rsync 3.1.2-2.1ubuntu1.6 breaks compatibility with unison 2.48.4-1ubuntu1 on Bionic

Rsync 3.1.2-2.1ubuntu1.6, when used by Unison using the copyprog option in Unison, produces an error message and fails to sync files. Rsync 3.1.2-2.1ubuntu1 succeeds in syncing files. I believe Rsync 3.1.2-2.1ubuntu1.5 succeeded but I no longer have the .deb file to test with.

Steps to reproduce:

Create a unison profile similar to the one given below, to sync a folder between a local machine and a remote ssh server. Both client and server need the same Ubuntu and Unison versions, and I used unpassphrased SSH keys for authentication. Place a test file in the folder to be synced, then run 'unison profilename'.

Note that I have redacted domains in the below.

Unison profile:

label = Test Profile
root = /root/unison-test
root = ssh://fs2b.our.domain.org.uk//root/unison-test

# run repeatedly and fully automatically
auto = true
batch = true
copyonconflict = true
#repeat = 60

# the copy program must be manually specified in order to sync ACLs
copythreshold=0
copyprog = rsync -a -A -X --rsh=ssh --inplace --compress
copyprogrest = rsync -a -A -X --rsh=ssh --partial --inplace --compress

# general settings
group = true
owner = true
#path = sharedfolder
#path = users
perms = -1
sortbysize = true
times = true

Unison output with rsync 3.1.2-2.1ubuntu1.6
root@fs72a:~# unison fs2b_unisontest -debug all
[startup] Preferences:
ui = graphic
host =
server = false
prefsdocs = false
doc =
version = false
silent = false
dumbtty = false
testserver = false
rest = fs2b_unisontest
showprev = false
selftest = false
confirmmerge = false
retry = 0
repeat =
contactquietly = false
key =
label = Syncs P and S drives between fs72a and fs2a
expert = false
height = 15
auto = true
maxthreads = 0
maxsizethreshold = -1
prefer =
force =
sortnewfirst = false
sortbysize = true
keeptempfilesaftermerge = false
diff = diff -u CURRENT2 CURRENT1
copyonconflict = true
backupdir =
maxbackups = 2
backups = false
backupsuffix =
backupprefix = .bak.$VERSION.
backuploc = central
copymax = 1
copyquoterem = default
copythreshold = 0
copyprogrest = rsync -a -A -X --rsh=ssh --partial --inplace --compress
copyprog = rsync -a -A -X --rsh=ssh --inplace --compress
rsync = true
fastcheck = default
ignorelocks = false
dumparchives = false
showarchive = false
rootsName =
ignorearchives = false
fastercheckUNSAFE = false
fat = false
allHostsAreRunningWindows = false
someHostIsRunningWindows = false
confirmbigdel = true
batch = true
root = ssh://fs2b.our.domain.org.uk//root/unison-test
root = /root/unison-test
killserver = false
halfduplex = false
stream = true
addversionno = false
servercmd =
sshargs =
rshargs =
rshcmd = rsh
sshcmd = ssh
xferbycopying = true
sshversion =
clientHostName = fs72a
ignoreinodenumbers = false
links-aux = true
links = default
times = true
group = true
owner = true
numericids = false
dontchmod = false
perms = -1
watch = true
rsrc-aux = false
rsrc = default
maxerrors = 1
unicodeCS = false
unicodeEnc = false
unicode = default
someHostIsInsensitive = false
ignorecase = default
timers = false
terse = false
logfile = /root/.unison/unison.log
log = true
debugtimes = false
debug = all
addprefsto =
Contacting server...
[remote] Shell connection: ssh (ssh, fs2b.our.domain.org.uk, -e, none, unison, -server)
[globals] Checking path '' for expansions
Connected [//fs2b//root/unison-test -> //fs72a//root/unison-test]
[startup] Roots:
        /root/unison-test
        ssh://fs2b.our.domain.org.uk//root/unison-test
  i.e.
        /root/unison-test
        ssh://fs2b.our.domain.org.uk//root/unison-test
  i.e. (in canonical order)
       /root/unison-test
       //fs2b//root/unison-test

[props] Setting permission mask to 7777 (777777777777777777777 and 7777)
[stasher] initBackupsLocal
[stasher] d = /
[stasher] Prefix and suffix regexps for backup filenames have been updated
[server: stasher] initBackupsLocal
[server: stasher] d = /
[server: stasher] Prefix and suffix regexps for backup filenames have been updated
Looking for changes
[ui] temp: Globals.paths =
[update] Loading archive from /root/.unison/areaa7785bb2204bed0cb7af3360b3c323
[update] Setting archive for //fs72a//root/unison-test
[server: update] Loading archive from /root/.unison/ar8a6c9477fdd76859c429c5bdd5afe2d9
[server: update] Setting archive for //fs2b//root/unison-test
[update] findOnRoot //fs2b//root/unison-test
[update] findOnRoot /root/unison-test
[update] findLocal /root/unison-test ()
[fpcache] opening cache file /root/.unison/fpeaa7785bb2204bed0cb7af3360b3c323 for input
[fpcache] read chunk of 1 files
[fpcache] opening cache file /root/.unison/fpeaa7785bb2204bed0cb7af3360b3c323 for output
[pred] ignore '' = false
[update] buildUpdateRec: /root/unison-test
[pred] immutable '' = false
[pred] ignore 'hello' = false
[update] buildUpdateRec: /root/unison-test/hello
[update] buildUpdate -> New file
[fpcache] cache hit for path hello
[xferhint] insertEntry: fspath=/root/unison-test, path=hello, fp=(8ddd8be4b179a529afa5f2ffae4b9858,)
[update] Setting archive for //fs72a//root/unison-test

  Waiting for changes from server
[server: update] findLocal /root/unison-test ()
[server: fpcache] opening cache file /root/.unison/fp8a6c9477fdd76859c429c5bdd5afe2d9 for input
[server: fpcache] opening cache file /root/.unison/fp8a6c9477fdd76859c429c5bdd5afe2d9 for output
[server: pred] ignore '' = false
[server: update] buildUpdateRec: /root/unison-test
[server: pred] immutable '' = false
[server: update] Setting archive for //fs2b//root/unison-test
Reconciling changes
[recon] reconcileAll
[recon] reconcile: 1 results
[update] Marking 0 paths equal
[pred] forcepartial 'hello' = false
[pred] preferpartial 'hello' = false
new file ----> hello
Propagating updates
UNISON 2.48.4 started propagating changes at 12:53:09.13 on 08 Mar 2023
[BGN] Copying hello from /root/unison-test to //fs2b//root/unison-test
[files] copy /root/unison-test hello ---> //fs2b//root/unison-test hello
[server: pred] follow 'hello' = false
[server: fspath] Os.findWorkingDir(/root/unison-test,hello) = (/root/unison-test,hello)
[update] updateArchive /root/unison-test hello
[pred] ignore 'hello' = false
[files] copyRec hello --> .unison.hello.8a6c9477fdd76859c429c5bdd5afe2d9.unison.tmp (really to hello)
[abort] Checking line 0
[copy] copyRegFile(/root/unison-test,hello) -> (//fs2b//root/unison-test,hello,/root/unison-test,.unison.hello.8a6c9477fdd76859c429c5bdd5afe2d9.unison.tmp,modified on 2023-03-08 at 11:59:12 size 13 --rw-r--r-- user=0 group=0)
[abort] Checking line 0
[server: copy] tryCopyMovedFile: -> .unison.hello.8a6c9477fdd76859c429c5bdd5afe2d9.unison.tmp /(8ddd8be4b179a529afa5f2ffae4b9858,)/
[server: xferhint] lookup: fp = (8ddd8be4b179a529afa5f2ffae4b9858,)
rsync -a -A -X --rsh=ssh --inplace --compress '/root/unison-test/hello' 'fs2b.our.domain.org.uk:'\''/root/unison-test/.unison.hello.8a6c9477fdd76859c429c5bdd5afe2d9.unison.tmp'\'''
[copy] transferFileUsingExternalCopyprog hello: returned...
rsync: change_dir#3 "/root//'/root/unison-test" failed: No such file or directory (2)
rsync error: errors selecting input/output files, dirs (code 3) at main.c(712) [Receiver=3.1.2]

Exited with status 3
[abort] Aborting line 0
Failed: External copy program did not create target file (or bad length): .unison.hello.8a6c9477fdd76859c429c5bdd5afe2d9.unison.tmp
100% 00:00 ETAFailed [hello]: External copy program did not create target file (or bad length): .unison.hello.8a6c9477fdd76859c429c5bdd5afe2d9.unison.tmp
UNISON 2.48.4 finished propagating changes at 12:53:09.66 on 08 Mar 2023
Saving synchronizer state
[update] Updating archives
[update] Saving archive in /root/.unison/sceaa7785bb2204bed0cb7af3360b3c323
[server: update] Saving archive in /root/.unison/sc8a6c9477fdd76859c429c5bdd5afe2d9
[update] Copying archive /root/.unison/tmeaa7785bb2204bed0cb7af3360b3c323 to /root/.unison/areaa7785bb2204bed0cb7af3360b3c323
[server: update] Copying archive /root/.unison/tm8a6c9477fdd76859c429c5bdd5afe2d9 to /root/.unison/ar8a6c9477fdd76859c429c5bdd5afe2d9
[update] Removing archive /root/.unison/tmeaa7785bb2204bed0cb7af3360b3c323
[server: update] Removing archive /root/.unison/tm8a6c9477fdd76859c429c5bdd5afe2d9
Synchronization incomplete at 12:53:09 (0 items transferred, 0 skipped, 1 failed)
  failed: hello
root@fs72a:~# [server: remote] Connection closed by the client

Unison output with rsync 3.1.2-2.1ubuntu1

root@fs72a:~# unison fs2b_unisontest -debug all
[startup] Preferences:
ui = graphic
host =
server = false
prefsdocs = false
doc =
version = false
silent = false
dumbtty = false
testserver = false
rest = fs2b_unisontest
showprev = false
selftest = false
confirmmerge = false
retry = 0
repeat =
contactquietly = false
key =
label = Syncs P and S drives between fs72a and fs2a
expert = false
height = 15
auto = true
maxthreads = 0
maxsizethreshold = -1
prefer =
force =
sortnewfirst = false
sortbysize = true
keeptempfilesaftermerge = false
diff = diff -u CURRENT2 CURRENT1
copyonconflict = true
backupdir =
maxbackups = 2
backups = false
backupsuffix =
backupprefix = .bak.$VERSION.
backuploc = central
copymax = 1
copyquoterem = default
copythreshold = 0
copyprogrest = rsync -a -A -X --rsh=ssh --partial --inplace --compress
copyprog = rsync -a -A -X --rsh=ssh --inplace --compress
rsync = true
fastcheck = default
ignorelocks = false
dumparchives = false
showarchive = false
rootsName =
ignorearchives = false
fastercheckUNSAFE = false
fat = false
allHostsAreRunningWindows = false
someHostIsRunningWindows = false
confirmbigdel = true
batch = true
root = ssh://fs2b.our.domain.org.uk//root/unison-test
root = /root/unison-test
killserver = false
halfduplex = false
stream = true
addversionno = false
servercmd =
sshargs =
rshargs =
rshcmd = rsh
sshcmd = ssh
xferbycopying = true
sshversion =
clientHostName = fs72a
ignoreinodenumbers = false
links-aux = true
links = default
times = true
group = true
owner = true
numericids = false
dontchmod = false
perms = -1
watch = true
rsrc-aux = false
rsrc = default
maxerrors = 1
unicodeCS = false
unicodeEnc = false
unicode = default
someHostIsInsensitive = false
ignorecase = default
timers = false
terse = false
logfile = /root/.unison/unison.log
log = true
debugtimes = false
debug = all
addprefsto =
Contacting server...
[remote] Shell connection: ssh (ssh, fs2b.our.domain.org.uk, -e, none, unison, -server)
[globals] Checking path '' for expansions
Connected [//fs2b//root/unison-test -> //fs72a//root/unison-test]
[startup] Roots:
        /root/unison-test
        ssh://fs2b.our.domain.org.uk//root/unison-test
  i.e.
        /root/unison-test
        ssh://fs2b.our.domain.org.uk//root/unison-test
  i.e. (in canonical order)
       /root/unison-test
       //fs2b//root/unison-test

[props] Setting permission mask to 7777 (777777777777777777777 and 7777)
[stasher] initBackupsLocal
[stasher] d = /
[stasher] Prefix and suffix regexps for backup filenames have been updated
[server: stasher] initBackupsLocal
[server: stasher] d = /
[server: stasher] Prefix and suffix regexps for backup filenames have been updated
Looking for changes
[ui] temp: Globals.paths =
[update] Loading archive from /root/.unison/areaa7785bb2204bed0cb7af3360b3c323
[update] Setting archive for //fs72a//root/unison-test
[server: update] Loading archive from /root/.unison/ar8a6c9477fdd76859c429c5bdd5afe2d9
[server: update] Setting archive for //fs2b//root/unison-test
[update] findOnRoot //fs2b//root/unison-test
[update] findOnRoot /root/unison-test
[update] findLocal /root/unison-test ()
[fpcache] opening cache file /root/.unison/fpeaa7785bb2204bed0cb7af3360b3c323 for input
[fpcache] read chunk of 1 files
[fpcache] opening cache file /root/.unison/fpeaa7785bb2204bed0cb7af3360b3c323 for output
[pred] ignore '' = false
[update] buildUpdateRec: /root/unison-test
[pred] immutable '' = false
[pred] ignore 'hello' = false
[update] buildUpdateRec: /root/unison-test/hello
[update] buildUpdate -> New file
[fpcache] cache hit for path hello
[xferhint] insertEntry: fspath=/root/unison-test, path=hello, fp=(8ddd8be4b179a529afa5f2ffae4b9858,)
[update] Setting archive for //fs72a//root/unison-test

  Waiting for changes from server
[server: update] findLocal /root/unison-test ()
[server: fpcache] opening cache file /root/.unison/fp8a6c9477fdd76859c429c5bdd5afe2d9 for input
[server: fpcache] opening cache file /root/.unison/fp8a6c9477fdd76859c429c5bdd5afe2d9 for output
[server: pred] ignore '' = false
[server: update] buildUpdateRec: /root/unison-test
[server: pred] immutable '' = false
[server: update] Setting archive for //fs2b//root/unison-test
Reconciling changes
[recon] reconcileAll
[recon] reconcile: 1 results
[update] Marking 0 paths equal
[pred] forcepartial 'hello' = false
[pred] preferpartial 'hello' = false
new file ----> hello
Propagating updates
UNISON 2.48.4 started propagating changes at 12:58:13.42 on 08 Mar 2023
[BGN] Copying hello from /root/unison-test to //fs2b//root/unison-test
[files] copy /root/unison-test hello ---> //fs2b//root/unison-test hello
[server: pred] follow 'hello' = false
[server: fspath] Os.findWorkingDir(/root/unison-test,hello) = (/root/unison-test,hello)
[update] updateArchive /root/unison-test hello
[pred] ignore 'hello' = false
[files] copyRec hello --> .unison.hello.8a6c9477fdd76859c429c5bdd5afe2d9.unison.tmp (really to hello)
[abort] Checking line 0
[copy] copyRegFile(/root/unison-test,hello) -> (//fs2b//root/unison-test,hello,/root/unison-test,.unison.hello.8a6c9477fdd76859c429c5bdd5afe2d9.unison.tmp,modified on 2023-03-08 at 11:59:12 size 13 --rw-r--r-- user=0 group=0)
[abort] Checking line 0
[server: copy] tryCopyMovedFile: -> .unison.hello.8a6c9477fdd76859c429c5bdd5afe2d9.unison.tmp /(8ddd8be4b179a529afa5f2ffae4b9858,)/
[server: xferhint] lookup: fp = (8ddd8be4b179a529afa5f2ffae4b9858,)
rsync -a -A -X --rsh=ssh --inplace --compress '/root/unison-test/hello' 'fs2b.our.domain.org.uk:'\''/root/unison-test/.unison.hello.8a6c9477fdd76859c429c5bdd5afe2d9.unison.tmp'\'''
[copy] transferFileUsingExternalCopyprog hello: returned...
[server: props] Setting permissions for /root/unison-test/.unison.hello.8a6c9477fdd76859c429c5bdd5afe2d9.unison.tmp to --rw-r--r-- (644/7777)
[server: xferhint] insertEntry: fspath=/root/unison-test, path=.unison.hello.8a6c9477fdd76859c429c5bdd5afe2d9.unison.tmp, fp=(8ddd8be4b179a529afa5f2ffae4b9858,)
[files] rename(root=//fs2b//root/unison-test, pathOld=.unison.hello.8a6c9477fdd76859c429c5bdd5afe2d9.unison.tmp, pathNew=hello)
[server: update] checkNoUpdates /root/unison-test hello
[server: update] buildUpdateRec: /root/unison-test/hello
[server: update] buildUpdate -> Absent and no archive
[server: files] Renaming .unison.hello.8a6c9477fdd76859c429c5bdd5afe2d9.unison.tmp to hello in /root/unison-test; root is /root/unison-test
[server: files] rename: moveFirst=false
[server: stasher] backup: /root/unison-test / hello
[server: pred] follow 'hello' = false
[server: fspath] Os.findWorkingDir(/root/unison-test,hello) = (/root/unison-test,hello)
[server: stasher] File hello in /root/unison-test does not exist, so no need to back up
[server: os] rename /root/unison-test/.unison.hello.8a6c9477fdd76859c429c5bdd5afe2d9.unison.tmp to /root/unison-test/hello
[server: files] Contents of /root/unison-test/hello after renaming = 8ddd8be4b179a529afa5f2ffae4b9858
[server: pred] backupcurr 'hello' = false
[server: xferhint] insertEntry: fspath=/root/unison-test, path=hello, fp=(8ddd8be4b179a529afa5f2ffae4b9858,)
[server: update] replaceArchiveLocal /root/unison-test hello
[server: update] Setting archive for //fs2b//root/unison-test
[update] updateArchive /root/unison-test hello
[pred] ignore 'hello' = false
[update] replaceArchiveLocal /root/unison-test hello
[update] Setting archive for //fs72a//root/unison-test
[pred] backupcurr 'hello' = false
[END] Copying hello
UNISON 2.48.4 finished propagating changes at 12:58:14.13 on 08 Mar 2023
Saving synchronizer state
[update] Updating archives
[update] Saving archive in /root/.unison/sceaa7785bb2204bed0cb7af3360b3c323
[server: update] Saving archive in /root/.unison/sc8a6c9477fdd76859c429c5bdd5afe2d9
[update] Copying archive /root/.unison/tmeaa7785bb2204bed0cb7af3360b3c323 to /root/.unison/areaa7785bb2204bed0cb7af3360b3c323
[server: update] Copying archive /root/.unison/tm8a6c9477fdd76859c429c5bdd5afe2d9 to /root/.unison/ar8a6c9477fdd76859c429c5bdd5afe2d9
[update] Removing archive /root/.unison/tmeaa7785bb2204bed0cb7af3360b3c323
[server: update] Removing archive /root/.unison/tm8a6c9477fdd76859c429c5bdd5afe2d9
Synchronization complete at 12:58:14 (1 item transferred, 0 skipped, 0 failed)
root@fs72a:~# [server: remote] Connection closed by the client