bug#68805: 29.2; Tramp: out-of-band copy-file fails when tramp-copy-keep-date is nil

[Sean Devlin <spd@toadstyle.org>]

Hi folks,

I created a custom Tramp method with support for out-of-band file
transfer using tramp-copy-program, and I left tramp-copy-keep-date
unset. When I try to copy a file to the remote using copy-file with the
keep-date argument set to t, the transfer succeeds, but Emacs raises an
error. If I set tramp-copy-keep-date to t, there is no error.

You can reproduce the error by modifying the scp method:

1. Emacs -Q
2. Evaluate these forms:

(setf (alist-get 'tramp-copy-keep-date 
		 (alist-get "scp" tramp-methods nil nil #'string-equal))
      '(nil))

(copy-file "/path/to/bigfile" "/scp:your-remote:bigfile" nil t)

3. Emacs will raise a file-missing error
4. Type q to dismiss the backtrace
5. Evaluate:

(dired "/scp:your-remote:")

6. Verify the file was transferred despite the error

I think the issue is due to caching. Before copying, Tramp checks to see
if the file exists, and it does not. After the transfer, Tramp tries to
check again because the keep-date argument was set, tramp-copy-keep-date
is nil or unset, and Tramp wants to set the file time manually. Instead
of actually doing this file existence check, Tramp instead gets the
property from cache that was set just prior to the file transfer. It
then raises a file-missing error.

Thanks,

Sean

In GNU Emacs 29.2 (build 1, aarch64-apple-darwin21.6.0, NS
appkit-2113.60 Version 12.6.6 (Build 21G646)) of 2024-01-18 built on
armbob.lan
Windowing system distributor 'Apple'
System Description:  macOS 14

Configured using:
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Memory information:
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