fedora: check existing spec file for rpmautospec

If we have a new file: assume yes. If there was an old file, convert
only when the old file had %autochangelog.

The regexp for %autochangelog is taken from
https://pagure.io/fedora-infra/rpmautospec/pull-request/219.

rust2rpm/__main__.py

@@ -5,6 +5,7 @@ from datetime import datetime, timezone
 import difflib
 import itertools
 import os
+import pathlib
 import re
 import shlex
 import shutil
@@ -258,9 +259,24 @@ def to_list(s):
         return []
     return list(filter(None, (l.strip() for l in s.splitlines())))
 
+def detect_rpmautospec(default_target, spec_file):
+    # We default to on only for selected distros for now…
+    if default_target not in {"fedora"}:
+        return False
+
+    try:
+        text = spec_file.read_text()
+    except FileNotFoundError:
+        # A new file, let's try the new thing.
+        return True
+
+    # We are redoing an existing spec file. Figure out if it had
+    # %autochangelog enabled before.
+    autochangelog_re = r"^\s*%(?:autochangelog|\{\??autochangelog\})\b"
+    return any(re.match(autochangelog_re, line) for line in text.splitlines())
+
 def main():
     default_target = get_default_target()
-    default_rpmautospec = default_target in {"fedora"}
 
     parser = argparse.ArgumentParser("rust2rpm",
                                      formatter_class=argparse.RawTextHelpFormatter)
@@ -279,7 +295,7 @@ def main():
                         help="Do initial patching of Cargo.toml")
     parser.add_argument("-s", "--store-crate", action="store_true",
                         help="Store crate in current directory")
-    parser.add_argument("-a", "--rpmautospec", action="store_true", default=default_rpmautospec,
+    parser.add_argument("-a", "--rpmautospec", action="store_true",
                         help="Use autorelease and autochangelog features")
     parser.add_argument("--all-features", action="store_true",
                         help="Activate all available features")
@@ -340,8 +356,22 @@ def main():
         raise ValueError("No bins and no libs")
     kwargs["include_devel"] = is_lib
 
+    if args.suffix is not None:
+        if metadata.name[-1].isdigit():
+            suffix = f'-{args.suffix}'
+        else:
+            suffix = args.suffix
+    else:
+        suffix = ""
+    kwargs["pkg_suffix"] = suffix
+    spec_file = pathlib.Path(f"rust-{metadata.name}{suffix}.spec")
+
     kwargs["auto_changelog_entry"] = not args.no_auto_changelog_entry
-    kwargs["rpmautospec"] = args.rpmautospec
+
+    if args.rpmautospec:
+        kwargs["rpmautospec"] = True
+    else:
+        kwargs["rpmautospec"] = detect_rpmautospec(default_target, spec_file)
 
     if args.target in {"fedora", "mageia", "opensuse"}:
         kwargs["include_build_requires"] = True
@@ -394,15 +424,6 @@ def main():
 
     kwargs["distconf"] = conf[args.target]
 
-    if args.suffix is not None:
-        if metadata.name[-1].isdigit():
-            suffix = f'-{args.suffix}'
-        else:
-            suffix = args.suffix
-    else:
-        suffix = ""
-    kwargs["pkg_suffix"] = suffix
-    spec_file = f"rust-{metadata.name}{suffix}.spec"
     spec_contents = template.render(md=metadata, patch_file=patch_file, **kwargs)
     if args.stdout:
         print(f"# {spec_file}")