[OTELS-985] omit hostname field from JSON payload when empty

pkg/logs/processor/encoder_test.go

@@ -269,6 +269,70 @@ func TestJsonEncoder(t *testing.T) {
 	})
 }
 
+func TestJsonEncoderHostnameOmitted(t *testing.T) {
+	logsConfig := &config.LogsConfig{Source: "Source"}
+	source := sources.NewLogSource("", logsConfig)
+
+	t.Run("empty hostname is omitted from JSON", func(t *testing.T) {
+		msg := newMessage([]byte("log message"), source, message.StatusInfo)
+		msg.State = message.StateRendered
+
+		err := JSONEncoder.Encode(msg, "")
+		assert.Nil(t, err)
+
+		var parsed map[string]any
+		assert.Nil(t, json.Unmarshal(msg.GetContent(), &parsed))
+		_, hasHostname := parsed["hostname"]
+		assert.False(t, hasHostname, "hostname key should be absent when hostname is empty")
+	})
+
+	t.Run("non-empty hostname is present in JSON", func(t *testing.T) {
+		msg := newMessage([]byte("log message"), source, message.StatusInfo)
+		msg.State = message.StateRendered
+
+		err := JSONEncoder.Encode(msg, "my-host")
+		assert.Nil(t, err)
+
+		var parsed map[string]any
+		assert.Nil(t, json.Unmarshal(msg.GetContent(), &parsed))
+		hostname, hasHostname := parsed["hostname"]
+		assert.True(t, hasHostname, "hostname key should be present when hostname is non-empty")
+		assert.Equal(t, "my-host", hostname)
+	})
+}
+
+func TestJsonServerlessInitEncoderHostnameOmitted(t *testing.T) {
+	logsConfig := &config.LogsConfig{Source: "Source"}
+	source := sources.NewLogSource("", logsConfig)
+
+	t.Run("empty hostname is omitted from JSON", func(t *testing.T) {
+		msg := newMessage([]byte("log message"), source, message.StatusInfo)
+		msg.State = message.StateRendered
+
+		err := JSONServerlessInitEncoder.Encode(msg, "")
+		assert.Nil(t, err)
+
+		var parsed map[string]any
+		assert.Nil(t, json.Unmarshal(msg.GetContent(), &parsed))
+		_, hasHostname := parsed["hostname"]
+		assert.False(t, hasHostname, "hostname key should be absent when hostname is empty")
+	})
+
+	t.Run("non-empty hostname is present in JSON", func(t *testing.T) {
+		msg := newMessage([]byte("log message"), source, message.StatusInfo)
+		msg.State = message.StateRendered
+
+		err := JSONServerlessInitEncoder.Encode(msg, "my-host")
+		assert.Nil(t, err)
+
+		var parsed map[string]any
+		assert.Nil(t, json.Unmarshal(msg.GetContent(), &parsed))
+		hostname, hasHostname := parsed["hostname"]
+		assert.True(t, hasHostname, "hostname key should be present when hostname is non-empty")
+		assert.Equal(t, "my-host", hostname)
+	})
+}
+
 func TestEncoderToValidUTF8(t *testing.T) {
 	// valid utf-8
 	assert.Equal(t, "", toValidUtf8(nil))

pkg/logs/processor/json.go

@@ -30,7 +30,7 @@ type jsonPayload struct {
 	Message   ValidUtf8Bytes `json:"message"`
 	Status    string         `json:"status"`
 	Timestamp int64          `json:"timestamp"`
-	Hostname  string         `json:"hostname"`
+	Hostname  string         `json:"hostname,omitempty"`
 	Service   string         `json:"service"`
 	Source    string         `json:"ddsource"`
 	Tags      string         `json:"ddtags"`

pkg/logs/processor/json_serverless_init.go

@@ -29,7 +29,7 @@ type jsonServerlessInitPayload struct {
 	Message   ValidUtf8Bytes `json:"message"`
 	Status    string         `json:"status"`
 	Timestamp int64          `json:"timestamp"`
-	Hostname  string         `json:"hostname"`
+	Hostname  string         `json:"hostname,omitempty"`
 	Service   string         `json:"service,omitempty"`
 	Source    string         `json:"ddsource"`
 	Tags      string         `json:"ddtags"`

releasenotes/notes/fix-otel-logs-empty-hostname-fargate-6b2fdadda066a5ab.yaml

@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    Fix OTel logs sent via the Datadog exporter on ECS Fargate showing an empty
+    string for the ``host`` reserved attribute instead of no host. The hostname
+    field is now omitted from the log payload when no hostname applies (e.g.
+    serverless/Fargate environments), so logs appear without a host rather than
+    with a blank one.