Only %prep once when running %generate_buildrequires multiple times

This saves time and IO and I believe this is what was originally intended.

While doing this I moved some code around. Namely:

 - moved the variables used only in the if dynamic_buildrequires block inside it
 - ensured the --nodeps for the final rpmbuild call is only added once

mock/py/mockbuild/backend.py

@@ -747,17 +747,18 @@ def get_command(mode, checkdeps=False):
             return command
 
         bd_out = self.make_chroot_path(self.buildroot.builddir)
-        max_loops = int(self.config.get('dynamic_buildrequires_max_loops'))
-        success = False
         dynamic_buildrequires = dynamic_buildrequires and self.config.get('dynamic_buildrequires')
         if dynamic_buildrequires:
+            max_loops = int(self.config.get('dynamic_buildrequires_max_loops'))
+            success = False
+            br_mode = ['-br']
             while not success and max_loops > 0:
                 # run rpmbuild+installSrpmDeps until
                 # * it fails
                 # * installSrpmDeps does nothing
                 # * or we run out of dynamic_buildrequires_max_loops tries
                 packages_before = self.buildroot.all_chroot_packages()
-                command = get_command(['-br'])
+                command = get_command(br_mode)
                 (output, returncode) = \
                     self.buildroot.doChroot(command,
                                             shell=False, logger=self.buildroot.build_log, timeout=timeout,
@@ -781,13 +782,14 @@ def get_command(mode, checkdeps=False):
                     success = True
                 for f_buildreqs in buildreqs:
                     os.remove(f_buildreqs)
-                if not sc:
-                    # We want to (re-)write src.rpm with dynamic BuildRequires,
-                    # but with short-circuit it doesn't matter
-                    mode = ['-ba']
-                # rpmbuild -br already does %prep, so we don't need waste time
-                # on re-doing it
-                mode += ['--noprep']
+                # The first rpmbuild -br already did %prep, so we don't need waste time
+                if '--noprep' not in br_mode:
+                    br_mode += ['--noprep']
+            if not sc:
+                # We want to (re-)write src.rpm with dynamic BuildRequires,
+                # but with short-circuit it doesn't matter
+                mode = ['-ba']
+            mode += ['--noprep']
 
         # When we used dynamic buildrequires, we must let rpmbuild check deps.
         # The rpmbuild -ba call executes the %generate_buildrequires once again

releng/release-notes-next/generate_buildrequires_prep.feature

@@ -0,0 +1,22 @@
+Only run the `%prep` section once when running `%generate_buildrequires`
+multiple times.
+Previously Mock run `%prep` repeatedly before each `%generate_buildrequires`
+round except for the last one.
+This was inconsistent and unnecessary slow/wasteful.
+
+When the original support for `%generate_buildrequires` landed into Mock,
+the intention was to only call `%prep` once.
+However when Mock added support for multiple rounds of
+`%generate_buildrequires`, `%prep` ended up only being skipped in the final
+`rpmbuild` call. This was an oversight.
+`%prep` is now only called once, as originally intended.
+
+Some RPM packages might be affected by the change, especially if a dirty
+working directory after running `%generate_buildrequires` affects the results
+of subsequent rounds of `%generate_buildrequires`.
+However, such behavior was undefined and quite buggy even previously,
+due to the lack of the `%prep` section in the final `rpmbuild` call.
+
+Packages that need to run commands before every round of
+`%generate_buildrequires` should place those commands in
+the `%generate_buildrequires` section itself rather than `%prep`.