[raster] ST_MapAlgebra checks callbacks parameters but not return type

[strk]

I tried using a callback only accepting 2 arguments and ST_MapAlgebra complained about that. But it did not complain about the callback returning an integer rather than a float8.

As the C code fetches the return value in binary form, I guess returning anything but "float8" would just not work, silently.

If this is confirmed, it would be nice for ST_MapAlgebra to also check the return type of the provided callback.

[dustymugs]

The check for return type will throw an Exception if not a double precision.

[strk]

I didnt' get an exception when returning integer. Looking at the code the return type check only prevents your callback from returning a SET.

[dustymugs]

I'll probably fix this as I fix #2803.

[dustymugs]

Fixed for -trunk in r12744. Fixed for -2.1 in r12745.

[strk]

Great, thanks!

nitpicking: the error printed on returning a SET still says you need to return a "double precision", while if you return a "text" (for example) the error tells you that also int,smallint and float4 are accepted.