[Tile-serving] [osm2pgsql-dev/osm2pgsql] as_linestring() and likely other geometry transform functions can only be called from the process_way() function, not custom functions. (Issue #2171)

[mboeringa]

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## What version of osm2pgsql are you using?

osm2pgsql version 1.11.0 (1.8.1-397-gcf66989f)

## What operating system and PostgreSQL/PostGIS version are you using?

Ubuntu 22.04
PostgreSQL 16.2 (Ubuntu 16.2-1.pgdg22.04+1), PostGIS 3.4


## Tell us something about your system

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## What did you do exactly?
I have been trying to convert an existing style (a derivative of Paul Norman's work for the flex style of OpenStreetMap carto: https://github.com/gravitystorm/openstreetmap-carto/pull/4431) that still used the now deprecated 'add_row()' function to use the new 'insert()'.

Although I think I have the syntax right now, I run into an issue with a check that osm2pgsql seems to do, I get an error message:

`ERROR: Failed to execute Lua function 'osm2pgsql.process_relation': osm2pgsql/flex-config/openstreetmap-carto.lua:1228: Error in 'as_linestring': The function as_linestring() can only be called from the process_way() function.`

It is true that Paul's work for that style uses a couple of helper functions which implement the actual insertion 'add_row/insert', and that the 'as_linestring()' function call thus is not inside the 'process_way' function.

While I could theoretically rewrite everything to be inside the 'process_way' function, this not only would bloat the function, but this limitation also hampers general Lua development.

It also kind of breaks "backward compatibility" with the deprecated 'add_row' function, as using that function in a helper function outside 'process_way' was never a problem and simply worked.

Could this limitation with 'insert' and required 'as_linestring/multipolygon' etc. be lifted and allow more flexible function and code design?

## What did you expect to happen?

Geometry transforms for the new 'insert' work outside the 'process_way' function.


## What did happen instead?

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(or link to, if it is too verbose) the log output. -->


## What did you do to try analyzing the problem?

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reporting. -->



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