Why does `:cdo print` not show full output?

Question

:cdo print command (intended to print every line in the quickfix list), shows generic file information instead of line output for some quickfix entries. Why is this?

The following example is taken from Practical Vim tip 109 "Call grep Without Leaving Vim". In the code samples, the source text files are in the code/grep directory.)

In the example, the quickfix list is populated with three entries across two text files:

:code/grep
:grep Waldo *
:cli
 1 department-store.txt:1: Waldo is beside the boot counter.
 2 goldrush.txt:6: Waldo is studying his clipboard.
 3 goldrush.txt:9: The penny farthing is 10 paces ahead of Waldo.

However, :cdo print outputs unexpected text "goldrush.txt" 9 lines --11%-- for the first entry instead of Waldo is beside the boot counter.:

(1 of 3): Waldo is beside the boot counter.
"goldrush.txt" 9 lines --11%--
(2 of 3): Waldo is studying his clipboard.
Waldo is studying his clipboard.
(3 of 3): The penny farthing is 10 paces ahead of Waldo.
The penny farthing is 10 paces ahead of Waldo.

Observations

1. The behavior of :cdo print is different from that of sister commands :{tab,arg,buf}do print, whose output includes all applicable lines, for instance :bufdo print:

   Waldo is beside the boot counter.
   A man has found gold.
   

2. Redirecting :cdo print to a register shows the expected output.

   :redir @a
   :cdo print
   :redir END
   

   A subsequent :echo @a reveals:

   "department-store.txt" 5 lines --20%-- ((2) of 2)
   (1 of 3): Waldo is beside the boot counter.
   Waldo is beside the boot counter.
   
   "goldrush.txt" 9 lines --11%-- (2 of 2)
   (2 of 3): Waldo is studying his clipboard.
   Waldo is studying his clipboard.
   (3 of 3): The penny farthing is 10 paces ahead of Waldo.
   The penny farthing is 10 paces ahead of Waldo.
   

   Compared to this, the direct output of :cdo print appears to be truncated.

---

Vim version: 8.2.800

Answer 1

:help :cdo explains that the command actually is comprised of several subcommands:

It works like doing this: :cfirst :{cmd} :cnext :{cmd} etc.

The "truncation" you observe are the file open messages of any new files from the quickfix list; these compete with the :print command for the (limited) output area and thus partially overwrite each other. If you don't see the same behavior with :argdo et al., this looks like an inconsistency in its implementation. You may want to open an issue at the bug tracker, or directly discuss this on the vim_dev mailing list. The :cdo command is a later addition; something might have been missed in its implementation, as it mainly is invoked for the side effects of the supplied {command}, and not for the output itself as with :print.

Update: This inconsistency has been fixed in version 8.2.0836. Thanks Yeggapan for picking this up and supplying a patch!