Re: RFR: JDK-8169718: nsk/jdb/locals/locals002: ERROR: Cannot find boolVar with expected value: false

[Chris Plummer]

On 6/6/18 8:51 AM, Daniel D. Daugherty wrote:
The two prompts are for two different things so I don't think
that removing one of the prompts is the right solution either.
In the case of an asynchronous command like 'cont', you get
the '<prompt=after-cont-command>' right away because the command
processor is ready for another command. You will also get the
'<prompt-after-breakpoint-event-location1>' prompt when the
breakpoint is hit because that identifies the thread that posted
the breakpoint event. If it takes a long time to hit the breakpoint,
then it will be a long time before the breakpoint prompt is seen.
Likewise, if the breakpoint is hit quickly, then the breakpoint
prompt will be seen more quickly and will sometimes interfere with
the command processor prompt. That's normal for an async command.

That was the point I was trying to make. I also felt that printing both 
prompts was appropriate.
One possible way to fix this kind of issue is to structure the
test flow to slightly delay the target thread so it takes longer
to get to the breakpoint. '<prompt=after-cont-command>' will have
a better chance to be printed before (and not colliding with) the
'<prompt-after-breakpoint-event-location1>' prompt.

So if the test program has:

    <some line of code>;  // breakpoint here

change it to something like:

    try { Thread.sleep(100); } catch (InterruptedException ie) {};
    <some line of code>;  // breakpoint here

Of course, I'm assuming here that the test driver code is looking
for the breakpoint to be hit (and not for '<prompt=after-cont-command>').
Jerry and I did find a few jdb based tests looking for the wrong 
prompt...
The issue is really with the delivery of the first prompt. It can be 
intermingled with the output from the breakpoint. Yes, your solution 
might work most of the time (still not 100% guaranteed) when dealing 
with breakpoints, but is not a solution when dealing with this same 
problem occurring while single stepping.

My suggestion was to have processing of breakpoints and single stepping 
events block until executeCommand() completed and had printed the first 
prompt. I'm not sure if this is feasible or practical.

thanks,

Chris

Dan


On 6/2/18 7:02 AM, gary.ad...@oracle.com wrote:
On 6/1/18 6:54 PM, Chris Plummer wrote:
Hi Gary,

I'm a bit unclear on one point. According to your description, 
should we be seeing two prompts for commands like "cont", and 
"step"? When you say we are expecting:

  "Breakpoint hit:" <location>
   <prompt>

Isn't it really:

   <prompt>
  "Breakpoint hit:" <location>
   <prompt>
Let me stretch out the log a little bit
The <prompt> is main[1] or Thread-1[1], etc.
and not distinguished for command or event completion.

<prompt>
   stop at <location1>
<prompt-after-stop-command>
   stop at <location2>
<prompt-after-stop-command>
   cont
<prompt=after-cont-command>
   cont
   "Breakpoint hit:" <location1>
<prompt-after-cont-command>
  "Breakpoint hit:"
<prompt-after-breakpoint-event-location1>
  <location2>
<prompt-after-breakpoint-event-location2>

The source of the command or event prompts
was verified with a stacktrace in the printPrompt()
method.

I figure this must be the case, because your fix removes the 
printing of a prompt, and we still need one prompt, so there must 
have been two to start with. I think in the above the last <prompt> 
is being produced sequentially right after the "Breakpoint hit:" 
output, so is never an issue. The first <prompt> comes from the 
printPrompt() call after issuing the "cont" command, and it's 
possible to hit the breakpoint and start producing more output 
before printPrompt() has completed, thus causing the problem you are 
seeing.
printPrompt() is a very quick operation and is not being interrupted.

But the output from the event handler is produced incrementally.
   - event header, e.g. "Breakpont hit:", "Step completed:", etc.
   - from vmInterrupted()
     - printCurrentLocation()
     - monitor command output (if any)
     - printPrompt()

There seems to be some purpose to printing this first prompt and I'm 
not so sure eliminating it is the right thing to do. I think it 
might be there to indicate that the command has been initiated, 
because there could be some delay before the breakpoint is hit. So 
the issue then is how to make sure this prompt appears in the input 
stream before any of the "Step completed:" or "Breakpoint hit:" 
output. Possibly event processing should be blocked until 
executeCommand() completes.
The primary purposes of prompts is for "user interactive" nudge to 
type another command.
But the failures are from tests that are using the presence of a 
prompt as an ACK of the
last command. This "prompt-protocol", does not include any sequence 
numbers ,
does not pair prompt to command or event, and does not block during
any multipart message formation.

To prevent the error we are observing we would have to buffer all of 
the event output

   "Breakpoint hit:" <location> <monitored-commands>
    <prompt for breakpoint event>

Another way to look at the current failure is to say the event 
handler output
has been interrupted by a command sent prematurely. This would never
be seen at user speeds typing in commands.

I should also mention any other output from the user application can 
also
interfere with the command and event output streams. I have seen
stray prompts in the logs from System.out.println(). That was the
reason earlier I was looking at changing the tests to look for
specific prompts before continuing to the next command.

\gra
thanks,

Chris

On 5/26/18 3:50 AM, gary.ad...@oracle.com wrote:
This is a review request for a previously closed test bug.
The test was recently moved to the open repos, and the
proposed fix is in the open code.

  Webrev: http://cr.openjdk.java.net/~gadams/8169718/webrev/


-------- Forwarded Message --------
Subject:     RFR: JDK-8169718: nsk/jdb/locals/locals002: ERROR: 
Cannot find boolVar with expected value: false
Date:     Fri, 25 May 2018 11:35:10 -0400
From:     Gary Adams <gary.ad...@oracle.com>
Reply-To:     gary.ad...@oracle.com





The jdb tests use stdin to send commands to a jdb process
and parses the stdout to determine if a command was
successful and when the process is prompting for new commands
to be sent.

Some commands are synchronous, so when the command is completed
a new prompt is sent back immediately.

Some commands are asynchronous, so there could be a delay
until a breakpoint is reached. The event handler then sends a prompt
when the application thread is stopped and new jdb commands can be 
sent.

The problem causing the intermittent failures was a corruption in the
output stream when prompts were being sent at the wrong times.

Instead of receiving
   "Breakpoint hit:" <location>
    <prompt>

the log contained
   "Breakpoint hit:" <prompt> <location>

Once out of sync, jdb commands were being sent prematurely
and the wrong values were being compared against expected behavior.
The simple fix proposed here recognizes that commands like "cont",
"step" and "next" are asynchronous commands and should not send back
a prompt immediately. Instead. the event handler will deliver the 
next prompt
when the next "Breakpoint hit:" or "Step completed:" state change 
occurs.

The bulk of the testing was done on windows-x64-debug builds where the
intermittent failures were observed in ~5 in 1000 testruns. The fix 
has
also been tested on linux-x64-debug, solaris-sparcv9-debug,
and macosx-x64-debug, even though the failures have never been 
reported
against those platforms.

Failures have been observed in many of the nsk/jdb tests with 
similar corrupted
output streams, but never directly associated with this issue before.

  redefine001, caught_exception002, locals002, eval001, next001,
  stop_at003, step002, print002, trace001, step_up001, read001,
  clear004, kill001, set001