*** THIS FILE IS THE FIRST ATTEMPT AND ITS MAIN RESULT IS WRONG — KEPT DELIBERATELY. ***

The volume lookup returned EMPTY, so the copy was empty, so postgres:17 initialised a FRESH 17
datadir and started happily. The line "PG_VERSION on the copy:" two lines below is blank and that
should have stopped the step; it did not. `17-...` is therefore NOT evidence that 17 accepts a 16
datadir — it never met one.

WHAT IT DID ACCIDENTALLY MEASURE, and this part IS real and useful: the MIRROR case. postgres:16
meeting a datadir written by 17 refuses in its own words —

    FATAL:  database files are incompatible with server
    DETAIL: The data directory was initialized by PostgreSQL version 17, which is not compatible
            with this version 16.15.

which is the sentence a household would meet if a 17 pin were ever put BACK to 16. The correct
measurement — 17 meeting 16 — is in `18-postgres-refusal-reproduced.txt`.

================================================================================================

INDEPENDENT REPRODUCTION of the PostgreSQL 16 -> 17 refusal
R-320: the product destroyed the original evidence (R-621), so it is reproduced here rather
than inferred. Plain docker, beside the product, on a COPY of a real 16 datadir.

the live docmost 16 datadir volume: 
PG_VERSION on the copy: 

=== now start postgres:17-alpine on that 16 datadir — the household's case, exactly
--- container state:
running=true exit=0 restarts=0
--- THE REFUSAL, VERBATIM:
2026-09-21 19:45:47.148 UTC [41] LOG:  database system is shut down
 done
server stopped

PostgreSQL init process complete; ready for start up.

2026-09-21 19:45:47.239 UTC [1] LOG:  starting PostgreSQL 17.11 on x86_64-pc-linux-musl, compiled by gcc (Alpine 15.2.0) 15.2.0, 64-bit
2026-09-21 19:45:47.239 UTC [1] LOG:  listening on IPv4 address "0.0.0.0", port 5432
2026-09-21 19:45:47.239 UTC [1] LOG:  listening on IPv6 address "::", port 5432
2026-09-21 19:45:47.245 UTC [1] LOG:  listening on Unix socket "/var/run/postgresql/.s.PGSQL.5432"
2026-09-21 19:45:47.253 UTC [55] LOG:  database system was shut down at 2026-09-21 19:45:47 UTC
2026-09-21 19:45:47.263 UTC [1] LOG:  database system is ready to accept connections

--- positive control: the SAME copy under postgres:16-alpine must start
running=false exit=1

2026-09-21 19:45:53.852 UTC [1] FATAL:  database files are incompatible with server
2026-09-21 19:45:53.852 UTC [1] DETAIL:  The data directory was initialized by PostgreSQL version 17, which is not compatible with this version 16.15.
--- and the data is still there, asked of the engine:
Error response from daemon: container ea0332381c9771265f47fd08988836c38afb697496096074305ed154a3ac8f3e is not running
(throwaway containers and the copied volume removed by name)