journal-file: let's make journal_file_copy_entry() robust against concurrent writing of the source

As usual, we need to protect ourselves against concurrent modification
of journal files. We a pretty good at that these days when reading
journal files. But journal_file_copy_entry() so far wasn't too good with
that. journal_file_append_data() so far returned EINVAL when you pass
invalid data to it. Since we pass the source data as-is in there, it's
going to fail if the journal source file is slightly invalid due to a
concurrent update.

Hence, we need to validate data gracefully here that we think comes from
a safe place, because actually it doesn't, it's directly copied from an
unsafe journal file.

Hence, let's introduce a clear error code here, and look for it in
journal_file_copy_entry(), and handle it gracefully.

Pretty sure this fixes #33372, but it's a race, so I don't know for
sure. If this remains reproducible we need to look at this again.

Fixes: #33372

src/libsystemd/sd-journal/journal-file.c

@@ -1842,8 +1842,11 @@ static int journal_file_append_data(
 
         assert(f);
 
+        /* Return a recognizable error for data that is submitted for *writing*. Note that we leave EBADMSG
+         * for bad messages we *read*. And EINVAL is too generic an error (which might be triggered by all
+         * kinds of other places). Hence use a separate EUCLEAN here. */
         if (!data || size == 0)
-                return -EINVAL;
+                return -EUCLEAN;
 
         hash = journal_file_hash_data(f, data, size);
 
@@ -1855,7 +1858,7 @@ static int journal_file_append_data(
 
         eq = memchr(data, '=', size);
         if (!eq)
-                return -EINVAL;
+                return -EUCLEAN;
 
         osize = journal_file_data_payload_offset(f) + size;
         r = journal_file_append_object(f, OBJECT_DATA, osize, &o, &p);
@@ -4456,10 +4459,11 @@ int journal_file_copy_entry(
                         return r;
                 assert(r > 0);
 
-                if (l == 0)
-                        return -EBADMSG;
-
                 r = journal_file_append_data(to, data, l, &u, &h);
+                if (r == -EUCLEAN) {
+                        log_debug_errno(r, "Entry item %"PRIu64" invalid, skipping over it: %m", i);
+                        continue;
+                }
                 if (r < 0)
                         return r;