Add log_merge_force_partial trigger for testing partial merges.

Add a trigger that forces btree_merge() to return -ERANGE after
modifying a leaf's worth of items, causing many small partial merges
per merge cycle. This is used by tests to reliably reproduce races
that depend on partial merges splicing items into fs_root while
finalized logs still exist.

The trigger check lives inside btree_merge() where it can observe
actual item modification progress, rather than overriding the
caller's dirty byte limit argument which applies to the whole
writer context.

Signed-off-by: Auke Kok <auke.kok@versity.com>

kmod/src/btree.c

@@ -2486,6 +2486,14 @@ int scoutfs_btree_merge(struct super_block *sb,
 			mitem = next_mitem(mitem);
 			free_mitem(&rng, tmp);
 		}
+
+		if (mitem && walk_val_len == 0 &&
+		    !(walk_flags & (BTW_INSERT | BTW_DELETE)) &&
+		    scoutfs_trigger(sb, LOG_MERGE_FORCE_PARTIAL)) {
+			ret = -ERANGE;
+			*next_ret = mitem->key;
+			goto out;
+		}
 	}
 
 	ret = 0;

kmod/src/triggers.c

@@ -46,6 +46,7 @@ static char *names[] = {
 	[SCOUTFS_TRIGGER_SRCH_MERGE_STOP_SAFE] = "srch_merge_stop_safe",
 	[SCOUTFS_TRIGGER_STATFS_LOCK_PURGE] = "statfs_lock_purge",
 	[SCOUTFS_TRIGGER_RECLAIM_SKIP_FINALIZE] = "reclaim_skip_finalize",
+	[SCOUTFS_TRIGGER_LOG_MERGE_FORCE_PARTIAL] = "log_merge_force_partial",
 };
 
 bool scoutfs_trigger_test_and_clear(struct super_block *sb, unsigned int t)

kmod/src/triggers.h

@@ -9,6 +9,7 @@ enum scoutfs_trigger {
 	SCOUTFS_TRIGGER_SRCH_MERGE_STOP_SAFE,
 	SCOUTFS_TRIGGER_STATFS_LOCK_PURGE,
 	SCOUTFS_TRIGGER_RECLAIM_SKIP_FINALIZE,
+	SCOUTFS_TRIGGER_LOG_MERGE_FORCE_PARTIAL,
 	SCOUTFS_TRIGGER_NR,
 };