Modeler - Feature iteration does not work

[esnyder-rve]

What is the bug or the crash?

When running a processing model with a Vector Features input, using the "Iterate over this layer..." option only runs correctly once. The remainder of iterations all "run", but with 0 algorithms being executed.

Steps to reproduce the issue

1. Create a new processing model
2. Add a Vector Features input
3. Add some processing algorithm (such as a buffer tool) & set that algorithms output as final
4. Run model
5. Set the "Iterate over this layer..." option
6. Execute model
7. See output

My output (abridged for conciseness):

Algorithm 'Iteration Test' starting…
Input parameters:
{ 'input_features' : QgsProcessingFeatureSourceDefinition(...), 'output' : 'TEMPORARY_OUTPUT' }

Executing iteration 1/6…
Prepare algorithm: native:buffer_1
Running Buffer [1/1]
Input Parameters:
{ INPUT: '...', DISSOLVE: False, ... }
Results:
{ OUTPUT: 'Buffered_35d8d698_40f2_4c78_bacb_905843d212f6' }
OK. Execution took 0.005 s (1 output).
Model processed OK. Executed 1 algorithm total in 0.011s.
Executing iteration 2/6…
Model processed OK. Executed 0 algorithms total in 0s.
Executing iteration 3/6…
Model processed OK. Executed 0 algorithms total in 0s.
Executing iteration 4/6…
Model processed OK. Executed 0 algorithms total in 0s.
Executing iteration 5/6…
Model processed OK. Executed 0 algorithms total in 0s.
Executing iteration 6/6…
Model processed OK. Executed 0 algorithms total in 0s.
Loading resulting layers
Execution completed in 0.09 seconds
Algorithm 'Iteration Test' finished

Versions

QGIS version	3.40.2-Bratislava
QGIS code revision	14826ca

Libraries
Qt version	5.15.13
Python version	3.12.8
GDAL/OGR version	3.9.3
PROJ version	9.5.0
EPSG Registry database version	v11.016 (2024-08-31)
GEOS version	3.13.0-CAPI-1.19.0
SQLite version	3.46.1
PDAL version	2.8.1
PostgreSQL client version	16.2
SpatiaLite version	5.1.0
QWT version	6.3.0
QScintilla2 version	2.14.1
OS version	Windows 11 Version 2009

Active Python plugins
db_manager	0.1.20
grassprovider	2.12.99
processing	2.12.99

Supported QGIS version

• I'm running a supported QGIS version according to the roadmap.

New profile

• I tried with a new QGIS profile

Additional context

This is a regression, as this works as-expected in QGIS 3.34.14 LTR.

The example I ran was running just the buffer tool in a model with all default values.
My test model is here: Iteration Test.zip

[agiudiceandrea]

It looks like the issue with the model execution in feature iteration mode occurs using QGIS >= 3.38 while it doesn't occur using QGIS <= 3.36.
@nyalldawson, may it be that the regression is related to #57178 and #57212 which have been introduced in QGIS 3.38?