FEATURE-ci-windows-stage-compiler: Capture bounded configure diagnostics

Retain the last actual configure check before fixture cleanup while preserving RuntimeError and installed-matrix tail visibility. Prepare a temporary manual Windows-only selector with unchanged native Windows steps; task remains In Progress pending one caller-owned native capture and proven repair.

Verified real-configure RED/GREEN and over-budget diagnostics, healthy eight-case fixture, unchanged test and production-driver invariants, four workflow tests, actionlint, shell syntax, Python syntax and whitespace.

Author: etr

.github/workflows/v3-package-validation.yml

@@ -5,10 +5,17 @@ on:
   pull_request:
     branches: [v3, master]
   workflow_dispatch:
+    # Temporary diagnostic selector; remove before merging this task into v3.
+    inputs:
+      windows_only:
+        description: 'Capture Windows diagnostics; other platform jobs are skipped'
+        type: boolean
+        default: false
 permissions:
   contents: read
 jobs:
   linux:
+    if: github.event_name != 'workflow_dispatch' || !inputs.windows_only
     runs-on: ubuntu-24.04
     timeout-minutes: 90
     steps:
@@ -44,6 +51,7 @@ jobs:
           retention-days: 14
           if-no-files-found: error
   macos:
+    if: github.event_name != 'workflow_dispatch' || !inputs.windows_only
     runs-on: macos-15
     timeout-minutes: 90
     steps:
@@ -148,6 +156,7 @@ jobs:
           retention-days: 14
           if-no-files-found: error
   freebsd:
+    if: github.event_name != 'workflow_dispatch' || !inputs.windows_only
     runs-on: ubuntu-24.04
     timeout-minutes: 180
     steps:

scripts/test-v3-stage-install.py

@@ -3,6 +3,7 @@
 import os
 from pathlib import Path
 import platform
+import re
 import shlex
 import shutil
 import subprocess
@@ -49,7 +50,21 @@ def setUpClass(cls):
              'RANLIB=' + cls.shell_path(cls.build / 'native-ranlib-boundary.sh')], cwd=cls.build,
             env=cls.environment, stdout=subprocess.PIPE, stderr=subprocess.STDOUT, text=True)
         if result.returncode:
-            raise RuntimeError(result.stdout)
+            # Preserve the failed check before class cleanup removes config.log.
+            try:
+                log = (cls.build / 'config.log').read_text(errors='replace')
+                core = log.split('## Cache variables.', 1)[0]
+                checks = list(re.finditer(r'^configure:\d+: checking ', core, re.MULTILINE))
+                diagnostic = core[checks[-1].start():] if checks else ''
+                diagnostic = diagnostic.split('\nconfigure: failed program was:', 1)[0]
+                excerpt = diagnostic.encode()
+                # The installed matrix retains only the final 3000 output bytes.
+                if len(excerpt) > 2000:
+                    excerpt = excerpt[:1000] + b'\n[configure check truncated]\n' + excerpt[-1000:]
+                diagnostic = '\nLast configure check (config.log):\n' + excerpt.decode(errors='replace')
+            except OSError as error:
+                diagnostic = '\nCannot read fixture config.log: ' + str(error)
+            raise RuntimeError(result.stdout + diagnostic)
         cls.installer = cls.build / 'native-install-boundary.sh'
         # A tiny real library reaches Automake/libtool's archive and DLL install
         # paths without compiling the unrelated protocol engines in each fixture.

specs/tasks/FEATURE-ci-windows-stage-compiler.md

@@ -0,0 +1,71 @@
+# FEATURE-ci-windows-stage-compiler: retain the native fixture compiler diagnostic
+
+Status: In Progress — diagnostic checkpoint ready; native compiler cause and repair pending.
+Base: `v3` at `6fc672326b9cad8f4aa068b9641fc03e21218e31`.
+
+The fresh native Windows package job `120607477636` and archive job
+`120608161470` reach the fixture's C compiler check, then report that the compiler
+cannot create executables. The fixture's configure stdout omits the actual
+compiler/linker stderr, and class cleanup removes its temporary `config.log`.
+The existing Windows package artifact `12526449139` retains the successful outer
+configure log, but not the failing inner fixture log. The native cause is unknown.
+
+The first checkpoint reports the last actual configure check before cleanup,
+excluding cache/output-variable sections and the failed source dump. Long check
+output retains its first and last 1000 bytes with a truncation marker. This keeps
+the command and diagnostic near the end of the exception for the installed
+matrix's final-3000-byte error report. Configure failure still raises
+`RuntimeError`, and normal temporary-directory cleanup still runs.
+
+Action items:
+
+- [x] Read primary CLAUDE.md/AGENTS.md, saved plan, literal journal, complete fixture,
+  existing workflow, and retained outer Windows configure/command context.
+- [x] Create the isolated Groundwork worktree from the exact v3 base; preserve
+  unrelated work.
+- [x] Prove RED with actual configure and a deterministic invalid clang option:
+  compiler stderr exists in `config.log` but is absent from the original exception.
+- [x] Add bounded failure reporting before cleanup; preserve the original failure.
+- [x] Prove GREEN with actual configure, including an over-budget real compiler
+  diagnostic; retain compiler command/stderr in the final 3000 output bytes.
+- [x] Preserve all eight original test-method ASTs, configured `D:/ci/prefix`, MSYS
+  shell/path handoffs, real libtool operations, and production Makefile drivers.
+- [x] Run the healthy eight-case fixture and affected syntax/workflow checks.
+- [x] Prepare a temporary branch-only `windows_only` manual dispatch input in the
+  existing package workflow; keep the Windows job and normal push/PR behavior
+  unchanged. The three other jobs are explicitly skipped for that selection.
+- [ ] Caller: review checkpoint, publish only this task branch, and dispatch one
+  manual Windows-only native control using `windows_only=true`.
+- [ ] Read the actual native diagnostic and adjudicate the cause. If the control
+  passes, record intermittency/uncertainty and stop; do not retry unchanged.
+- [ ] Resume this same task for a proven minimal repair with relevant TDD.
+- [ ] Remove every temporary workflow change, restoring the exact base workflow
+  before final validation and merge into v3.
+- [ ] Caller: complete required Groundwork validation and finalization.
+
+Checkpoint verification:
+
+- Bootstrap passes with the existing .PHONY / AC_OUTPUT warnings.
+- Original healthy fixture: all 8 tests pass in 19.173s.
+- Actual-configure RED fails because the emitted exception omits clang's
+  `unknown argument` diagnostic; the original `RuntimeError` and cleanup are
+  observed. GREEN retains the actual command and compiler stderr in the final
+  3000 bytes; the same proof with 80 invalid options verifies bounded truncation.
+- Changed healthy fixture: all 8 tests pass in 20.183s.
+- Python syntax, original eight test ASTs, unchanged setup outside failure
+  reporting, unchanged MSYS handoffs and production driver bytes pass.
+- All 4 existing package workflow tests pass. Actionlint 1.7.7 passes with its
+  optional shellcheck integration disabled (shellcheck is unavailable locally).
+  `bash -n` passes for all 18 existing shell step bodies. Parsed workflow
+  comparison confirms the entire Windows job, action pins, privileges and normal
+  workflow are unchanged apart from the temporary dispatch/job selectors.
+- `git diff --check` passes. CLAUDE.md's C++ cpplint does not apply to these Python,
+  YAML and task-record changes; no Python linter is documented.
+
+Evidence and the task-local executable probes are retained in the primary
+checkout at `.debug/ci-windows-stage-compiler-lbpdh7hb/implementation/`; the literal
+journal is `.debug/ci-windows-stage-compiler-lbpdh7hb/journal.md`.
+No native Windows execution, compiler/flag/runtime repair, full validation,
+publication or merge is claimed at this checkpoint. Native capture, the accepted
+repair, workflow restoration and final checks remain incomplete. Other platform
+failures and earlier bounded investigations are outside this checkpoint.