[squeak-dev] Transcript error when forceUpdate: false (?)

Marcel Taeumel marcel.taeumel at hpi.de
Thu Jan 28 08:05:16 UTC 2021

Hi Levente.

Thanks für clarification. We are talking about more than one problem here. :-) I did just address the issue with errors/debuggers happening due to UI process disruptions. You are also concerned about the order of contents in the Transcript's contents. Both are very important and required for claiming that it would be thread-safe.

(Which reminds me that I find it very strange that the UI components (e.g. PluggableTextMorph) do also check the Transcript's internal characterLimit. I would expect an append-only behavior when a Transcript window is open.)

Anyway, your proposal about a safelyNextPut(All): looks good. :-) +1

Am 27.01.2021 23:05:21 schrieb Levente Uzonyi <leves at caesar.elte.hu>:
Hi Marcel,

On Tue, 26 Jan 2021, Marcel Taeumel wrote:

> Hi Jaromir,
> please take a look at Collections-mt.923 (inbox). Maybe those changes would satisfy all your current needs in this regard. :-)
> @all: Would Transcript now be thread-safe? Did I miss something?

Not at all. The problem is not #endEntry but all the other
methods writing to the stream e.g.: #nextPutAll:.
The real solution is to replace the stream-like API with one that provides
an easy way to write messages in a thread-safe way. E.g.:

Transcript showStream: [ :stream |
stream nextPutAll: 'Hello'; space; nextPutAll: ' World!'; cr ]

Which guarantees that 'Hello World!' appears on the Transcript without
being mixed with other messages.


Transcript show: 'Hello {1}!{2}' format: { 'World'. String cr }.

The implementations could be e.g.:

showStream: aBlock

| string |
string := String streamContents: aBlock.
self safelyNextPutAll: string

show: aString format: arguments

| string |
string := aString format: arguments.
self safelyNextPutAll: string

The last challenge is to implement #safelyNextPutAll: which involves
making Transcript not be a TranscriptStream.
Transcript should encapsulate the stream and use a Mutex (not a global one
because its pointless) to provide thread-safety while writing the
characters on it or reading its contents. E.g.:

safelyNextPutAll: aString

mutex critical: [
stream nextPutAll: aString ]

For backwards compatibility, the stream-API methods must be provided for a
while but those methods should be thread-safe on their own. E.g.:

nextPutAll: aString

self safelyNextPutAll: aString

nextPut: aCharacter

self safelyNextPutAll: aCharacter asString


> Best,
> Marcel
> Am 25.01.2021 21:38:36 schrieb jaromir :
> Well, I tried deferring the whole Transcript endEntry machinery to the UI
> doOneCycle (bypassing the changed: #appendEntryLater mechanism) for
> #forceUpdate = false only ... and it seems to avoid the problem!
> TranscriptStream >> endEntry
> deferredEntry ifNil: [ false ]. "this is a new instance variable"
> self semaphore critical:[
> self class forceUpdate
> ifTrue: [self changed: #appendEntry; reset]
> ifFalse: [deferredEntry := true].
> TranscriptStream >> flushDeferredEntry
> "This is run every UI cycle in doOneCycleNowFor:"
> deferredEntry ifTrue: [
> self class forceUpdate: true.
> self endEntry.
> deferredEntry := false.
> self class forceUpdate: false.
> ]
> doOneCycleNowFor: aWorld
> "... the whole body remains unchanged except:"
> capturingGesture ifFalse:
> [aWorld runStepMethods.
> Transcript flushDeferredEntry. "this is printing for #forceUpdate =
> false"
> self displayWorldSafely: aWorld].
> For #forceUpdate = true the endEntry mechanism remains unchanged and failing
> as before...
> --
> Sent from: http://forum.world.st/Squeak-Dev-f45488.html

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