dnode_is_dirty: check dnode and its data for dirtiness

Over its history this the dirty dnode test has been changed between
checking for a dnodes being on `os_dirty_dnodes` (`dn_dirty_link`) and
`dn_dirty_record`.

  de198f2 Fix lseek(SEEK_DATA/SEEK_HOLE) mmap consistency
  2531ce3 Revert "Report holes when there are only metadata changes"
  ec4f9b8 Report holes when there are only metadata changes
  454365b Fix dirty check in dmu_offset_next()
  66aca24 SEEK_HOLE should not block on txg_wait_synced()

Also illumos/illumos-gate@c543ec060d illumos/illumos-gate@2bcf0248e9

It turns out both are actually required.

In the case of appending data to a newly created file, the dnode proper
is dirtied (at least to change the blocksize) and dirty records are
added.  Thus, a single logical operation is represented by separate
dirty indicators, and must not be separated.

The incorrect dirty check becomes a problem when the first block of a
file is being appended to while another process is calling lseek to skip
holes. It can happen that the dnode part is undirtied, while dirty
records are still on the dnode for the next txg. In this case,
`lseek(fd, 0, SEEK_DATA)` would not know that the file is dirty, and
would go to `dnode_next_offset()`. Since the object has no data blocks
yet, it returns `ESRCH`, indicating no data found, which results in
`ENXIO` being returned to `lseek()`'s caller.

Since coreutils 9.2, `cp` performs sparse copies by default, that is, it
uses `SEEK_DATA` and `SEEK_HOLE` against the source file and attempts to
replicate the holes in the target. When it hits the bug, its initial
search for data fails, and it goes on to call `fallocate()` to create a
hole over the entire destination file.

This has come up more recently as users upgrade their systems, getting
OpenZFS 2.2 as well as a newer coreutils. However, this problem has been
reproduced against 2.1, as well as on FreeBSD 13 and 14.

This change simply updates the dirty check to check both types of dirty.
If there's anything dirty at all, we immediately go to the "wait for
sync" stage, It doesn't really matter after that; both changes are on
disk, so the dirty fields should be correct.

Signed-off-by: Rob Norris <robn@despairlabs.com>

module/zfs/dnode.c

@@ -1778,15 +1778,24 @@ dnode_try_claim(objset_t *os, uint64_t object, int slots)
 }
 
 /*
- * Checks if the dnode contains any uncommitted dirty records.
+ * Checks if the dnode itself is dirty, or is carrying any uncommitted records.
+ * Its important to check both, as some operations (eg appending to a file) can
+ * dirty both as a single logical unit, but they are not synced out atomically,
+ * so checking one and not the other can result an object appearing to be clean
+ * mid-way through a commit.
+ *
+ * Do not change this lightly! If you get it wrong, dmu_offset_next() can
+ * detect a hole where there is really data, leading to silent corruption.
  */
+
 boolean_t
 dnode_is_dirty(dnode_t *dn)
 {
 	mutex_enter(&dn->dn_mtx);
 
 	for (int i = 0; i < TXG_SIZE; i++) {
-		if (multilist_link_active(&dn->dn_dirty_link[i])) {
+		if (multilist_link_active(&dn->dn_dirty_link[i]) ||
+		    list_head(&dn->dn_dirty_records[i])) {
 			mutex_exit(&dn->dn_mtx);
 			return (B_TRUE);
 		}