`agx`: "read exposure" leaves a stale `curve_gamma` in history

Found by bots via code analysis, 2026-08-25.

Severity: latent. No wrong render while auto_gamma is on, because the pipe
recomputes the value. The stale parameter becomes live the moment the user turns
auto_gamma off, and the image then jumps.

Summary

agx's gui_changed() recomputes curve_gamma from the pivot whenever auto_gamma is
enabled — but the whole body is skipped during a GUI update, and
_read_exposure_params_callback() triggers exactly that path.

The chain:

1. _read_exposure_params_callback() (src/iop/agx.c:1158-1167) changes
   p->range_black_relative_ev / p->range_white_relative_ev via
   _adjust_relative_exposure_from_exposure_params(), then calls dt_iop_gui_update(self)
   and immediately dt_dev_add_history_item().
2. dt_iop_gui_update() wraps module->gui_update() in DT_ENTER_GUI_UPDATE()
   (src/develop/imageop.c:2442).
3. agx's gui_update() ends with gui_changed(self, NULL, NULL) (:2259).
4. gui_changed() guards its entire body with if(!DT_IN_GUI_UPDATE()) (:1762), so the
   block that recomputes the gamma is skipped:

    if(g && p->auto_gamma)                       // :1798-1804
    {
      tone_mapping_params_t tone_mapping_params;
      _set_log_mapping_params(self->params, &tone_mapping_params);
      _adjust_pivot(self->params, &tone_mapping_params);
      dt_bauhaus_slider_set(g->curve_gamma, tone_mapping_params.curve_gamma);
    }

History is then committed with the old p->curve_gamma.

Why it does not show up immediately

While auto_gamma is on, the pipe ignores p->curve_gamma — _adjust_pivot()
(src/iop/agx.c:782-800) recomputes it:

  if(p->auto_gamma)
    tone_mapping_params->curve_gamma =
        pivot_x > 0.f && p->curve_pivot_y_linear_output > 0.f
        ? log2f(p->curve_pivot_y_linear_output) / log2f(tone_mapping_params->pivot_x)
        : p->curve_gamma;

so p->curve_gamma is only a fallback for a degenerate pivot. The slider is also hidden
in this state (gtk_widget_set_visible(g->curve_gamma, !p->auto_gamma), :1742).

The actual consequence

p->curve_gamma sits in history holding a value that no longer matches the pivot. When
the user disables auto_gamma, that stale value becomes the live one and the rendering
jumps discontinuously — with no edit that explains it.

Suggested fix

Recompute curve_gamma in the callback itself rather than relying on gui_changed()
running, or hoist the if(g && p->auto_gamma) block out of the !DT_IN_GUI_UPDATE()
guard. The latter needs care: the block calls dt_bauhaus_slider_set(), whose
value-changed handler is what writes p->curve_gamma, and that handler is itself
suppressed during a GUI update — so hoisting alone would move the slider without updating
the parameter. Writing p->curve_gamma directly and then setting the slider inside
DT_ENTER_GUI_UPDATE()/DT_LEAVE_GUI_UPDATE() is the clearer form.

Check the other callers of _update_pivot_x() for the same pattern.

Environment

Present in 56898c27b8. Static analysis only — nothing verified at runtime.