Add DB subscription (#114)

Adds a struct that can subscribe to changes in a database query,
starting from some cursor onwards. Features:
- Poll based on an interval or on an input notifier channel
- Serve updates via an output channel
- Stops polling when context is closed
- Non-racy unit tests (hopefully)

Also adds a flexible DB query against the gateway_envelopes table that
can query based on a range of params. Given the size of the query, I
should add a unit test for it eventually.

README.md

@@ -108,3 +108,5 @@ We use [sqlc](https://docs.sqlc.dev/en/latest/index.html) to generate the code f
 ```sh
 dev/generate
 ```
+
+If needed, there is a sqlc [playground](https://play.sqlc.dev/p/f6eebe941750560934cefa943c77f63497debc828c487e8d1771fb6d83773246) for experimenting with how the query syntax translates into Go code.

dev/test

@@ -3,7 +3,7 @@ set -e
 
 ulimit -n 2048
 
-go test ./... "$@"
+go test -timeout 3s `go list ./... | grep -v -e 'pkg/abis' -e 'pkg/config' -e 'pkg/proto' -e 'pkg/mock' -e 'pkg/testing'` "$@"
 
 if [ -n "${RACE:-}" ]; then
     echo

pkg/db/queries.sql

@@ -14,7 +14,22 @@ WHERE
 
 -- name: InsertGatewayEnvelope :execrows
 SELECT
-	insert_gateway_envelope(@originator_id, @sequence_id, @topic, @originator_envelope);
+	insert_gateway_envelope(@originator_id, @originator_sequence_id, @topic, @originator_envelope);
+
+-- name: SelectGatewayEnvelopes :many
+SELECT
+	*
+FROM
+	gateway_envelopes
+WHERE (sqlc.narg('topic')::BYTEA IS NULL
+	OR topic = @topic)
+AND (sqlc.narg('originator_node_id')::INT IS NULL
+	OR originator_node_id = @originator_node_id)
+AND (sqlc.narg('originator_sequence_id')::BIGINT IS NULL
+	OR originator_sequence_id > @originator_sequence_id)
+AND (sqlc.narg('gateway_sequence_id')::BIGINT IS NULL
+	OR id > @gateway_sequence_id)
+LIMIT sqlc.narg('row_limit')::INT;
 
 -- name: InsertStagedOriginatorEnvelope :one
 SELECT

pkg/db/subscription.go

@@ -0,0 +1,101 @@
+package db
+
+import (
+	"context"
+	"fmt"
+	"time"
+
+	"go.uber.org/zap"
+)
+
+type PollableDBQuery[ValueType any] func(ctx context.Context, lastSeenID int64, numRows int32) (results []ValueType, lastID int64, err error)
+
+// Poll whenever notified, or at an interval if not notified
+type PollingOptions struct {
+	Interval time.Duration
+	Notifier <-chan bool
+	NumRows  int32
+}
+
+type DBSubscription[ValueType any] struct {
+	ctx        context.Context
+	log        *zap.Logger
+	lastSeenID int64
+	options    PollingOptions
+	query      PollableDBQuery[ValueType]
+	updates    chan<- []ValueType
+}
+
+func NewDBSubscription[ValueType any](
+	ctx context.Context,
+	log *zap.Logger,
+	query PollableDBQuery[ValueType],
+	lastSeenID int64,
+	options PollingOptions,
+) *DBSubscription[ValueType] {
+	return &DBSubscription[ValueType]{
+		ctx:        ctx,
+		log:        log,
+		lastSeenID: lastSeenID,
+		options:    options,
+		query:      query,
+		updates:    nil,
+	}
+}
+
+func (s *DBSubscription[ValueType]) Start() (<-chan []ValueType, error) {
+	if s.updates != nil {
+		return nil, fmt.Errorf("Already started")
+	}
+	if s.options.NumRows <= 0 || s.log == nil {
+		return nil, fmt.Errorf("Required params not provided")
+	}
+	updates := make(chan []ValueType)
+	s.updates = updates
+
+	go func() {
+		s.poll()
+
+		timer := time.NewTimer(s.options.Interval)
+		for {
+			timer.Reset(s.options.Interval)
+			select {
+			case <-s.ctx.Done():
+				s.log.Info("Context done; stopping subscription")
+				close(s.updates)
+				return
+			case <-s.options.Notifier:
+				s.poll()
+			case <-timer.C:
+				s.poll()
+			}
+		}
+	}()
+
+	return updates, nil
+}
+
+func (s *DBSubscription[ValueType]) poll() {
+	// Repeatedly query page by page until no more results
+	for {
+		results, lastID, err := s.query(s.ctx, s.lastSeenID, s.options.NumRows)
+		if err != nil {
+			s.log.Error(
+				"Error querying for DB subscription",
+				zap.Error(err),
+				zap.Int64("lastSeenID", s.lastSeenID),
+				zap.Int32("numRows", s.options.NumRows),
+			)
+			// Did not update lastSeenID; will retry on next poll
+			break
+		}
+		if len(results) == 0 {
+			break
+		}
+		s.lastSeenID = lastID
+		s.updates <- results
+		if int32(len(results)) < s.options.NumRows {
+			break
+		}
+	}
+}