m_into_nodes

from qgis.core import QgsGeometry, QgsPoint, QgsProject

layer = QgsProject.instance().mapLayersByName('test')[0]

layer.startEditing()

for f in layer.getFeatures():
    geom = f.geometry()

    # sécurité : seulement LineString
    if geom.isMultipart():
        continue

    pts = geom.constGet().points()
    dist = 0.0
    new_pts = []

    for i, p in enumerate(pts):
        if i > 0:
            dist += pts[i-1].distance(p)
        new_pts.append(QgsPoint(p.x(), p.y(), p.z(), dist))

    f.setGeometry(QgsGeometry.fromPolyline(new_pts))
    layer.updateFeature(f)

layer.commitChanges()