perf: Parse FDv2 payloads in a single jsonstream pass

go.mod

@@ -8,7 +8,7 @@ require (
 	github.com/gregjones/httpcache v0.0.0-20171119193500-2bcd89a1743f
 	github.com/launchdarkly/ccache v1.1.0
 	github.com/launchdarkly/eventsource v1.10.0
-	github.com/launchdarkly/go-jsonstream/v3 v3.1.1
+	github.com/launchdarkly/go-jsonstream/v3 v3.1.2-0.20260729181618-28aa1e35cda3
 	github.com/launchdarkly/go-ntlm-proxy-auth v1.0.3
 	github.com/launchdarkly/go-sdk-common/v3 v3.5.0
 	github.com/launchdarkly/go-sdk-events/v3 v3.6.2

go.sum

@@ -19,8 +19,8 @@ github.com/launchdarkly/ccache v1.1.0 h1:voD1M+ZJXR3MREOKtBwgTF9hYHl1jg+vFKS/+VA
 github.com/launchdarkly/ccache v1.1.0/go.mod h1:TlxzrlnzvYeXiLHmesMuvoZetu4Z97cV1SsdqqBJi1Q=
 github.com/launchdarkly/eventsource v1.10.0 h1:H9Tp6AfGu/G2qzBJC26iperrvwhzdbiA/gx7qE2nDFI=
 github.com/launchdarkly/eventsource v1.10.0/go.mod h1:J3oa50bPvJesZqNAJtb5btSIo5N6roDWhiAS3IpsKck=
-github.com/launchdarkly/go-jsonstream/v3 v3.1.1 h1:ugupp2eNtwVbr69KCdeUrm1vUf1/3ju4Wdliaob95uY=
-github.com/launchdarkly/go-jsonstream/v3 v3.1.1/go.mod h1:ZBjhKq8mhArCtqotGRGnteY6eXpNm1GaOdUSZHh+ZjM=
+github.com/launchdarkly/go-jsonstream/v3 v3.1.2-0.20260729181618-28aa1e35cda3 h1:wT4WhK2sqM4j0nJwa+eO7g+XCblsT5myDhiadFaXeA8=
+github.com/launchdarkly/go-jsonstream/v3 v3.1.2-0.20260729181618-28aa1e35cda3/go.mod h1:ZBjhKq8mhArCtqotGRGnteY6eXpNm1GaOdUSZHh+ZjM=
 github.com/launchdarkly/go-ntlm-proxy-auth v1.0.3 h1:i3V0N+R0Fd2nXfGEVKCBIZ8kyttZ+SRKvBG8cdcphO4=
 github.com/launchdarkly/go-ntlm-proxy-auth v1.0.3/go.mod h1:kU5uMfNSTpYE6fIzmAXjFxUdmnaDPUEQ5zKm3RVKUsY=
 github.com/launchdarkly/go-ntlmssp v1.0.3 h1:rFxOnnEJ2DzJ+NU0plhXqnldJUwn3wWJFTWKCmaiQdE=

internal/datasourcev2/event_parsing.go

@@ -0,0 +1,51 @@
+package datasourcev2
+
+import (
+	"encoding/json"
+
+	"github.com/launchdarkly/go-server-sdk/v7/subsystems"
+	"github.com/launchdarkly/go-server-sdk/v7/subsystems/ldstoretypes"
+)
+
+// This file contains the parts of FDv2 protocol event decoding that are shared by both build
+// variants. The event decoders themselves are split by build tag:
+//
+//   - event_parsing_default.go: a single-pass jsonstream decoder using jreader.RawValue/Offset for
+//     zero-copy raw capture. Each event, and in particular each put-object item's JSON, is scanned
+//     once. Its scalar decoders are written to match encoding/json's acceptance rules (see below).
+//   - event_parsing_easyjson.go: a reflection-based decode (encoding/json) for the
+//     launchdarkly_easyjson build, so that no new code depends on the easyjson token reader, which
+//     is planned for removal from go-jsonstream.
+//
+// The two variants accept and reject the same payloads. The easyjson build decodes each scalar
+// event with encoding/json into the subsystems types; the default build's jsonstream decoders are
+// written to mirror that behavior exactly -- including rejecting non-integer version/target values
+// on the plain-int types and matching Selector's float64-based (truncating) version handling. Both
+// produce a parsedPutObject carrying the item's raw bytes and, for recognized kinds, its parsed
+// form, so the ChangeSet's collections are assembled without a later re-parse.
+
+const errNoKnownPollingEvents = "didn't receive any known protocol events in polling payload"
+
+// parsedPutObject is the result of decoding a put-object event's data.
+type parsedPutObject struct {
+	kind    subsystems.ObjectKind
+	key     string
+	version int
+	// object is the item's raw JSON. In the default build it references the input buffer passed
+	// to parsePutObjectEventData and is valid only as long as that buffer is.
+	object json.RawMessage
+	// item is the parsed representation of object, set only when hasItem is true. hasItem is
+	// false when the kind is unrecognized, in which case the raw bytes are still retained for
+	// forwards compatibility.
+	item    ldstoretypes.ItemDescriptor
+	hasItem bool
+}
+
+// addTo adds the put to a change-set builder, carrying the parsed item along when there is one.
+func (p parsedPutObject) addTo(builder *subsystems.ChangeSetBuilder) {
+	if p.hasItem {
+		builder.AddParsedPut(p.kind, p.key, p.version, p.object, p.item)
+	} else {
+		builder.AddPut(p.kind, p.key, p.version, p.object)
+	}
+}

internal/datasourcev2/event_parsing_benchmark_test.go

@@ -0,0 +1,43 @@
+package datasourcev2
+
+import (
+	"context"
+	"testing"
+)
+
+// These benchmarks guard the single-pass polling-payload parser against regressions, and keep the
+// previous reflection-based decode path measurable for comparison. The dominant cost of client
+// initialization over FDv2 is parsing the initial polling payload, so this path is
+// performance-sensitive: every SDK start pays it once per payload.
+
+func BenchmarkParsePollingPayload(b *testing.B) {
+	body := makePollingBody(b, 2000)
+	b.SetBytes(int64(len(body)))
+	b.ReportAllocs()
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		changeSet, err := parsePollingPayload(context.Background(), body)
+		if err != nil {
+			b.Fatal(err)
+		}
+		if _, err := changeSet.Collections(); err != nil {
+			b.Fatal(err)
+		}
+	}
+}
+
+func BenchmarkParsePollingPayloadReflectionReference(b *testing.B) {
+	body := makePollingBody(b, 2000)
+	b.SetBytes(int64(len(body)))
+	b.ReportAllocs()
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		changeSet, err := parsePollingPayloadReflectionReference(body)
+		if err != nil {
+			b.Fatal(err)
+		}
+		if _, err := changeSet.Collections(); err != nil {
+			b.Fatal(err)
+		}
+	}
+}