[ArturWieczorek]

Since 13.7.2.1 (migration 2-0049) epoch is a view: db-sync stores finished epochs in epoch_finalized and computes the current one from block and tx. The three epoch scripts in scripts/ still assumed a table. Found while testing 13.8.0.0.

What went wrong

• fix-epoch-table.sql tries to INSERT ... ON CONFLICT into the view, fails with cannot insert into view "epoch", and still exits 0: the script didn't stop on errors, so the COMMIT just rolled the failed transaction back.
• check-and-fix-epoch-table.sh asks "Apply fix?" and then runs that fix.
• With epoch sync not enabled (disable_epoch, or db-sync not started since the upgrade) the view is empty, so validate-epoch-table.sql lists every epoch as MISSING.

Changes, one per commit:

1. fix-epoch-table.sql stops at the first error, and refuses up front on a view, before BEGIN:
   ERROR: epoch is a view since db-sync 13.7.2.1, so this script cannot repair it
   HINT: Finished epochs are stored in epoch_finalized, which db-sync maintains; a mismatch reported by validate-epoch-table.sql points to a db-sync bug.
   On the old table it works as before, and a failed run now exits non-zero.
2. validate-epoch-table.sql: on a view with epoch sync not enabled, it says so and exits 0 instead of listing every epoch as MISSING.
3. check-and-fix-epoch-table.sh: on a view it doesn't offer the fix. If epoch sync is not enabled it says so and exits 0 without validating. If there are mismatches it prints them, says the script can't repair them, and exits 1.
4. check-and-fix-epoch-table.sh: with no terminal (closed stdin) the prompt hit end of input, and set -e ended the script with exit 1 and no message. No input now counts as "no", and an answer without a final newline is still taken.
5. check-and-fix-epoch-table.sh: the header claimed it requires PGPASSFILE, could take the database name "via psql_args", and read "field 5" of the pgpass line. None of that matched the code.
6. scripts/README.md: the epoch section says what applies since 13.7.2.1.
7. check-and-fix-epoch-table.sh took only the database name from the pgpass file, so with a pgpass for another host, port or user it checked (and after "y" fixed) a same-named database on the default server. It now takes host, port, database and user from the first pgpass line, as db-sync does; PGHOST/PGPORT/PGDATABASE/PGUSER still win, and a * field is left to psql.
8. validate-epoch-table.sql now also reports start_time / end_time mismatches. The fix rewrites both, but validate never flagged them, so the wrapper said "No mismatches" for an epoch it would have changed.
9. Run without -v epoch_no, fix-epoch-table.sql and validate-epoch-table.sql now stop with missing -v epoch_no: run psql -v epoch_no=<epoch, or -1 for all epochs> ... and exit 3, instead of a raw syntax error (validate even exited 0).

Why not repair epoch_finalized instead: db-sync already maintains it with the same recompute from block and tx. It upserts at each epoch boundary, deletes from the rollback epoch on, and backfills at startup. The old fix also writes the current epoch, which would hide it from epoch_current until the next boundary.

Testing

• Throwaway databases built from the real 13.7.1.0 (table) and 13.8.0.0 (view) schemas: 16 scenarios covering both forms, epoch sync on, off or not set up, mismatches, the answers "y" and "n", and no input. The new scenarios fail on master and pass with these commits, and removing any of the changes makes its scenario fail again.
• Commits 7-9: 5 more scenarios (21 in total), red before each commit and green after, plus a pgpass test with a second PostgreSQL server holding a same-named database (before: the wrapper fixed the wrong one; now the one the pgpass names). validate on real preview / preprod (13.7.1.0 table, 13.8.0.0 view) still reports 0 rows with the time columns included.
• shellcheck is clean.
• Real databases, read-only:

┌─────────────────────────────────────────────────────┬──────────────────────────────────┬───────────────────────┬───────────────────────┐
│                      Database                       │               fix                │       validate        │        wrapper        │
├─────────────────────────────────────────────────────┼──────────────────────────────────┼───────────────────────┼───────────────────────┤
│ preprod / preview 13.8.0.0, preprod 13.7.2.1 (view) │ refuses, exit 3, nothing written │ exit 0, no mismatches │ exit 0, no mismatches │
├─────────────────────────────────────────────────────┼──────────────────────────────────┼───────────────────────┼───────────────────────┤
│ preprod / preview 13.7.1.0 (table)                  │ not run (writes)                 │ exit 0, no mismatches │ exit 0, no mismatches │
└─────────────────────────────────────────────────────┴──────────────────────────────────┴───────────────────────┴───────────────────────┘

scripts/README.md is also changed by #2183, in a different section, and the two merge cleanly.