Fix %pyproject_buildrequires -w when the build backend is already installed and pip isn't

Fixes: https://bugzilla.redhat.com/2169855

pyproject-rpm-macros.spec

@@ -191,6 +191,8 @@ export HOSTNAME="rpmbuild"  # to speedup tox in network-less mock, see rhbz#1856
 - Fixes: rhbz#2244282
 - Show a better error message when %%pyproject_install finds no wheel
 - Fixes: rhbz#2242452
+- Fix %%pyproject_buildrequires -w when the build backend is already installed and pip isn't
+- Fixes: rhbz#2169855
 
 * Wed Sep 13 2023 Python Maint <python-maint@redhat.com> - 1.10.0-1
 - Add %%_pyproject_check_import_allow_no_modules for automated environments

pyproject_buildrequires.py

@@ -328,6 +328,11 @@ def generate_run_requirements_wheel(backend, requirements, wheeldir):
     # Reuse the wheel from the previous round of %pyproject_buildrequires (if it exists)
     wheel = find_built_wheel(wheeldir)
     if not wheel:
+        # pip is already echoed from the macro
+        # but we need to explicitly restart if has not yet been installed
+        # see https://bugzilla.redhat.com/2169855
+        requirements.add('pip >= 19', source='%pyproject_buildrequires -w')
+        requirements.check(source='%pyproject_buildrequires -w')
         import pyproject_wheel
         returncode = pyproject_wheel.build_wheel(
             wheeldir=wheeldir,

pyproject_buildrequires_testcases.yaml

@@ -365,6 +365,7 @@ Run dependencies with extras and build wheel option:
     setuptools: 50
     wheel: 1
     pyyaml: 1
+    pip: 20
   include_runtime: true
   build_wheel: true
   extras:
@@ -375,6 +376,7 @@ Run dependencies with extras and build wheel option:
     python3dist(wheel)
     python3dist(wheel)
     python3dist(setuptools) >= 40
+    python3dist(pip) >= 19
     python3dist(py) >= 1.5
     python3dist(six) >= 1.10
     python3dist(setuptools)