dmrg: refuse MEASURE_* parameters with a clear error instead of SIGSEGV

[skilledwolf]

Problem

The classic dmrg binary crashes with SIGSEGV on every input that requests MEASURE_LOCAL, MEASURE_AVERAGE, MEASURE_CORRELATIONS, or MEASURE_STRUCTURE_FACTOR: dmtk::System::rotate_hami dereferences the result of System::operator()(op) without a null check, and the lookup legitimately returns 0 while a measurement operator's site is not yet covered by the current block layout during early final_sweep iterations near the chain endpoints. Diagnosed with lldb: BasicOp::is_diagonal called on this == 0x0 via rotate_hami.

A skip-null guard at the call sites is not a fix: it compiles but silently drops legitimate measurement terms wholesale (a Sz_total=2 local-Sz sum comes out as 0.15 instead of 2). Correctly fixing the legacy path needs the block operator-set bookkeeping re-derived so measurement operators are guaranteed present at every iteration.

Fix

Until someone does that surgery, fail fast in DMRGTask::dostep with an actionable message when any MEASURE_* parameter is present. Since #92 removed the mps_optim application (which supported the same MEASURE_* syntax), the message suggests dropping the MEASURE_* parameters (energy/entropy still work) or using an ALPS release that still ships mps_optim.

How to reproduce

Save as parm:

LATTICE="open chain lattice"
MODEL="spin"
local_S=1/2
CONSERVED_QUANTUMNUMBERS="Sz"
Sz_total=0
J=1
L=8
SWEEPS=2
NUMBER_EIGENVALUES=1
MAXSTATES=32
MEASURE_LOCAL[Local Sz]=Sz
{}

parameter2xml parm
dmrg parm.in.xml ; echo "exit=$?"

• on master: exit=139 (SIGSEGV) mid-sweep, empty stderr
• with the fix: clean nonzero exit with a single-line explanation of what is unsupported and why
• delete the MEASURE_LOCAL line and the identical run completes unchanged (exit=0) — the guard only fires when a MEASURE_* parameter is present

The same crash reproduces with MEASURE_AVERAGE, MEASURE_CORRELATIONS, and MEASURE_STRUCTURE_FACTOR (the old dmrg-03/dmrg-04 tutorial inputs were public reproducers before the tutorials were removed).

Provenance

Diagnosed and guarded in a downstream ALPS modernization fork, where the guard is pinned by an API regression test asserting the non-signal exit code and the actionable stderr text.

[egull]

@afeiguin give me a sign of life here – do you have capacity to deal with this or should we sign up others?