Hi Levente,

On Tue, Mar 31, 2020 at 3:33 PM Levente Uzonyi <leves at caesar.elte.hu> wrote:

> Hi Eliot,
>
> On Mon, 30 Mar 2020, Eliot Miranda wrote:
>
> > Hi Levente,
> >
> >> On Mar 30, 2020, at 2:21 PM, Levente Uzonyi <leves at caesar.elte.hu>
> wrote:
> >>
> >> Hi Eliot,
> >>
> >>> On Mon, 30 Mar 2020, Eliot Miranda wrote:
> >>>
> >>> Well, that's not what I meant by a search.  However, as Levente
> pointed out, textual searches should be surrounded with
> CurrentReadOlySouceFiles cacheDuring:.  I think this is an awful
> implementation and would implement it
> >>> very differently but that's the work-around we have in place now,
> >>
> >> How would you implement it?
> >>
> >> <history>
> >> When I introduced CurrentReadOnlySourceFiles, I wanted to solve the
> issue of concurrent access to the source files.
> >> I had the following options:
> >> 1) controlled access to a shared resource (a single read-only copy of
> the SourceFiles array) with e.g. a Monitor
> >> 2) same as 1) but with multiple copies pooled
> >> 3) exceptions to define the scope and lifetime of the resources (the
> read-only copies) within a process
> >>
> >> I chose the third option because it was possible to introduce it
> without exploring and rewriting existing users: you could leave all code
> as-is
> >> and sprinke CurrentReadOnlySourceFiles cacheDuring: [ ... ] around code
> that needed better performance.
> >> It's obviously not a perfect solution, but I still think it was the
> best available at the time.
> >>
> >> Later ProcessLocalVariables were added to Trunk. Which could be used to
> solve the concurrent access issue by using process-local copies of the
> source files. The only challenge is to release them after they are not
> needed any more. Perhaps a high priority process could do that after a few
> minutes of inactivity. Or we could just let them linger and see if they
> cause any problems.
> >> </history>
> >
> > I think the key issue (& this from a discussion here with Bert) is
> access time source in the debugger while one is debugging file access.  As
> the debugger asks for source so the file pointer is moved and hence screws
> up the access one is trying to debug.
>
> I don't think that's the only issue. Have a look at the senders of
> #readOnlyCopy. Many of them were added 10+ years ago, well before
> CurrentReadOnlySourceFiles was introduced. Most of those could use
> CurrentReadOnlySourceFiles too but are unrelated to the debugger.
>

Yes, but IIRC that issue was to separate the writable file from the
read-only file.  I remember dealing with this when working on Newspeak in
2007/2008. So SourceFiles can easily maintain a writable file and a
read-only copy of the file for both sources and changes and do writes
through the writable one.


> >
> > So I would provide something like
> >   SourceFiles withSubstituteCopiesWhile: aBlock
> > which would install either copies of the files or read-only copies of
> the files for the duration of the block, and have the debugger use the
> method around its access to method source.
> >
> > The IDE is essentially single threaded as far as code modification goes,
> even if this isn’t enforced. There is no serialization on adding/removing
> methods and concurrent access can corrupt method dictionaries, and that
> limitation is fine in practice.  So having the same restriction on source
> file access is fine too (and in fact I think the restriction already
> applies; if one were to fork compiles then source update to the changes
> file could get corrupted too).
> >
> > So I think not using read-only copies to read source, and having the
> debugger use copies for its access would be a good lightweight solution.
>
> I agree with what you wrote about method changes, but reading the sources
> concurrently is still a possibility, especially when multiple UI processes
> can exist at the same time (e.g. that's what opening a debugger does,
> right?).
>

My assertion is that the IDE is essentially single0-threaded and this
doesn't;t have to be supported.  In any case, concurrent access will work
if processes of the same priority level are cooperating.  But I just
answered the debugger issue.  I'm assuming that the debugger guards all its
source access by substituting a different file.  So it, and only it,
accesses the sources files through copies, and it, and only it pays the
cost for substituting the copies.  Normal queries can use a single
read-only copy.  That gives us the functionality of cacheDuring: without
having to invoke it.


So let me reiterate.

SourceFiles is modified to have a single writable version of the changes
file and a single read-only version of sources nd changes files.  Source
code is read through the readable copy and new source written through the
writable copy.  Whenever the debugger accesses source it does so through a
method that first saves the files, substitutes copies in SourcesFiles,
evaluates its block (that will access source through the copies), and then
ensures that the original files are restored.  There can be error checking
for writing to the changes file in the debugger while writes are in
progress to the original writable changes file, although I'm not sure this
is necessary; folks debugging source file access usually know what they're
doing.

The result is that
- normal source reading does not require creating a read-only copy; it
already exists.
- the debugger does not interfere with source access because it is careful
to use copies and leave the originals undisturbed
- CurrentReadOnlySourceFiles and cacheDuring: can be discarded




>
> Levente
>
> >
> >> Levente
> >
> > Eliot
>


-- 
_,,,^..^,,,_
best, Eliot
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