Generate python3dist(toml) BuildRequires directly from the macro

The macro checks if pyproject.toml exists and echoes the dependency early.

For projects with pyproject.toml, this saves one installation round.
Previously, the installation steps by %generate_buildrequires were:

 1. (python3-devel +) pip + packaging
 2. toml
 3. parsed dependencies from pyproject.toml
 4. ...

Now they are:

 1. (python3-devel +) pip + packaging + toml
 2. parsed dependencies from pyproject.toml
 3. ...

For projects without pyproject.toml, the number of rounds remains the same:

 1. (python3-devel +) pip + packaging
 2. setuptools + wheel
 3. ...

This is also more consistent:
The Python script now only outputs dependencies of the probed project,
it no longer partially outputs dependencies for itself.

macros.pyproject

@@ -85,6 +85,9 @@ fi
 echo 'python%{python3_pkgversion}-devel'
 echo 'python%{python3_pkgversion}dist(pip) >= 19'
 echo 'python%{python3_pkgversion}dist(packaging)'
+if [ -f pyproject.toml ]; then
+  echo 'python%{python3_pkgversion}dist(toml)'
+fi
 # Check if we can generate dependencies on Python extras
 if [ "%{py_dist_name []}" == "[]" ]; then
   extras_flag=%{?!_python_no_extras_requires:--generate-extras}

pyproject-rpm-macros.spec

@@ -106,6 +106,7 @@ export HOSTNAME="rpmbuild"  # to speedup tox in network-less mock, see rhbz#1856
 %changelog
 * Tue Feb 02 2021 Miro Hrončok <mhroncok@redhat.com> - 0-37
 - Remove support for Python 3.7 from %%pyproject_buildrequires
+- Generate python3dist(toml) BR with pyproject.toml earlier to avoid extra install round
 
 * Wed Jan 27 2021 Fedora Release Engineering <releng@fedoraproject.org> - 0-36
 - Rebuilt for https://fedoraproject.org/wiki/Fedora_34_Mass_Rebuild

pyproject_buildrequires.py

@@ -152,10 +152,13 @@ def get_backend(requirements):
     except FileNotFoundError:
         pyproject_data = {}
     else:
+        try:
             # lazy import toml here, not needed without pyproject.toml
-        requirements.add('toml', source='parsing pyproject.toml')
-        requirements.check(source='parsing pyproject.toml')
             import toml
+        except ImportError as e:
+            print_err('Import error:', e)
+            # already echoed by the %pyproject_buildrequires macro
+            sys.exit(0)
         with f:
             pyproject_data = toml.load(f)
 

pyproject_buildrequires_testcases.yaml

@@ -12,7 +12,8 @@ Nothing installed yet:
   pyproject.toml: |
     # empty
   expected: |
-    python3dist(toml)
+    python3dist(setuptools) >= 40.8
+    python3dist(wheel)
   result: 0
 
 Insufficient version of setuptools:
@@ -23,7 +24,6 @@ Insufficient version of setuptools:
   pyproject.toml: |
     # empty
   expected: |
-    python3dist(toml)
     python3dist(setuptools) >= 40.8
     python3dist(wheel)
   result: 0
@@ -48,7 +48,6 @@ Default build system, empty setup.py:
     # empty
   setup.py: |
   expected: |
-    python3dist(toml)
     python3dist(setuptools) >= 40.8
     python3dist(wheel)
     python3dist(wheel)
@@ -93,7 +92,6 @@ Build system dependencies in pyproject.toml with extras:
         "py3 ; python_version > '3.0'",
     ]
   expected: |
-    python3dist(toml)
     python3dist(foo)
     python3dist(bar) > 5
     python3dist(bar[baz]) > 5
@@ -124,7 +122,6 @@ Build system dependencies in pyproject.toml without extras:
         "multi[extras1,extras2] == 6.0",
     ]
   expected: |
-    python3dist(toml)
     python3dist(bar) > 5
     python3dist(multi) = 6
     python3dist(setuptools) >= 40.8