[Tracking] Complete the set -u hardening (v2.3.0 unbound-variable regressions)

[Zordrak]

Summary

Enabling set -uo pipefail in v2.3.0 (bin/terraform.sh:8)
exposed several pre-existing unguarded variable references that only crash on
specific, previously-tolerated code paths. This is a tracking issue grouping
the individual defects so they can be resolved together as a single, coherent
"complete the set -u hardening" pass and verified end-to-end.

This tracker is not a separate defect — the actual bugs and their fixes live in
the child issues below.

Child issues

• Unbound variable access breaks deployment #49 — Unbound variable access breaks deployment (${@} unbound at
  bin/terraform.sh:284 when no -- args are supplied)
• Bootstrap without -e crashes with "environment: unbound variable" under set -u #50 — Bootstrap without -e crashes with environment: unbound variable
• Uninitialised tf_var_file_paths array crashes with "unbound variable" when no tfvars files exist #51 — Uninitialised tf_var_file_paths array crashes with
  unbound variable when no tfvars files exist
• Bootstrap with -a shell crashes with "component: unbound variable" under set -u #52 — Bootstrap with -a shell crashes with component: unbound variable

Why group them

All four share a single cause class: set -u was switched on without a full
sweep for references that can be reached while unset. They are distinct root
causes with distinct fixes, but they interact and are cheapest to fix and
verify as one unit:

• Verification ordering: Unbound variable access breaks deployment #49 crashes earliest (line 284, argument
  parsing). Until it is fixed, a bootstrap run cannot even reach Bootstrap without -e crashes with "environment: unbound variable" under set -u #50/Bootstrap with -a shell crashes with "component: unbound variable" under set -u #52, so
  those cannot be verified end-to-end in isolation. Unbound variable access breaks deployment #49 is effectively a
  prerequisite for validating the others via the documented bootstrap workflow.
• Shared theme in bootstrap mode: Bootstrap without -e crashes with "environment: unbound variable" under set -u #50 (environment) and Bootstrap with -a shell crashes with "component: unbound variable" under set -u #52 (component)
  are the same pattern — variables declared only in the non-bootstrap branch
  (bin/terraform.sh:316-331) but referenced bare
  in bootstrap mode.
• Same fix family: Uninitialised tf_var_file_paths array crashes with "unbound variable" when no tfvars files exist #51 is the array analogue (missing =() initialiser),
  while the v2.3.0 hardening commit already demonstrated the intended guard
  style at bin/terraform.sh:357
  ([ -n "${environment:-}" ]) — it was simply applied to one line and no
  others.

Suggested approach

Treat this as one focused hardening PR (or a short series) that:

1. Fixes Unbound variable access breaks deployment #49 first so the bootstrap path is reachable for testing.
2. Ensures environment and component are always defined (even if empty) so
   the existing [ -n "${var}" ] presence checks behave, and audits the S3
   path constructions in bootstrap mode (secrets/remote-vars lookups) — these
   arguably should be skipped entirely when bootstrap == 'true'.
3. Initialises tf_var_file_paths as =() and guards the zero-element
   cat "${tf_var_file_paths[@]}" at bin/terraform.sh:606.
4. Resolves the shell-in-bootstrap message (Bootstrap with -a shell crashes with "component: unbound variable" under set -u #52), likely by switching to the
   always-set ${component_name}.
5. Does a final belt-and-braces sweep for any remaining bare ${var}
   references reachable while unset, so we don't file a fifth issue next month.

Verification

After the fixes, the documented README bootstrap workflow (no -e, no -c,
minimal/empty etc/) should run cleanly:

bin/terraform.sh -p myproject -b myproject-tfscaffold -r eu-west-2 --bootstrap -a plan
bin/terraform.sh -p myproject -b myproject-tfscaffold -r eu-west-2 --bootstrap -a apply

…with no unbound variable aborts, and without needing a dummy -- <arg>
workaround.

Related

Individual defects: #49, #50, #51, #52.

[Zordrak]

Claimed by feature-implementer. Delivering all four set -u fixes (#49, #50, #51, #52) plus a full sweep in a single PR on branch fix/53-setu-hardening.