fix(json): use value alias in Document.UnmarshalJSON to prevent infinite recursion with encoding/json/v2 (#4748)

The pattern 'type Alias *Document' does not strip methods under
encoding/json/v2 (GOEXPERIMENT=jsonv2), causing UnmarshalJSON to call
itself infinitely until the goroutine stack overflows (1GB limit).

Change to 'type Alias Document' with (*Alias)(d) cast — the standard
Go pattern that works correctly with both encoding/json v1 and v2.

Adds a regression test that uses debug.SetMaxStack to shrink the
goroutine stack limit to 8MB, making the overflow happen in milliseconds
rather than minutes if the recursion is reintroduced.

Ref: https://github.com/golang/go/issues/75361

Signed-off-by: Benjamin Grandfond <benjamin.grandfond@docker.com>

syft/format/syftjson/model/document.go

@@ -17,8 +17,8 @@ type Document struct {
 }
 
 func (d *Document) UnmarshalJSON(data []byte) error {
-	type Alias *Document
+	type Alias Document
-	aux := Alias(d)
+	aux := (*Alias)(d)
 
 	if err := json.Unmarshal(data, aux); err != nil {
 		return fmt.Errorf("could not unmarshal syft JSON document: %w", err)

syft/format/syftjson/model/document_test.go

@@ -2,12 +2,48 @@ package model
 
 import (
 	"encoding/json"
+	"runtime/debug"
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
 
+// TestDocumentUnmarshalJSON_NoInfiniteRecursion guards against the regression
+// where encoding/json/v2 (GOEXPERIMENT=jsonv2) would call UnmarshalJSON
+// recursively on the alias type, causing a goroutine stack overflow.
+// See: https://github.com/golang/go/issues/75361
+func TestDocumentUnmarshalJSON_NoInfiniteRecursion(t *testing.T) {
+	data := `{
+		"artifacts": [
+			{"id": "1", "name": "pkg-a", "version": "1.0", "type": "npm", "foundBy": "cataloger", "locations": [], "licenses": [], "language": "javascript", "cpes": [], "purl": "pkg:npm/pkg-a@1.0"},
+			{"id": "2", "name": "pkg-b", "version": "2.0", "type": "gem", "foundBy": "cataloger", "locations": [], "licenses": [], "language": "ruby",      "cpes": [], "purl": "pkg:gem/pkg-b@2.0"}
+		],
+		"schema": {"version": "16.0.0", "url": "https://example.com"},
+		"descriptor": {"name": "syft", "version": "1.0.0"}
+	}`
+
+	// Shrink the max goroutine stack to 8MB so that infinite recursion
+	// (golang/go#75361 — encoding/json/v2 re-dispatching to UnmarshalJSON
+	// via type Alias *Document) overflows quickly rather than after minutes.
+	old := debug.SetMaxStack(8 * 1024 * 1024)
+	defer debug.SetMaxStack(old)
+
+	done := make(chan error, 1)
+	go func() {
+		var doc Document
+		done <- json.Unmarshal([]byte(data), &doc)
+	}()
+
+	select {
+	case err := <-done:
+		require.NoError(t, err)
+	case <-time.After(5 * time.Second):
+		t.Fatal("json.Unmarshal did not complete — likely infinite recursion in UnmarshalJSON")
+	}
+}
+
 func TestDocumentUnmarshalJSON_SchemaDetection(t *testing.T) {
 	tests := []struct {
 		name     string