Define compat shims for %patchN when building for EL <= 10

Thanks Ray Strode for the idea of searching the spec file for used %patchN macros.

Signed-off-by: Miro Hrončok <miro@hroncok.cz>

src/centpkg/__init__.py

@@ -16,10 +16,12 @@
 # the full text of the license.
 
 
+import os
 import re
 import warnings
 
 import git
+import rpm
 from pyrpkg import Commands, rpkgError
 from pyrpkg.utils import cached_property
 
@@ -201,6 +203,55 @@ class Commands(Commands):
                     self.branch_merge,
                 )
 
+    def _define_patchn_compatiblity_macros(self):
+        """
+        RPM 4.19 deprecated the %patchN macro. RPM 4.20 removed it completely.
+        The macro works on c8s, c9s, c10s, but does not work on Fedora 41+.
+        We can no longer even parse RPM spec files with the %patchN macros.
+        When we build for old streams, we define the %patchN macros manually as %patch -P N.
+        Since N can be any number including zero-prefixed numbers,
+        we regex-search the spec file for %patchN uses and define only the macros found.
+        """
+        # Only do this on RPM 4.19.90+ (4.19.9x were pre-releases of 4.20)
+        if tuple(int(i) for i in rpm.__version_info__) < (4, 19, 90):
+            return
+        # Only do this when building for CentOS Stream version with RPM < 4.20
+        try:
+            if int(self._distval.split("_")[0]) > 10:
+                return
+        except ValueError as e:
+            self.log.debug(
+                "Cannot parse major dist version as int: %s",
+                self._distval.split("_")[0],
+                exc_info=e,
+            )
+            return
+        defined_patchn = False
+        try:
+            specfile_path = os.path.join(self.layout.specdir, self.spec)
+            with open(specfile_path, "rb") as specfile:
+                # Find all uses of %patchN in the spec files
+                # Using a benevolent regex: commented out macros, etc. match as well
+                for patch in re.findall(rb"%{?patch(\d+)\b", specfile.read()):
+                    # We operate on bytes becasue we don't know the spec encoding
+                    # but the matched part only includes ASCII digits
+                    patch = patch.decode("ascii")
+                    self._rpmdefines.extend(
+                        [
+                            "--define",
+                            # defines parametric macro %patchN which passes all arguments to %patch -P N
+                            "patch%s(-) %%patch -P %s %%{?**}" % (patch, patch),
+                        ]
+                    )
+                    defined_patchn = True
+        except OSError as e:
+            self.log.debug("Cannot read spec.", exc_info=e)
+        if defined_patchn:
+            self.log.warn(
+                "centpkg defined %patchN compatibility shims to parse the spec file. "
+                "%patchN is obsolete, use %patch -P N instead."
+            )
+
     # redefined loaders
     def load_rpmdefines(self):
         """
@@ -245,6 +296,7 @@ class Commands(Commands):
             "--eval",
             "%%undefine %s" % self._distunset,
         ]
+        self._define_patchn_compatiblity_macros()
         self.log.debug("RPMDefines: %s" % self._rpmdefines)
 
     def construct_build_url(self, *args, **kwargs):