sagemathgh-39527: Test on CI that update-meson is properly ran
    
Fixes sagemath#39172


Side note, can we just include the `__init__.py` to be tracked by git?

### 📝 Checklist

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- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [x] I have linked a relevant issue or discussion.
- [x] I have created tests covering the changes. (not applicable)
- [x] I have updated the documentation and checked the documentation
preview. (no documentation change? Do we need to mention this in
developer guide somewhere?)

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on. For example,
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<!-- - sagemath#12345: short description why this is a dependency -->
<!-- - sagemath#34567: ... -->
    
URL: sagemath#39527
Reported by: user202729
Reviewer(s): Tobias Diez, user202729

Author: Release Manager

.github/workflows/ci-meson.yml

@@ -72,6 +72,19 @@ jobs:
           # Use --no-deps and pip check below to verify that all necessary dependencies are installed via conda
           pip install --no-build-isolation --no-deps --config-settings=builddir=builddir . -v
 
+      - name: Check update-meson
+        # this step must be after build, because meson.build creates a number of __init__.py files
+        # that is needed to make tools/update-meson.py run correctly
+        shell: bash -l {0}
+        run: |
+          python3 tools/update-meson.py
+          if ! ./tools/test-git-no-uncommitted-changes; then
+            git add --intent-to-add .  # also show newly created files in git diff
+            git status
+            git diff
+            false
+          fi
+
       - name: Verify dependencies
         shell: bash -l {0}
         run: pip check

.gitignore

@@ -466,3 +466,8 @@ src/sage/libs/mpfr/__init__.py
 src/sage/libs/mpc/__init__.py
 src/sage/calculus/transforms/__init__.py
 src/sage/calculus/__init__.py
+
+# Temporary files generated by Meson CI (needed to make test pass because
+# ci-meson.yml runs a `./tools/test-git-no-uncommitted-changes` step)
+/.ccache
+/setup-miniconda-patched-environment-*.yml

Makefile

@@ -254,13 +254,7 @@ TEST_TARGET = $@
 TEST = ./sage -t --logfile=$(TEST_LOG) $(TEST_FLAGS) --optional=$(TEST_OPTIONAL) $(TEST_FILES)
 
 test-git-no-uncommitted-changes:
-	@UNCOMMITTED=$$(git status --porcelain); \
-	if [ -n "$$UNCOMMITTED" ]; then \
-	    echo "Error: the git repo has uncommitted changes:"; \
-	    echo "$$UNCOMMITTED"; \
-	    echo; \
-	    exit 1; \
-	fi
+	./tools/test-git-no-uncommitted-changes
 
 test: all
 	@echo '### make $(TEST_TARGET): Running $(TEST)' >> $(TEST_LOG)

src/sage/data_structures/meson.build

@@ -42,9 +42,7 @@ foreach name, pyx : extension_data
   )
 endforeach
 
-extension_data_cpp = {
-  'pairing_heap' : files('pairing_heap.pyx'),
-}
+extension_data_cpp = {'pairing_heap' : files('pairing_heap.pyx')}
 
 foreach name, pyx : extension_data_cpp
   py.extension_module(

tools/test-git-no-uncommitted-changes

@@ -0,0 +1,10 @@
+#!/usr/bin/env sh
+# Test that there is no uncommitted changes in the repository. Return failure if there is.
+# Can also be invoked with `make test-git-no-uncommitted-changes` from top level.
+UNCOMMITTED="$(git status --porcelain)";
+if [ -n "$UNCOMMITTED" ]; then
+	echo "Error: the git repo has uncommitted changes:";
+	echo "$UNCOMMITTED";
+	echo;
+	exit 1;
+fi