Enforce macro canonical-vs-chapter-copy identity via sync script and CI

The repository ships a copy of apple-linux-convergence.S in each
chapter directory that demonstrates assembly (11 copies at last count,
plus the canonical one in macros/) so that readers browsing or
downloading a single chapter from GitHub have the macros sitting
right next to the sources that use them. That self-containment is
worth keeping. Manual synchronization of 12 copies on every macro
edit is not: all 11 are currently byte-identical to the canonical,
but the first drift is a matter of when, not if, and diagnosing
"which chapter broke when I added a new macro" after the fact is
a bad time.

This commit turns "the copies are in sync" from a hope into a
machine-enforced invariant:

- scripts/sync-macros.sh: walks macros/*.S, finds every file with
  the same basename anywhere else in the repo (excluding .git/ and
  macros/ itself), and overwrites any copy that differs. Idempotent;
  prints only the files it actually changed plus a summary. Uses
  only POSIX tools (find, cmp, cp, basename) plus bash builtins
  under a #!/usr/bin/env bash shebang. Verified working under both
  macOS bash 3.2.57 and zsh 5.9 on clean-tree and drift-repair
  paths.

- .github/workflows/check-macros.yml: runs the sync script on every
  push and pull request, then fails the job if git diff --exit-code
  shows the script produced any uncommitted change. The failure
  message tells the author exactly what to do (run the script
  locally, commit the result).

- macros/README.md: new "Source of truth" section marking the
  chapter copies as derived artifacts, pointing editors at the
  sync script, and stating that CI enforces the invariant.

Rejected alternatives:

- Symlinking each chapter copy to macros/apple-linux-convergence.S.
  Cheapest option (zero infrastructure) and git handles symlinks
  natively, but Windows checkouts without Developer Mode replace
  the symlink with a plain-text file containing the target path.
  This book's audience is overwhelmingly Linux and Apple Silicon,
  so the Windows hazard is mostly theoretical, but a sync-and-check
  approach works in every clone environment and makes the
  source-of-truth relationship explicit rather than implicit in a
  filesystem feature.

- Having each chapter .include the canonical file via a relative
  path. Breaks the "self-contained chapter" property the copies
  exist to preserve; a reader who downloads one chapter gets a
  broken build because macros/ is not beside it.

- Making the copies build-time artifacts (generated by make, not
  committed). Same problem: a reader browsing one chapter on
  GitHub no longer sees the macro file they need.

Tests:

- ./scripts/sync-macros.sh run on the current tree reports
  "macros already in sync (11 chapter copies checked)" and exits 0.
- Injecting a trailing-line perturbation into a chapter copy and
  re-running the script: detects the drift, reports "synced: <path>",
  and restores the file to canonical. Verified under both bash and
  zsh, both paths.

.github/workflows/check-macros.yml

@@ -0,0 +1,41 @@
+# check-macros.yml
+#
+# Verifies that every chapter-level copy of a macro file is byte-identical
+# to its canonical version in macros/. Runs scripts/sync-macros.sh and
+# fails the job if the script produced any change that was not already
+# committed.
+#
+# Rationale: the repo ships one copy of each macro file per chapter so
+# that each chapter directory remains self-contained on GitHub. Those
+# copies are derived artifacts; macros/*.S is the single source of
+# truth. This check turns "copies are in sync" from a convention into
+# a machine-enforced invariant, so drift cannot sneak into main.
+#
+# To fix a failure locally:
+#     ./scripts/sync-macros.sh
+#     git add -u
+#     git commit --amend --no-edit    # or a new commit, whichever you prefer
+
+name: Check macro sync
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+jobs:
+  check-macros:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Run sync script
+        run: ./scripts/sync-macros.sh
+
+      - name: Fail if any chapter copy drifted from canonical
+        run: |
+          if ! git diff --exit-code; then
+            echo "::error::Chapter macro copies drifted from the canonical files in macros/."
+            echo "::error::Run ./scripts/sync-macros.sh locally and commit the result."
+            exit 1
+          fi

macros/README.md

@@ -6,6 +6,28 @@ on Apple Silicon and Linux machines without change.
 
 The work is ongoing and subject to change.
 
+## Source of truth
+
+The files in this directory (`macros/*.S`) are the **canonical**
+versions of the macros. Every chapter directory that demonstrates
+assembly code keeps a copy of `apple-linux-convergence.S` alongside
+its sources, so that a reader browsing or downloading a single
+chapter on GitHub has the macros sitting right next to the `.S`
+files that use them.
+
+Those chapter-level copies are **derived artifacts**. Do not edit
+them. Edit the file here in `macros/`, then run:
+
+```
+./scripts/sync-macros.sh
+```
+
+from the repository root to propagate the change to every chapter
+copy. A GitHub Actions job (`.github/workflows/check-macros.yml`)
+re-runs the sync script on every push and pull request and fails
+the build if any copy has drifted from canonical, so this invariant
+cannot silently break.
+
 There are limits to what these macros can do. Variadic functions such as
 `printf()` must be handled via parallel code paths (i.e. use of `#if`).
 

scripts/sync-macros.sh

@@ -0,0 +1,69 @@
+#!/usr/bin/env bash
+#
+# sync-macros.sh
+#
+# Propagate canonical macro files from macros/ into the per-chapter
+# copies scattered throughout the repository.
+#
+# Source of truth:
+#     Any file in macros/*.S at the repository root.
+#
+# Targets:
+#     Every file elsewhere in the repository whose basename matches a
+#     file in macros/. The chapter copies exist so that each chapter
+#     directory remains self-contained (a reader can download a single
+#     chapter from GitHub and have the macros it needs sitting right
+#     next to the source files). That self-containment is valuable;
+#     manual synchronization of ~12 copies is not. This script lets us
+#     have both.
+#
+# Operation:
+#     For each canonical file, find every copy with the same basename
+#     outside macros/ and overwrite it if (and only if) it differs.
+#     Prints one line per file actually modified; prints nothing beyond
+#     a final summary if everything was already in sync.
+#
+# Intended use:
+#     Run after editing any file in macros/, then commit the changes
+#     (the canonical file plus all updated copies). CI verifies this
+#     was done by re-running the script and requiring `git diff
+#     --exit-code` to be clean.
+#
+# Perry Kivolowitz
+# A Gentle Introduction to Assembly Language
+
+set -euo pipefail
+
+REPO_ROOT="$(git rev-parse --show-toplevel)"
+cd "$REPO_ROOT"
+
+if [ ! -d macros ]; then
+    echo "error: macros/ directory not found at repo root" >&2
+    exit 1
+fi
+
+changed=0
+checked=0
+
+for canonical in macros/*.S; do
+    [ -f "$canonical" ] || continue
+    name="$(basename "$canonical")"
+
+    # Find every file in the repo with this basename, excluding the
+    # canonical location itself and anything under .git/. The -type f
+    # filter skips any stray symlinks a prior experiment may have left.
+    while IFS= read -r copy; do
+        checked=$((checked + 1))
+        if ! cmp -s "$canonical" "$copy"; then
+            cp "$canonical" "$copy"
+            echo "synced: $copy  <-  $canonical"
+            changed=$((changed + 1))
+        fi
+    done < <(find . -name "$name" -not -path './macros/*' -not -path './.git/*' -type f)
+done
+
+if [ "$changed" -eq 0 ]; then
+    echo "macros already in sync ($checked chapter copies checked)"
+else
+    echo "synced $changed of $checked chapter copies"
+fi