Fix memtest ISO validation false negatives

bible-local/decisions/2026-04-01-memtest-build-strategy.md

@@ -37,6 +37,27 @@ Additional evidence from the archived `easy-bee-nvidia-v3.17-dirty-amd64` logs d
 
 So the assumption "the current normal binary hook path is late enough to patch final memtest artifacts" is also false.
 
+Correction after inspecting the real `easy-bee-nvidia-v3.20-5-g76a9100-amd64.iso`
+artifact dated 2026-04-01:
+
+- the final ISO does contain `boot/memtest86+x64.bin`
+- the final ISO does contain `boot/memtest86+x64.efi`
+- the final ISO does contain memtest menu entries in both `boot/grub/grub.cfg`
+  and `isolinux/live.cfg`
+- so `v3.20-5-g76a9100` was **not** another real memtest regression in the
+  shipped ISO
+- the regression was in the build-time validator/debug path in `build.sh`
+
+Root cause of the false alarm:
+
+- `build.sh` treated "ISO reader command exists" as equivalent to "ISO reader
+  successfully listed/extracted members"
+- `iso_list_files` / `iso_extract_file` failures were collapsed into the same
+  observable output as "memtest content missing"
+- this made a reader failure look identical to a missing memtest payload
+- as a result, we re-entered the same memtest investigation loop even though
+  the real ISO was already correct
+
 ## Known Failed Attempts
 
 These approaches were already tried and should not be repeated blindly:
@@ -79,6 +100,8 @@ Any future memtest fix must explicitly identify:
 - where the memtest binaries are reliably available at build time
 - which exact build stage writes the final bootloader configs that land in the ISO
 - and a post-build proof from a real ISO, not only from intermediate workdir files
+- whether the ISO inspection step itself succeeded, rather than merely whether
+  the validator printed a memtest warning
 
 ## Decision
 
@@ -107,6 +130,10 @@ Current implementation direction:
   - injects memtest binaries there
   - ensures final bootloader entries there
   - reruns late binary stages (`binary_checksums`, `binary_iso`, `binary_zsync`) after the patch
+- also treat ISO validation tooling as part of the critical path:
+  - install a stable ISO reader in the builder image
+  - fail with an explicit reader error if ISO listing/extraction fails
+  - do not treat reader failure as evidence that memtest is missing
 
 ## Consequences
 
@@ -114,4 +141,6 @@ Current implementation direction:
 - Future memtest changes must also begin by reading the failed-attempt list above.
 - We should stop re-introducing "prefer built-in live-build memtest" as a default assumption without new evidence.
 - Memtest validation in `build.sh` is not optional; it is the acceptance gate that prevents another silent regression.
+- But validation output is only trustworthy if ISO reading itself succeeded. A
+  "missing memtest" warning without a successful ISO read is not evidence.
 - If we change memtest strategy again, we must update this ADR with the exact build evidence that justified the change.