feat: query iterator (#244)

* QueryIter returns go iterator for query results

* Iterator checks context

- Check context for error on each iteration.
- Test that canceling context stops iteration.

* ensure context is canceled

Author: gammazero

datastore.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"errors"
 	"io"
+	"iter"
 
 	query "github.com/ipfs/go-datastore/query"
 )
@@ -94,6 +95,50 @@ type Read interface {
 	Query(ctx context.Context, q query.Query) (query.Results, error)
 }
 
+// QueryIter returns a go iterator that allows ranging over query results.
+// The range yields two values, a result Entry and an error. If an error is
+// returned then iteration stops.
+//
+// Example:
+//
+//	qry := query.Query{
+//		Prefix: keyPrefix,
+//	}
+//	var foundVal []byte
+//	for ent, err := range QueryIter(dstore, qry) {
+//		if err != nil {
+//			return err
+//		}
+//		if ent.Key == lookingFor {
+//			foundVal = ent.Val
+//			break
+//		}
+//	}
+func QueryIter(ctx context.Context, ds Read, q query.Query) iter.Seq2[query.Entry, error] {
+	return func(yield func(query.Entry, error) bool) {
+		results, err := ds.Query(ctx, q)
+		if err != nil {
+			yield(query.Entry{}, err)
+			return
+		}
+		defer results.Close()
+
+		for result := range results.Next() {
+			if ctx.Err() != nil {
+				yield(query.Entry{}, ctx.Err())
+				return
+			}
+			if result.Error != nil {
+				yield(query.Entry{}, result.Error)
+				return
+			}
+			if !yield(result.Entry, nil) {
+				return
+			}
+		}
+	}
+}
+
 // Batching datastores support deferred, grouped updates to the database.
 // `Batch`es do NOT have transactional semantics: updates to the underlying
 // datastore are not guaranteed to occur in the same iota of time. Similarly,

test/basic_tests.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"context"
 	"crypto/rand"
+	"errors"
 	"fmt"
 	"reflect"
 	"strings"
@@ -403,7 +404,8 @@ func randValue() []byte {
 }
 
 func subtestQuery(t *testing.T, ds dstore.Datastore, q dsq.Query, count int) {
-	ctx := context.Background()
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
 
 	var input []dsq.Entry
 	for i := 0; i < count; i++ {
@@ -511,6 +513,58 @@ func subtestQuery(t *testing.T, ds dstore.Datastore, q dsq.Query, count int) {
 		}
 	}
 
+	// Test QueryIter for same results.
+	actual = actual[:0]
+	for ent, err := range dstore.QueryIter(ctx, ds, q) {
+		if err != nil {
+			t.Fatal("query result error: ", err)
+		}
+		actual = append(actual, ent)
+	}
+	if len(actual) != len(expected) {
+		t.Fatalf("expected %d results from QueryIter, got %d", len(expected), len(actual))
+	}
+	if len(q.Orders) == 0 {
+		dsq.Sort([]dsq.Order{dsq.OrderByKey{}}, actual)
+	}
+	for i := range actual {
+		if actual[i].Key != expected[i].Key {
+			t.Errorf("for result %d, expected key %q, got %q", i, expected[i].Key, actual[i].Key)
+			continue
+		}
+		if !q.KeysOnly && !bytes.Equal(actual[i].Value, expected[i].Value) {
+			t.Errorf("value mismatch for result %d (key=%q)", i, expected[i].Key)
+		}
+		if q.ReturnsSizes && actual[i].Size <= 0 {
+			t.Errorf("for result %d, expected size > 0 with ReturnsSizes", i)
+		}
+	}
+
+	const cancelAt = 1
+	if len(actual) > cancelAt {
+		// Test that query iterator stops when context is canceled.
+		var i int
+		for ent, err := range dstore.QueryIter(ctx, ds, q) {
+			if err != nil {
+				if !errors.Is(err, context.Canceled) {
+					t.Fatal("query result error: ", err)
+				}
+				t.Log("err at:", i, err)
+				continue
+			}
+			if ent.Key == "" {
+				t.Fatal("entry has empty key")
+			}
+			i++
+			if i == cancelAt {
+				cancel()
+			}
+		}
+		if i != cancelAt {
+			t.Fatal("expected iteration to be canceled at", cancelAt, "canceled at", i)
+		}
+	}
+
 	t.Log("deleting all keys")
 	for _, e := range input {
 		if err := ds.Delete(ctx, dstore.RawKey(e.Key)); err != nil {