BUG: `assemble(Constant * Cofunction(Function))` fails

[jrmaddison]

Describe the bug
Assembly of a multiple of a cofunction action fails.

Steps to Reproduce

import firedrake as fd

mesh = fd.UnitIntervalMesh(1)
space = fd.FunctionSpace(mesh, "DG", 0)
u = fd.Function(space, name="u")
b = fd.assemble(fd.conj(fd.TestFunction(space)) * fd.dx)

fd.assemble(b(u))  # OK
fd.assemble(2 * b(u))  # OK
fd.assemble(fd.Constant(2) * b(u))  # Fails

leads to

Traceback (most recent call last):
  File "/home/jmaddis2/build/firedrake/examples/assemble_bug.py", line 10, in <module>
    fd.assemble(fd.Constant(2) * b(u))  # Fails
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "petsc4py/PETSc/Log.pyx", line 250, in petsc4py.PETSc.Log.EventDecorator.decorator.wrapped_func
  File "petsc4py/PETSc/Log.pyx", line 251, in petsc4py.PETSc.Log.EventDecorator.decorator.wrapped_func
  File "/home/jmaddis2/build/firedrake/firedrake/firedrake/adjoint_utils/assembly.py", line 58, in wrapper
    output.form = form
    ^^^^^^^^^^^
AttributeError: 'Zero' object has no attribute 'form'

[pbrubeck]

It seems that we are trying to be clever and simplifying for the case when the result is zero, which happens when either u or b are zero. Setting them to nonzero gives no error:

from firedrake import *
mesh = UnitIntervalMesh(1)
space = FunctionSpace(mesh, "DG", 0)
u = Function(space).assign(1)
b = Cofunction(space.dual()).assign(1)

assemble(Constant(2) * action(b, u))

Although this fails on a mesh with two elements!

AttributeError                            Traceback (most recent call last)
Cell In[7], line 8
      5 u = Function(space).assign(1)
      6 b = Cofunction(space.dual()).assign(1)
----> 8 assemble(Constant(2) * action(b, u))

File petsc4py/PETSc/Log.pyx:250, in petsc4py.PETSc.Log.EventDecorator.decorator.wrapped_func()

File petsc4py/PETSc/Log.pyx:251, in petsc4py.PETSc.Log.EventDecorator.decorator.wrapped_func()

File /scratch/brubeckmarti/firedrake/src/firedrake/firedrake/adjoint_utils/assembly.py:60, in annotate_assemble.<locals>.wrapper(form, *args, **kwargs)
     57         block.add_output(output.block_variable)
     58 else:
     59     # Assembled a 2-form
---> 60     output.form = form
     62 return output

AttributeError: 'Product' object has no attribute 'form'

[pbrubeck]

I think that the Constant is not being numerically evaluated by assemble

[pbrubeck]

Passing a tensor seems to fix the issue:

from firedrake import *
mesh = UnitIntervalMesh(2)
space = FunctionSpace(mesh, "DG", 0)
u = Function(space).assign(1)
b = Cofunction(space.dual()).assign(1)

tensor = Constant(0)
assemble(Constant(2) * action(b, u), tensor=tensor)