On 9/21/2015 12:19 PM, Semyon Sadetsky wrote:
On 9/21/2015 11:27 AM, Alexandr Scherbatiy wrote:
18.09.2015 19:22, Semyon Sadetsky пишет:
On 9/18/2015 6:51 PM, Alexander Scherbatiy wrote:
On 9/16/2015 5:14 PM, Semyon Sadetsky wrote:
On 9/16/2015 1:38 PM, Alexander Scherbatiy wrote:
On 9/15/2015 10:37 PM, Semyon Sadetsky wrote:
On 9/15/2015 6:16 PM, Alexander Scherbatiy wrote:
On 9/11/2015 7:24 PM, Semyon Sadetsky wrote:
The deadlock that you described exists only because
AbstractDocument uses locks in the UndoableEdit.undo()/redo()
methods.
Applications that use UndoManager and do not use lock in the
UndoableEdit.undo()/redo() methods do not have deadlock. They
worked fine before the fix and can lost data consistency after
the fix. This is definitely the regression.
You mean scenario when a document that does not support
synchronization but anyway is modified from several threads.
You can expect that such scenario is functional only if such
document is a single atomic field.
I updated the fix
http://cr.openjdk.java.net/~ssadetsky/8030702/webrev.04/ take it
into account.
This looks better. There are just some comments:
- The 'inProgress' variable in
UndoManager.undo()/redo()/undoOrRedo() methods should have
synchronization.
Is it possible to move 'if (inProgress)' check into
tryUndoOrRedo() method similarly to as it was used in the version
2 of the fix?
- UndoManager line 489: why not to use the original check from
the undoOrRedo() method "if (indexOfNextAdd == edits.size())" to
pick up undo or redo action?
- UndoManager line 516: An undoable edit can be requested two
times for the ANY action because the 'undo' variable can have old
value in the second synchronized block.
Even the logic is right it is better to take an edit based
on the 'action' variable.
- UndoManager.undoOrRedo() throws CannotUndoException now
instead of CannotRedoException if the redo action is not possible.
- It is possible to get rid of the 'done ' variable in
UndoManager.tryUndoOrRedo() just simply have an infinity loop.
- It is possible to use Boolean values TRUE, FALSE or null for
three-state logic. But it is up to you to choose enums or Boolean
in the fix.
I made the changes:
http://cr.openjdk.java.net/~ssadetsky/8030702/webrev.05/ except
for the last one. Actually triple Boolean logic is a bad style.
There are two main synchronized block in the tryUndoOrRedo()
method: one to look up an undoable edit lock and the second which
use the undoable edit lock.
In the version 02 of the fix the original code from undo() and
redo() methods were moved to these two blocks.
It is not clear why don't you want to do the same (just call
tryUndoOrRedo() with necessary argument for all
UndoManager.undo()/redo()/undoOrRedo() methods) in the latest fix?
Splitting logic like:
-------------------
420 public void undo() throws CannotUndoException {
421 if (!tryUndoOrRedo(Action.UNDO)) {
422 synchronized (this) {
423 super.undo();
424 }
425 }
426 }
-------------------
always have a question that before the fix the super.undo() was
called only for '!inProgress' condition but now the 'inProgress'
can be changed when super.undo() is called.
inProgress can be changed when super.undo() is called because it is
protected by synchronized block.
Imagine that undo() is called with inProgress = true, then it is
blocked by the concurrent document change, and after the retry it
finds inProgress= false, so it cannot undo by usual way anymore,
because the UndoManager is converted into a single edit and it
should undo using super.undo().
I am talking about slightly different thing.
This is the code for the undo() method before the fix.
-------------------
410 public synchronized void undo() throws CannotUndoException {
411 if (inProgress) {
412 UndoableEdit edit = editToBeUndone();
...
416 undoTo(edit);
417 } else {
418 super.undo();
419 }
420 }
-------------------
Checking the 'inProgress' variable and calling undoTo(edit) and
super.undo() is done under the same synchronized block.
Let's slightly modify the code:
-------------------
public void undo() throws CannotUndoException {
boolean res = true;
synchronized (this) {
if (inProgress) {
UndoableEdit edit = editToBeUndone();
...
undoTo(edit);
It is not possible to execute undoTo() here because it will cause the
deadlock we are trying to fix.
May be with the updated sample it would be cleaner.
This is just a some method which uses a synchronization:
----------------------
public synchronized void someMethod() {
if (inProgress) {
// do something
} else {
// do something else
}
}
----------------------
This is an updated method:
-------------------
public void someMethod() {
boolean res = true;
[some synchronization]
if (inProgress) {
// do something
} else {
res = false;
}
[end of some synchronization]
[some synchronization]
if (!res) {
do something else
}
[end of some synchronization]
}
-------------------
The problem with the updated method is that inProgress variable can
have a value different from 'res' and the 'do something else' action can
be executed even inProgress is true.
} else {
res = false;
}
}
synchronized (this) {
if (!res) {
super.undo();
}
}
}
-------------------
Now the 'inProgress' variable checking and undoTo() is done on the
first synchronized block.
The result of the inProgress is used in the second synchronized
block. But the 'inProgress' variable
can be changed between these two blocks and we can't relay on the
'res' value.
Instead of 'res' the original 'inProgress' value should be checked
in the second synchronized block and if it is true the undoTo(edit)
should be called again instead of super.undo().
Above you provided the code logic bore the fix which is reworked
because it does not work. Why do we need to discuss it?
We need to discuss this because the inProgress variable in your
latest fix is used exactly as in the provided sample.
UndoManager line: 473 tryUndoOrRedo(action) returns false if
inProgress is false.
UndoManager line: 422 super.undo() is called if
tryUndoOrRedo(action) returns false despite the real 'inProgress'
variable value.
Thanks,
Alexandr.
Thanks,
Alexandr.
Thanks,
Alexandr.
Thanks,
Alexandr.
Thanks,
Alexandr.
Thanks,
Alexandr.
Thanks,
Alexandr.
Also synchronization is needed to provide coherency with
the state of the document. If those two are not provided
the document can be corrupted. This is enough most usages.
Thanks,
Alexandr.
Thanks,
Alexandr.
Thanks,
Alexandr.
Thanks,
Alexandr.
It is
I still think that updating the
UndoableManager for one particular
AbstarctManager class can be made only
after investigation of other possibilities.
You could start with choosing behavior
which you want to achieve or to keep, like:
- fix the deadlock
- atomic undo() method
- serialization
- immediate roll back action
- abstract document consistency after
undo() action
- ...
We need to pay attention to the deadlock
at first of cause.
Serialization and consistency are
achieved. Any concrete doubts?
immediate roll back action -- ?what is that?
"if user starts a long edit operation
and press undo after that he expects when
the long edit is finished it will be rolled
back immediately." - what ever does it mean.
Got it. It will work within the fairness. We
have discussed this allready.
I sacrificed undo/redo call atomicity
because you did not like doc references in
undo manager. I think it is not important
for the most multithreaded undo/redo
scenarios.
Could you give more details about it.
Which doc references do you mean?
Your statement a dozen iterations ago was:
"There should be a way to solve these
problems without storing links of external
classes inside the UndoManager."
I guess you used "link" term for references.
I would recommend to use standard
terminology: reference to the object,
dependency on the class, etc... to avoid
misunderstanding.
Usually "link" is in a browser document or a
tool that produces executables after
compilation.
and look which of the following
approaches can better solve them (where
the fist is more preferred and the last
is less preferred case):
- using UndoManager as is without
adding links from some specific classes
to it
- provide an API for UndoManager to
work with UndoableEdit-s which have
synchronization for undo/redo methods
- adding links of external classes
directly to UndoManager
What do you mean under link term? A
reference or dependency?
There are two options. If UndoManager
is a class designed to be only used with
the AbstractDocument and placed in the
javax.swing.text package it definitly can
have special code to handle an abstract
document instance in a special way.
If UndoManager is a general purpose
class, it looks strange that it handles
some special classes in different way as
all others. It usually mean that there are
some design problems in this class. That is
why I just asked to look at other ways at
first. Only if other solutions are not
suitable it has sense to look at the way
that you are provided.
Correct. I introduced extra dependency. It
is optional, but anyway. Of cause there is a
design problem in undo javax.swing.undo
package. But I cannot rewrite the API
because we will get a compatibility problem
then. I mentioned this several times in this
thread.
We are constrained by compatibility
requirements. UndoManager is a broadly
used class we cannot change the API so
drastically.
I think that you can generalize your
solution just adding an internal interface
like sun.swing.UndoableEditLock.
Every UndoableEdit which implements this
interface can provide a lock for its
synchronization.
If this will work it can be made public
in some days so other application can also
have proper synchronization for their
undo/redo actions.
OK. I added it.
http://cr.openjdk.java.net/~ssadetsky/8030702/webrev.02/
- We can return a public class that
implements an internal interface, but we
can't expose an internal API in the public
class definition.
May be it is possible to wrap an
UndoableEdit to the UndoableEditLockSupport
in the UndoableEditEvent or in some other place.
- The similar code is used both in the
UndoManager.undo() and redo() methods. Is it
possible to move this code to one method that
does undo or redo depending on the given
argument?
OK. accepted.
http://cr.openjdk.java.net/~ssadetsky/8030702/webrev.03/
- UndoManager.undo/redo methods
In your previous fix inProgress variable
and the super call were used under the lock. It
may leads to some synchronization issues if you
decide to omit it.
- UndoManager.tryUndoOrRedo()
It is possible to get rid of 'done'
variable just using an infinity loop and return
the exact result where the loop is terminated.
-
AbstractDocument.DefaultDocumentEventUndoableWrapper
implements both UndoableEdit and
UndoableEditLockSupport interfaces but
UndoableEditLockSupport already extends
UndoableEdit.
- "@since 1.9" javadoc for
DefaultDocumentEventUndoableWrapper.lockEdit()/unlockEdit()
methods really belongs to the
UndoableEditLockSupport methods.
In this case there is no need for
{@inheritDoc} tag.
Thanks,
Alexandr.
Thanks,
Alexandr.
Thanks,
Alexandr.
"adding links of external classes directly
to UndoManager" - Sorry, did not catch
what are you about? Could you clarify?
--Semyon
Thanks,
Alexandr.
--Semyon
There is a mistake in your scenario
steps: fireUndoableEditUpdate() is
called before the freeing the lock
(see
AbstractDocument.handleInsertString()
method).
Yet another argument do not do
this from the user experience:
if user starts a long edit
operation and press undo after
that he expects when the long
edit is finished it will be
rolled back immediately.
It is not true. The first
process adds his undo edit to
the UndoManager. While a user
trying to press undo the second
long process can be started.
That is what led to this issue
because when undo is in progress
document writing should be allowed.
Sorry but I didn't see why is "It
not true"? Then what is your
expectation when you press undo
button while edit is not finished
yet and there is no way to abort it?
It would be good if it works
as you described. But it does not
work in this way with or without
your fix.
undo() action has writeLock in
AbstractDocument and because of it
is always executed after insert
string action.
If a user sees that undo is
available, he can call it but the
second long insertString process
can start earlier and acquire the
writeLock.
That is what we are going to fix.
And this does work after this fix.
Undo call will be blocked by the
long edit until the last is done
without any deadlocks. And when
edit is done undo() will acquire
the lock and prevent any new edits
until undo() is done. Please
provide a scenario when in your
opinion it does not wok.
The first process starts for 5
minutes. When it is finished a user
sees that he can press undo. While
he is pressing undo button, the
second long process starts for 10
minutes and acquire the write lock.
The user presses undo but he needs
to wait 10 more minutes until the
second process is finished.
Actually, if two or more threads are
waiting for a monitor it is not
determined which one will get the
control after the signal. To order
that the ReentrantLock API could be
used but AbstractDocument uses
wait/notify for locking. I think it
is not worth to dig so deep. It does
not cause any issues
The issue that is considered is
"if user starts a long edit operation
and press undo after that he expects
when the long edit is finished it will
be rolled back immediately."
If you are agree that it is not
always possible to do the roll back
"immediately" there is no point to
discussion.
I agree. On that level it is not
possible to predict the order exactly
in such scenario. But the state of the
document will be consistent. And it is
possible to have it predictable using
lock fairness.
because undo() always get the last
edit anyway. If it will be important
for somebody to preserve the
execution order on that level of
details we will fix it.
So undo should be executed
after the edit is fully
performed because the
corresponding UndoableEdit
which undos this edit can be
produced only after the edit is
done.
I think at first we need to
look on the situation
externally rather than
concentrate on implementation
questions like in which class
do references go.
Yes, please look on this
situation from a user point of
view which wants to implement
simple Java Painter.
But could you describe this
scenario? Just steps when this
simple Painter fails under the
proposed fix?I
Note, if this Painter's content
is not an AbstarctDocument it
will work as before the fix.
Any application that uses
UndoManager and wants to have the
same synchronization (have the
same lock both for UndoableEdit
adding and undo() method
execution) will have the same
deadlock problems.
As I have already written:
---------------
Consider someone writes Java
Painter application where it is
possible to draw lines and images
and uses UndoManager for undo/redo
actions.
He might want that it was
possible to work with copied
images. He can get lock on ctrl+v
action, process an image, prepare
UndoableEdit and notify the
UndoManager.
He also can use lock/unlock in
the undo action to have a
consistent state with the
processed image. If someone calls
undo action during the image
processing and gets a deadlock
does it mean that link from Java
Painter need to be added to the
UndoManager?
---------------
Still do not understand the steps
for your Painter scenario. A link
(reference?) can be added if it is
required to implement
functionality. If the content is
not an AbstarctDocument it may be
required to implement custom
UndoManager to support the same
behavior.
What is the difference between
the AbstractDocument and other
classes (in Swing or user defined)?
Do you mean that the UndoManager is
intended only to be used with
AbstractDocument and it shouldn't be
used in other cases where undo/redo
actions are required for non text data?
No, undo manager can be used with any
classes. But since we have it
assigned to AbstarctDocument so often
we need to do our best to make undo
manager working with it correctly
because users do not like deadlocks
usualy. For other classes we cannot
provide synchronization by default
because there is no API to get the
lock. So it remains up to user how to
provide the undo manager
synchronization with the object it
controls for other classes
What we should do just to
understand that the same deadlock can
happen in an user applications because
he wants to use the same
synchronization both for the data
processing and for the undo action. If
so, there should be two investigations:
1. Is it possible to achieve the
requested goals without changing
UndoManager? In other words The
UndoManager should be used in proper
way as it is required by its design.
2. Is it possible to update the
UndoManager API to provide
functionality that meets new requests?
With API change it is reachable. But I
would preserve the current API as less
constrained. If we add some methods for
locking we will determine the way how a
user should synchronize his undoable
content. And user may not need any
synchronization at all. We should keep
in mind this opportunity as well.
Only after this discussion there can
be a reason to look to other ways.
Thanks,
Alexandr.
I think our undo manager
implementation do not pretend to be
used as the global undo manager for
big complex applications and it
cannot cover all possible undo
approaches. But some basic
functionality can be provided and it
should be usable. Without edits
serialization approach it is not
usable for multithreaded use. So
either we do not pretend to provide a
multithreaded undo manager and remove
all synchronize keywords from
UndoManager class, either we need to
support serialization approach which
does not cause deadlocks.
Thanks,
Alexandr.
I don't see a contradiction here,
could you point on it more precisely?
Thanks,
Alexandr.
Thanks,
Alexandr.
Thanks,
Alexandr.
--Semyon
Thanks,
Alexandr.
--Semyon
On 7/30/2015 5:27 PM,
Alexander Scherbatiy wrote:
Consider someone writes
Java Painter application
where it is possible to
draw lines and images and
uses UndoManager for
undo/redo actions.
He might want that it
was possible to work with
copied images. He can get
lock on ctrl+v action,
process an image, prepare
UndoableEdit and notify
the UndoManager.
He also can use
lock/unlock in the undo
action to have a
consistent state with the
processed image. If
someone calls undo action
during the image
processing and gets a
deadlock does it mean that
link from Java Painter
need to be added to the
UndoManager?
Thanks,
Alexandr.
It looks like
AbstractDocument
violates UndoManager
synchronization contract
when it both use lock to
work with UndoManager
and in the implemented
undo() method.
Thanks,
Alexandr.
--Semyon
