Function _startChildSpan

packages/core/src/tracing/trace.ts:548–600

* Creates a new `Span` while setting the current `Span.id` as `parentSpanId`. * This inherits the sampling decision from the parent span.

(
  parentSpan: Span,
  scope: Scope,
  spanArguments: SentrySpanArguments,
  isolationScope: Scope,
)

 * This inherits the sampling decision from the parent span.
 */
function _startChildSpan(
  parentSpan: Span,
  scope: Scope,
  spanArguments: SentrySpanArguments,
  isolationScope: Scope,
): Span {
  const { spanId, traceId } = parentSpan.spanContext();
  const _isTracingSuppressed = isTracingSuppressed(scope);
  const sampled = _isTracingSuppressed ? false : spanIsSampled(parentSpan);

  const childSpan = sampled
    ? new SentrySpan({
        ...spanArguments,
        parentSpanId: spanId,
        traceId,
        sampled,
      })
    : new SentryNonRecordingSpan({ traceId });

  addChildSpanToSpan(parentSpan, childSpan);

  setCapturedScopesOnSpan(childSpan, scope, isolationScope);

  const client = getClient();

  if (!client) {
    return childSpan;
  }

  if (hasSpanStreamingEnabled(client) && spanIsNonRecordingSpan(childSpan)) {
    if (spanIsNonRecordingSpan(parentSpan) && parentSpan.dropReason) {
      // We land here if the parent span was a segment span that was ignored (`ignoreSpans`).
      // In this case, the child was also ignored (see `sampled` above) but we need to
      // record a client outcome for the child.
      childSpan.dropReason = parentSpan.dropReason;
      client.recordDroppedEvent(parentSpan.dropReason, 'span');
    } else if (!_isTracingSuppressed) {
      // Otherwise, the child is not sampled due to sampling of the parent span,
      // hence we record a sample_rate client outcome for the child.
      childSpan.dropReason = 'sample_rate';
      client.recordDroppedEvent('sample_rate', 'span');
    }
  }

  client.emit('spanStart', childSpan);
  // If it has an endTimestamp, it's already ended
  if (spanArguments.endTimestamp) {
    client.emit('spanEnd', childSpan);
    client.emit('afterSpanEnd', childSpan);
  }

  return childSpan;
}

function getParentSpan(scope: Scope, customParentSpan: Span | null | undefined): SentrySpan | undefined {
  // always use the passed in span directly