt();
> 750 while (true) {
> 751 String ln = in.readLine();
> 752 if (ln == null) {
> 753 MessageOutput.println("Input stream closed.");
> 754 ln = "quit";
> 755 }
>
> jdk/src/jdk.jdi/share/classes/com/sun/tools/example/debug/tty/Env.java
>
> 79 static void shutdown(String message) {
> 80 if (connection != null) {
> 81 try {
> 82 connection.disposeVM();
> 83 } catch (VMDisconnectedException e) {
> 84 // Shutting down after the VM has gone away. This is
> 85 // not an error, and we just ignore it.
> 86 }
> 87 }
> 88 if (message != null) {
> 89 MessageOutput.lnprint(message);
> 90 MessageOutput.println();
> 91 }
> 92 System.exit(0);
> 93 }
>
> -Sharath Ballal
>
> *From:*Staffan Larsen
> *Sent:* Thursday, April 28, 2016 1:17 PM
> *To:* Sharath Ballal
> *Cc:* serviceability-dev at openjdk.java.net
> *Subject:* Re: RFR: JDK-8154144 Tests in com/sun/jdi fails 
> intermittently with "jdb input stream closed prematurely
>
> Hi Sharath,
>
> Can you explain more how this help with the problem in the bug?
>
> It looks like you are trying to avoid a race by not printing the 
> "Input stream closed.” message while shutting down. You added this:
>
> *136             ((TTY)notifier).setShuttingDown(true);*
>   137             Env.shutdown(shutdownMessageKey);
>
> The call on line 137 will result in a System.exit(0) call if I am 
> reading the code right. So adding the shutdown flag to line 136 will 
> maybe make the race smaller, but isn’t really solving it?
>
> What kind of testing have you run this fix through?
>
> Thanks,
>
> /Staffan
>
>     On 28 apr. 2016, at 09:22, Sharath Ballal
>     <sharath.ballal at oracle.com <mailto:sharath.ballal at oracle.com>> wrote:
>
>     Hi,
>
>     Pls review the change for bug
>
>     JDK-8154144 <https://bugs.openjdk.java.net/browse/JDK-8154144>-
>     Tests in com/sun/jdi fails intermittently with "jdb input stream
>     closed prematurely
>
>     Webrev:
>
>     http://cr.openjdk.java.net/~sballal/8154144/webrev.00/
>     <http://cr.openjdk.java.net/%7Esballal/8154144/webrev.00/>
>
>     -Sharath Ballal
>

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