Adds capability to have an updatable data source via ogr connection

[carolinux]

Not doing it like this gives us a read only data source the next time we try to access it (if for some reason we invalidate the connection - for instance via forceReload )
@m-kuhn

[m-kuhn]

@manisandro what do you think about this?

[manisandro]

@carolinux In what scenario is a writeable connection from the connection pool required? If memory serves me right, for all operations which alter the dataset, a spearate OGR handle is opened in read/write mode. The connection pool connections should only be used for read-only operations?

[carolinux]

I came across this when using the forceReload capability. There the handle
is invalidated and the new handle is created from this method.

On Mon, Jan 4, 2016 at 12:17 PM, Sandro Mani notifications@github.com
wrote:

@carolinux https://github.com/carolinux In what scenario is a writeable
connection from the connection pool required? If memory serves me right,
for all operations which alter the dataset, a spearate OGR handle is opened
in read/write mode. The connection pool connections should only be used for
read-only operations?

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#2618 (comment).

[manisandro]

forceReload invalidates all handles, correct. But where is a handle from the connection pool used for altering the dataset?

[carolinux]

I am writing a plugin where I want to write to an OGR layer via QGIS. (But
also the OGR layer underlying source may change on its own hence the need
for forceReload). So if I have forceReload'ed once, then the layer becomes
read-only.

On Mon, Jan 4, 2016 at 12:55 PM, Sandro Mani notifications@github.com
wrote:

forceReload invalidates all handles, correct. But where is a handle from
the connection pool used for altering the dataset?

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#2618 (comment).

[carolinux]

xlsx is an ogr layer type that supports read/write btw

On Mon, Jan 4, 2016 at 12:57 PM, Caroline Alexiou carolinegr@gmail.com
wrote:

I am writing a plugin where I want to write to an OGR layer via QGIS. (But
also the OGR layer underlying source may change on its own hence the need
for forceReload). So if I have forceReload'ed once, then the layer becomes
read-only.

On Mon, Jan 4, 2016 at 12:55 PM, Sandro Mani notifications@github.com
wrote:

forceReload invalidates all handles, correct. But where is a handle from
the connection pool used for altering the dataset?

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#2618 (comment).

[manisandro]

This behaviour needs to be investigated. The connections in the connection pool should remain read-only.

[m-kuhn]

Maybe it's related to calling forceReload on a layer which is currently in edit mode?

[carolinux]

Why should they remain read-only?

On Mon, Jan 4, 2016 at 1:01 PM, Matthias Kuhn notifications@github.com
wrote:

Maybe it's related to call forceReload on a layer which is currently in
edit mode?

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#2618 (comment).

[manisandro]

Because connections from the connection pool should only be used for read-only operations. Connections for altering the dataset are created separately, and the code around them ensures that the connection pool handles are invalidated when the dataset is altered.

[carolinux]

Alright, I misunderstood something. It indeed needs to be investigated how to re-open a datasource in r/w mode, but this PR wouldn't solve this then. Closing.