page server: Restore base backup from S3 at page server startup

This includes a "launch.sh" script that I've been using to initialize
and launch the Postgres + Page Server combination.

launch.sh

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+#!/bin/sh
+#
+# Set up a simple Compute Node + Page Server combination locally.
+#
+# NOTE: This doesn't clean up between invocations. You'll need to manually:
+#
+# - Kill any previous 'postgres' and 'pageserver' processes
+# - Clear the S3 bucket
+# - Remove the 'zenith-pgdata' directory
+
+
+set -e
+
+# Set up some config.
+#
+# CHANGE THESE ACCORDING TO YOUR S3 INSTALLATION
+export S3_REGION=auto
+export S3_ENDPOINT=https://localhost:9000
+export S3_ACCESSKEY=minioadmin
+export S3_SECRET=pikkunen
+export S3_BUCKET=zenith-testbucket
+
+
+COMPUTE_NODE_PGDATA=zenith-pgdata
+
+
+# 1. Initialize a cluster.
+initdb -D $COMPUTE_NODE_PGDATA -U zenith
+
+echo "port=65432" >> $COMPUTE_NODE_PGDATA/postgresql.conf
+echo "log_connections=on" >> $COMPUTE_NODE_PGDATA/postgresql.conf
+
+# Use a small shared_buffers, so that we hit the Page Server more
+# easily.
+echo "shared_buffers = 1MB" >> $COMPUTE_NODE_PGDATA/postgresql.conf
+
+# TODO: page server should use a replication slot, or some other mechanism
+# to make sure that the primary doesn't lose data that the page server still
+# needs. (The WAL safekeepers should ensure that)
+echo "wal_keep_size=10GB" >> $COMPUTE_NODE_PGDATA/postgresql.conf
+
+# Tell the Postgres server how to connect to the Page Server
+echo "page_server_connstring='host=localhost port=5430'" >> $COMPUTE_NODE_PGDATA/postgresql.conf
+
+
+# 2. Run zenith_push to push a base backup fo the database to an S3 bucket. The
+# Page Server will read it from there
+zenith_push -D $COMPUTE_NODE_PGDATA
+
+
+# 3. Launch page server
+rm -rf /tmp/pgdata-dummy
+initdb -N -D /tmp/pgdata-dummy
+PGDATA=/tmp/pgdata-dummy ./target/debug/pageserver  &
+
+# 4. Start up the Postgres server
+postgres -D $COMPUTE_NODE_PGDATA &
+
+
+echo "ALL SET! You can now connect to Postgres with something like:"
+echo ""
+echo 'psql "dbname=postgres host=localhost user=zenith port=65432"'