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+
+# `postgresqlTestHook` {#sec-postgresqlTestHook}
+
+This hook starts a PostgreSQL server during the `checkPhase`. Example:
+
+```nix
+{ stdenv, postgresql, postgresqlTestHook }:
+stdenv.mkDerivation {
+
+  # ...
+
+  checkInputs = [
+    postgresql
+    postgresqlTestHook
+  ];
+}
+```
+
+If you use a custom `checkPhase`, remember to add the `runHook` calls:
+```nix
+  checkPhase ''
+    runHook preCheck
+
+    # ... your tests
+
+    runHook postCheck
+  ''
+```
+
+## Variables {#sec-postgresqlTestHook-variables}
+
+The hook logic will read a number of variables and set them to a default value if unset or empty.
+
+Exported variables:
+
+ - `PGDATA`: location of server files.
+ - `PGHOST`: location of UNIX domain socket directory; the default `host` in a connection string.
+ - `PGUSER`: user to create / log in with, default: `test_user`.
+ - `PGDATABASE`: database name, default: `test_db`.
+
+Bash-only variables:
+
+ - `postgresqlTestUserOptions`: SQL options to use when creating the `$PGUSER` role, default: `LOGIN`.
+ - `postgresqlTestSetupSQL`: SQL commands to run as database administrator after startup, default: statements that create `$PGUSER` and `$PGDATABASE`.
+ - `postgresqlTestSetupCommands`: bash commands to run after database start, defaults to running `$postgresqlTestSetupSQL` as database administrator.
+ - `postgresqlEnableTCP`: set to `1` to enable TCP listening. Flaky; not recommended.
+ - `postgresqlStartCommands`: defaults to `pg_ctl start`.
+
+## TCP and the Nix sandbox {#sec-postgresqlTestHook-tcp}
+
+`postgresqlEnableTCP` relies on network sandboxing, which is not available on macOS and some custom Nix installations, resulting in flaky tests.
+For this reason, it is disabled by default.
+
+The preferred solution is to make the test suite use a UNIX domain socket connection. This is the default behavior when no `host` connection parameter is provided.
+Some test suites hardcode a value for `host` though, so a patch may be required. If you can upstream the patch, you can make `host` default to the `PGHOST` environment variable when set. Otherwise, you can patch it locally to omit the `host` connection string parameter altogether.
+
+::: {.note}
+The error `libpq: failed (could not receive data from server: Connection refused` is generally an indication that the test suite is trying to connect through TCP.
+:::