Optimize how spec files are parsed

Attempt to parse an abridged version of the spec file first, i.e. one
cut off at the `%prep` line, which should still contain everything
needed to parse epoch, version and autorelease flags. Only if that
fails, attempt to parse the full spec file.

This speeds up things tremendously in the case of spec files using
macros which run subprocesses, such as certain Python macros. Even spec
files not affected by this are parsed (slightly) faster if shortened
accordingly.

Thanks to Miro Hrončok and Ben Beasley for analyzing the issue and
suggesting this optimization.

Fixes: fedora-infra#22

Signed-off-by: Nils Philippsen <nils@redhat.com>

rpmautospec/pkg_history.py

@@ -7,7 +7,7 @@
 from functools import reduce
 from pathlib import Path
 from shutil import SpecialFileError
-from tempfile import TemporaryDirectory
+from tempfile import NamedTemporaryFile, TemporaryDirectory
 from typing import Any, Optional, Sequence, Union
 
 import pygit2
@@ -104,7 +104,7 @@ def _get_rpmverflags(cls, path: str, name: Optional[str] = None) -> Optional[str
 
         python_version = str(sys.version_info[0]) + "." + str(sys.version_info[1])
 
-        rpm_cmd = [
+        rpm_cmd_base = (
             "rpm",
             "--define",
             "_invalid_encoding_terminates_build 0",
@@ -123,15 +123,33 @@ def _get_rpmverflags(cls, path: str, name: Optional[str] = None) -> Optional[str
             "--qf",
             query,
             "--specfile",
-            f"{name}.spec",
-        ]
-
-        call = subprocess.run(
-            rpm_cmd,
-            cwd=path,
-            stdout=subprocess.PIPE,
-            stderr=subprocess.PIPE,
         )
+
+        with specfile.open(mode="rb") as unabridged, NamedTemporaryFile(
+            mode="wb", prefix=f"rpmautospec-abridged-{name}-", suffix=".spec", delete=False
+        ) as abridged:
+            # Attempt to parse a shortened version of the spec file first, to speed up processing in
+            # certain cases. This includes all lines before `%prep`, i.e. in most cases everything
+            # which is needed to make RPM parsing succeed and contain the info we want to extract.
+            for line in unabridged:
+                if line.strip() == b"%prep":
+                    break
+                abridged.write(line)
+            abridged.flush()
+
+            for spec_candidate in (abridged.name, f"{name}.spec"):
+                call = subprocess.run(
+                    rpm_cmd_base + (spec_candidate,),
+                    cwd=path,
+                    stdout=subprocess.PIPE,
+                    stderr=subprocess.PIPE,
+                )
+                if not call.returncode:
+                    # Parsing this candidate spec file succeeded. In the case of the abridged spec
+                    # file, we don’t need to parse the full spec file. In the case of the latter,
+                    # breaking out explicity doesn’t make a difference.
+                    break
+
         if call.returncode != 0:
             log.debug("rpm query for %r failed: %s", query, call.stderr.decode("utf-8", "replace"))
             return None