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);
} 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().
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
