pg_upgrade from 17.10 to 18.4 on Ubuntu fails: Finding the real data directory for the target cluster sh: 1: (null)/postgres: not found

Dear Friends,

Today a pg_upgrade failed with this awkward message:

Finding the real data directory for the source cluster        ok
Finding the real data directory for the target cluster        sh: 1:
(null)/postgres: not found

I checked the source code of pg_upgrade.c and thus options.c and found
that the (null) above stems from an uninitialised variable called cmd,
declared in line 433 of option.c

Line 468 reads:

    snprintf(cmd, sizeof(cmd), "\"%s/postgres\" -D \"%s\" -C
data_directory",
             cluster->bindir, cluster->pgconfig);

Calling the postgres binary (from the package, didn't compile myself)
works fine and gives me the correct real data directory:

    $ /usr/lib/postgresql/18/bin/postgres -D /etc/postgresql/18/main -C
data_directory
    /var/lib/postgresql/18/main

So how come that cmd is NULL in line 468? I actually cannot find where
the variable was given any content, so it's NULL. This generates the
error message with "(null)/postgres not found"

Obviously, cmd should contain the path of the oldbin directory, which is
to be derived from the path of the pg_upgrade being called. I didn't use
the -B option because the documentation says that it's optional and will
use the directory of pg_upgrade.

https://www.postgresql.org/docs/current/pgupgrade.html tells me:
|   -B| /|bindir|/
|   --new-bindir=|/|bindir
|/         the new PostgreSQL executable directory; default is the
directory where pg_upgrade resides

Obviously, this doesn't work as described.

When giving the path to the new bin directory with -B, pg_upgrade works
fine.

Please correct the code so that cmd gets initialised or alter the
documentation making the option -B mandatory.

Kind Regards,

Holger

--

Holger Jakobs, Bergisch Gladbach, Germany