[Vm-dev] Re: [squeak-dev] FileDoesNotExistException on existing changes file during trunk update

Eliot Miranda eliot.miranda at gmail.com
Tue Sep 2 18:30:41 UTC 2014

Hi Bert,

On Tue, Sep 2, 2014 at 11:23 AM, Bert Freudenberg <bert at freudenbergs.de>

> On 02.09.2014, at 17:03, Eliot Miranda <eliot.miranda at gmail.com> wrote:
> Hi Bert,
> On Mon, Sep 1, 2014 at 7:13 AM, Bert Freudenberg <bert at freudenbergs.de>
> wrote:
>> On 01.09.2014, at 15:56, Eliot Miranda <eliot.miranda at gmail.com> wrote:
>> >> Another thought is that given the abundance of memory these days, we
>> might cache both sources and changes in main memory (which would also speed
>> up full-text searches).
>> >
>> > Pharo is planning to eliminate them altogether which is more coherent
>> than caching them.
>> That's another discussion, but it might be a step towards that.
>> >  But IMO the solution is easy, maintain a *single* read-only copy of
>> the sources and changes files in SourceFilesArray (or whatever the class is
>> called; I'm on my phone) and read source through them instead of reopen ing
>> the damn things all the time.
>> Except that Squeak files do not maintain an independent file position
>> pointer, so reading from different positions in the same file is not
>> thread-safe. That's why the file is opened again.
> Can you explain more.  I don't understand.
> I think we're in violent agreement ;) This was a comment on the image-side
> file handling, not about the VM. (Although we might need better file prims.
> Tim appears to have ideas)
>  As I see it every file instance has its own FILE structure so I don't
> understand how this can be so.  Files are derived from
> FilePlugin's primitiveFileOpen function.  That is implemented in terms
> of fileOpenNamesizewritesecure, which allocates a ByteArray to hold state
> (so no two Smalltalk files share state) and then uses sqFileOpen to open
> the underlying file and fill in the state.  The C library implementation
> of sqFileOpen in
> platforms/Cross/plugins/FilePlugin/sqFilePluginBasicPrims.c uses fopen et
> al, again creating unique state.
> Precisely. Every Squeak file-open also opens an OS file via fopen(). The
> number of files a process can open is limited. That is why we run out of
> file handles unless the files gets closed properly.
>  So I don't see how having a writable Smalltalk file and a separate
> read-only Smalltalk file on the sources and changes can result in other
> than two separate independent file pointers.
> What I was getting at is that it would be much better to actually open the
> file only once (or twice, once for writing and once for read-only) and then
> maintain a file pointer in the image independently of the OS's file
> position. That is how we could share a single read-only file for many
> readers.

Right, now we are in agreement.  That's what I like.  1 read-only copy, one
writeable copy for the changes file, and presumably a single read-only file
for the sources file.

> The problem is that the OS file also has a file position, and the file
> positions we maintain in the image would easily get out of sync with that.

Potentially, but don't do that.  If the image wants to update the file
position it must also use primSetPosition:to: to keep the OS file pointer
up-to-date.  There's a thread-safety issue, but that's to be kept
orthogonal for now (the current situation also has such issues).

There *is* an issue with the structure of file access in the debugger.  If
> one were to step through execution of accessing source from e.g. the
> read-only sources file then the very act of fetching sources for the
> methods being displayed would confuse the state in the file one was
> observing through the debugger.  But that's hardly a new situation (one can
> get into a similar situation with the current setup), and the debugger
> could ease the situation by wrapping source access with something that
> reset the file's buffer etc.
>> > Then the file's own buffers will provide done caching.  Annoying that I
>> write this code in 2008 for newspeak but we still rely on the mad "run the
>> GC to finalize files when open fails" approach.
>> Well, you writing this for newspeak does not immediately benefit Squeak.
>> But if you point us to the code maybe someone can port it to Squeak?
> I already did a year or two ago and it got shot down on the debugger
> grounds I reiterated above.
> Well, maybe it's time to revisit, then.

Will do.

- Bert -

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