shared datasets

[April Shimitz]

Frank,

I have a few changes for the GDAL core. First is just a minor bug in the
GDALDataset Create method. The return of GetDescription() is getting
checked at line 185 against NULL, but GetDescription returns the
description string or an empty string. I think Create should be comparing
the return to an empty string, not NULL.

For LAS, we have implemented a wrapper layer around the GDAL calls to make
our transition from our old image I/O library to the GDAL library a
smoother process. In the wrapper layer we implemented some routines to
track the open datasets so files don't get opened multiple times. I just
found a bug in one of those routines and was trying to fix it when I came
across your GDALOpenShared function. The dataset sharing you've implemented
does has mostly the same functionality we had implemented in our wrapper
layer, so I decided to just use that instead. There were a couple glitches
I found when trying to use it though - functionality I need but couldn't
find implemented anywhere (I could have just missed it if it is already).
First, I need some way to find out what datasets are currently open quicker
than dumping the datasets to a file (GDALDumpOpenDatasets) then parsing the
file to retrieve the datasets. So I implemented a public version of the
GetOpenDatasets routine (GDALGetOpenDatasets). The other thing was that I
needed the ability to determine the access method used to open the file
described by a given dataset. That was not available publicly either, so I
implemented a GDALGetAccess routine. I have attached a single patch file
containing these modifications.

Let me know when (and if) these routines get incorporated into the GDAL
core and if you make any changes to them.

Thanks,
April Shimitz

(See attached file: gdalcore.patch)
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