upgrade: Use the in-place pg_upgrade, not a full dump/restore.

pg_upgradecluster has two possibilities for `--method`: `dump`, and
`upgrade`.  The former is the default, and does a `pg_dump` of all of
the databases in the old cluster and feeds them into the new cluster.
This is a sure-fire way of getting the same information in both
databases, but may be extremely slow on large databases, and is
guaranteed to fail on servers whose databases take up >50% of their
disk.

The `--method=upgrade` method, by contrast, uses pg_upgrade to copy
the raw database data file over to the new cluster, and then fiddles
with their internal structure as needed by the upgrade to let them be
correct for the new version[1].  This is slightly faster than the
dump/load method, since it skips the serialization step, but still
requires that there be enough space on disk for both old and new
versions at once.  `pg_upgrade` is currently supported for all
versions of PostgreSQL from 8.4 to 12.

Using `pg_upgrade` incurs slightly more risk, but since the it is
widely used by now, using it in the relatively-controlled Zulip server
environment is reasonable.  The expected worst failure is failure to
upgrade, not corruption or data loss.

Additionally passing `--link` uses hardlinks to link the data files
into both the old and new directories simultaneously.  This resolve
both the runtime of the operation, as well as the disk space usage.
The only potential downside to this is that as soon as writes have
occurred on the upgraded cluster, the old cluster can no longer be
started.  Since this tooling intends to remove the old cluster
immediately after the upgrade completes successfully, this is not a
significant drawback.

Switch to using `--method=upgrade --link`.  This technique spits out
two shell scripts which are expected to be run after completion of the
upgrade; one re-analyzes the statistics, the other does an `rm -rf` of
the data where it is still hardlinked in the old cluster.  Extract the
location of these scripts from parsing the `pg_upgradecluster` output;
since the path is not static, we must rely on it being relatively easy
to parse.  The risk of the path changing is lower, and has more
obvious failure modes, than inserting the current contents of these
upgrade steps into the overall `upgrade-postgres`.

[1] https://www.postgresql.org/docs/12/pgupgrade.html

scripts/setup/upgrade-postgres

@@ -43,12 +43,35 @@ pg_ctlcluster "$UPGRADE_FROM" main stop
     rm -rf "$TEMP_CONF_DIR"
 )
 
-pg_upgradecluster "$UPGRADE_FROM" main
+# Capture the output so we know where the path to the post-upgrade scripts is
+UPGRADE_LOG=$(mktemp "/var/log/zulip/postgres-upgrade-$UPGRADE_FROM-$UPGRADE_TO.XXXXXXXXX.log")
+pg_upgradecluster "$UPGRADE_FROM" main --method=upgrade --link | tee "$UPGRADE_LOG"
+SCRIPTS_PATH=$(grep -o "/var/log/postgresql/pg_upgradecluster-$UPGRADE_FROM-$UPGRADE_TO-main.*" "$UPGRADE_LOG" || true)
 
 crudini --set /etc/zulip/zulip.conf postgresql version "$UPGRADE_TO"
+
+# Update the statistics
+[ -n "$SCRIPTS_PATH" ] && su postgres -c "$SCRIPTS_PATH/analyze_new_cluster.sh"
+
 "$ZULIP_PATH"/scripts/zulip-puppet-apply -f
 
 service memcached restart
 
 pg_dropcluster "$UPGRADE_FROM" main
 apt remove -y "postgresql-$UPGRADE_FROM"
+if [ -n "$SCRIPTS_PATH" ]; then
+    su postgres -c "$SCRIPTS_PATH/delete_old_cluster.sh"
+    rm -rf "$SCRIPTS_PATH"
+else
+    set +x
+    echo
+    echo
+    echo ">>>>> pg_upgradecluster succeeded, but post-upgrade scripts path could not"
+    echo "      be parsed out!  Please read the pg_upgradecluster output to understand"
+    echo "      the current status of your cluster:"
+    echo "          $UPGRADE_LOG"
+    echo "      and report this bug with the Postgres $UPGRADE_FROM -> $UPGRADE_TO upgrade to:"
+    echo "          https://github.com/zulip/zulip/issues"
+    echo
+    echo
+fi