tools/analyze-dump-sort: a helper to compare two 'systemd-analyze dump' outputs

Lines in the dumps are ordered by some pseudo-random hashmap entry order, which
makes it hard to diff two outputs. This sort the entries alphabetically, and
also sorts items within the entries, and supresses timestamps and other fields
which always vary.

We could sort the output inside of systemd itself, but it'd make things more
complex, and we probably don't need output to be sorted in most cases. It also
wouldn't be enough, because timestamps and such would still need to be ignored
to do a nice diff. So I think doing the sorting and suppression in a python
helper is a better approach.

tools/analyze-dump-sort.py

@@ -0,0 +1,78 @@
+#!/usr/bin/python
+# SPDX-License-Identifier: LGPL-2.1-or-later
+
+"""
+A helper to compare 'systemd-analyze dump' outputs.
+
+systemd-analyze dump >/var/tmp/dump1
+(reboot)
+tools/analyze-dump-sort.py /var/tmp/dump1 → this does a diff from dump1 to current
+
+systemd-analyze dump >/var/tmp/dump2
+tools/analyze-dump-sort.py /var/tmp/{dump1,dump2} → this does a diff from dump1 to dump2
+"""
+
+import argparse
+import tempfile
+import subprocess
+
+def sort_dump(sourcefile, destfile=None):
+    if destfile is None:
+        destfile = tempfile.NamedTemporaryFile('wt')
+
+    units = {}
+    unit = []
+
+    same = []
+
+    for line in sourcefile:
+        line = line.rstrip()
+
+        header = line.split(':')[0]
+        if 'Timestamp' in header or 'Invocation ID' in header or 'PID' in header:
+            line = header + ': …'
+
+        if line.startswith('->'):
+            if unit:
+                units[unit[0]] = unit
+            unit = [line]
+        elif line.startswith('\t'):
+            assert unit
+
+            if same and same[0].startswith(header):
+                same.append(line)
+            else:
+                unit.extend(sorted(same, key=str.lower))
+                same = [line]
+        else:
+            print(line, file=destfile)
+
+    if unit:
+        units[unit[0]] = unit
+
+    for unit in sorted(units.values()):
+        print('\n'.join(unit), file=destfile)
+
+    destfile.flush()
+    return destfile
+
+def parse_args():
+    p = argparse.ArgumentParser(description=__doc__)
+    p.add_argument('one')
+    p.add_argument('two', nargs='?')
+    p.add_argument('--user', action='store_true')
+    return p.parse_args()
+
+if __name__ == '__main__':
+    opts = parse_args()
+
+    one = sort_dump(open(opts.one))
+    if opts.two:
+        two = sort_dump(open(opts.two))
+    else:
+        user = ['--user'] if opts.user else []
+        two = subprocess.run(['systemd-analyze', 'dump', *user],
+                             capture_output=True, text=True, check=True)
+        two = sort_dump(two.stdout.splitlines())
+    with subprocess.Popen(['diff', '-U10', one.name, two.name], stdout=subprocess.PIPE) as diff:
+        subprocess.Popen(['less'], stdin=diff.stdout)