[FIX] forwardport: don't break forward porting on huge conflicts

On forward-porting, odoo/odoo#170183 generates a conflict on pretty
much every one of the 1111 files it touches, because they are
modify/delete conflicts that generates a conflict message over 200
bytes per file, which is over 200kB of output.

For this specific scenario, the commit message was being passed
through arguments to the `git` command, resulting in a command line
exceeding `MAX_ARG_STRLEN`[^1]. The obvious way to fix this is to pass
the commit message via stdin as is done literally in the line above
where we just copy a non-generated commit message.

However I don't think hundreds of kbytes worth of stdout[^2] is of any
use, so shorten that a bit, and stderr while at it.

Don't touch the commit message size for now, possibly forever, but
note that some log diving reveals a commit with a legit 18kB message
(odoo/odoo@42a3b704f7) so if we want to
restrict that the limit should be at least 32k, and possibly 64. But
it might be a good idea to make that limit part of the ready / merge
checks too, rather than cut things off or trigger errors during
staging.

Fixes #900

[^1]: Most resources on "Argument list too long" reference `ARG_MAX`,
    but on both my machine and the server it is 2097152 (25% of the
    default stack), which is ~10x larger than the commit message we
    tried to generate. The actual limit is `MAX_ARG_STRLEN` which
    can't be queried directly but is essentially hard-coded to
    PAGE_SIZE * 32 = 128kiB, which tracks.

[^2]: Somewhat unexpectedly, that's where `git-cherry-pick` sends the
    conflict info.

forwardport/models/project.py

@@ -32,11 +32,10 @@ from odoo.osv import expression
 from odoo.tools.misc import topological_sort, groupby
 from odoo.tools.appdirs import user_cache_dir
 from odoo.addons.base.models.res_partner import Partner
-from odoo.addons.runbot_merge import git
+from odoo.addons.runbot_merge import git, utils
 from odoo.addons.runbot_merge.models.pull_requests import Branch
 from odoo.addons.runbot_merge.models.stagings_create import Message
 
-
 DEFAULT_DELTA = dateutil.relativedelta.relativedelta(days=3)
 
 _logger = logging.getLogger('odoo.addons.forwardport')
@@ -407,23 +406,25 @@ class PullRequests(models.Model):
                 err = err.rstrip() + '\n----------\nstatus:\n' + status
             else:
                 err = 'status:\n' + status
+
             # if there was a single commit, reuse its message when committing
             # the conflict
-            # TODO: still add conflict information to this?
             if len(commits) == 1:
                 msg = root._make_fp_message(commits[0])
-                conf.with_config(input=str(msg).encode()) \
-                    .commit(all=True, allow_empty=True, file='-')
             else:
-                conf.commit(
-                    all=True, allow_empty=True,
-                    message="""Cherry pick of %s failed
+                out = utils.shorten(out, 8*1024, '[...]')
+                err = utils.shorten(err, 8*1024, '[...]')
+                msg = f"""Cherry pick of {h} failed
 
 stdout:
-%s
+{out}
 stderr:
-%s
-""" % (h, out, err))
+{err}
+"""
+
+            conf.with_config(input=str(msg).encode()) \
+                .commit(all=True, allow_empty=True, file='-')
+
             return (h, out, err, [c['sha'] for c in commits]), working_copy
 
     def _cherry_pick(self, working_copy):

forwardport/tests/test_conflicts.py

@@ -1,3 +1,4 @@
+import random
 import re
 import time
 from operator import itemgetter
@@ -177,6 +178,94 @@ More info at https://github.com/odoo/odoo/wiki/Mergebot#forward-port
         'i': 'a',
     }
 
+def test_massive_conflict(env, config, make_repo):
+    """If the conflict is large enough, the commit message may exceed ARG_MAX
+     and trigger E2BIG.
+    """
+    # CONFLICT (modify/delete): <file> deleted in <commit> (<title>) and modified in HEAD.  Version HEAD of <file> left in tree.
+    #
+    # 107 + 2 * len(filename) + len(title) per conflicting file.
+    # - filename: random.randbytes(10).hex() -> 20
+    # - title: random.randbytes(20).hex() -> 40
+    # -> 701 (!) files
+
+    files = []
+    while len(files) < 1500:
+        files.append(random.randbytes(10).hex())
+
+    # region setup
+    project = env['runbot_merge.project'].create({
+        'name': "thing",
+        'github_token': config['github']['token'],
+        'github_prefix': 'hansen',
+        'fp_github_token': config['github']['token'],
+        'fp_github_name': 'herbert',
+        'branch_ids': [
+            (0, 0, {'name': 'a', 'sequence': 100}),
+            (0, 0, {'name': 'b', 'sequence': 80}),
+        ],
+    })
+
+    repo = make_repo("repo")
+    env['runbot_merge.events_sources'].create({'repository': repo.name})
+
+    repo_id = env['runbot_merge.repository'].create({
+        'project_id': project.id,
+        'name': repo.name,
+        'required_statuses': "default",
+        'fp_remote_target': repo.name,
+        'group_id': False,
+    })
+    env['res.partner'].search([
+        ('github_login', '=', config['role_reviewer']['user'])
+    ]).write({
+        'review_rights': [(0, 0, {'repository_id': repo_id.id, 'review': True})]
+    })
+
+    with repo:
+        # create branch with a ton of empty files
+        repo.make_commits(
+            None,
+            Commit(
+                random.randbytes(20).hex(),
+                tree=dict.fromkeys(files, "xoxo"),
+            ),
+            ref='heads/a',
+        )
+
+        # removes all those files in the next branch
+        repo.make_commits(
+            'a',
+            Commit(
+                random.randbytes(20).hex(),
+                tree=dict.fromkeys(files, "content!"),
+            ),
+            ref='heads/b',
+        )
+    # endregion setup
+
+    with repo:
+        # update all the files
+        repo.make_commits(
+            'a',
+            Commit(random.randbytes(20).hex(), tree={'a': '1'}),
+            Commit(random.randbytes(20).hex(), tree={'x': '1'}, reset=True),
+            ref='heads/change',
+        )
+        pr = repo.make_pr(target='a', head='change')
+        repo.post_status('refs/heads/change', 'success')
+        pr.post_comment('hansen rebase-ff r+', config['role_reviewer']['token'])
+    env.run_crons()
+
+    with repo:
+        repo.post_status('staging.a', 'success')
+    env.run_crons()
+
+    # we don't actually need more, the bug crashes the forward port entirely so
+    # the PR is never even created
+    _pra_id, _prb_id = env['runbot_merge.pull_requests'].search([], order='number')
+
+
 def test_conflict_deleted(env, config, make_repo):
     prod, other = make_basic(env, config, make_repo)
     # remove f from b

runbot_merge/utils.py

@@ -3,7 +3,7 @@ import itertools
 import time
 
 
-def shorten(text_ish, length):
+def shorten(text_ish, length, cont='...'):
     """ If necessary, cuts-off the text or bytes input and appends ellipsis to
     signal the cutoff, such that the result is below the provided length
     (according to whatever "len" means on the text-ish so bytes or codepoints
@@ -12,11 +12,10 @@ def shorten(text_ish, length):
     if len(text_ish or ()) <= length:
         return text_ish
 
-    cont = '...'
     if isinstance(text_ish, bytes):
         cont = cont.encode('ascii') # whatever
     # add enough room for the ellipsis
-    return text_ish[:length-3] + cont
+    return text_ish[:length-len(cont)] + cont
 
 BACKOFF_DELAYS = (0.1, 0.2, 0.4, 0.8, 1.6)
 def backoff(func=None, *, delays=BACKOFF_DELAYS, exc=Exception):