SpanProcess.onStart does not have access to span attributes

[anuraaga]

What happened?

Steps to Reproduce

Access span.attributes in a SpanProcessor.onStart method

Expected Result

Read attributes

Actual Result

Attributes always empty

Additional Details

Span processor is called in span constructor

opentelemetry-js/packages/opentelemetry-sdk-trace-base/src/Span.ts

Line 123 in 5ce32c0

this._spanProcessor.onStart(this, context);

But attributes are set after

https://github.com/open-telemetry/opentelemetry-js/blob/main/packages/opentelemetry-sdk-trace-base/src/Tracer.ts#L150

Presumably it would be better to call the spanprocessor fromTracer.startSpan instead of constructor. Alternatively, attributes could be passed into the constructor though backwards compatibility may be challenging.

OpenTelemetry Setup Code

No response

package.json

No response

Relevant log output

No response

[ArtAhmetaj]

So I checked the issue and want to be a contributor to this. I checked both your options on either calling the spanprocessor or modifying the constructor:

• Calling the span processor
  Seems nice in hindsight but I fear that the span class is being used by others directly without it being used through the tracer (I am new to the project so I am assuming that based on also the @deprecated decorators), so if I push this functionality to the tracer that means that when using span directly the .onStart will not be triggered, unless I just expose this as a span method with the ability to override the on start hooks, if that functionality is fine then that would be okay.

  const span = new Span(
      this,
      context,
      name,
      spanContext,
      spanKind,
      parentSpanId,
      links,
      options.startTime
    );
    // Set initial span attributes. The attributes object may have been mutated
    // by the sampler, so we sanitize the merged attributes before setting them.
    const initAttributes = sanitizeAttributes(
      Object.assign(attributes, samplingResult.attributes)
    );
    span.setAttributes(initAttributes);
    span.registerProcessor(): 

Seems really clunky tho...

• Adding the data in the constructor
  A simple solution when I had a look first on it:

constructor(
    parentTracer: Tracer,
    context: Context,
    spanName: string,
    spanContext: SpanContext,
    kind: SpanKind,
    parentSpanId?: string,
    links: Link[] = [],
    startTime?: TimeInput,
    _deprecatedClock?: unknown, // keeping this argument even though it is unused to ensure backwards compatibility,
    initAttributes?: Attributes
  ) {
    this.name = spanName;
    this._spanContext = spanContext;
    this.parentSpanId = parentSpanId;
    this.kind = kind;
    this.links = links;
    if(initAttributes){
      this.setAttributes(initAttributes);
    }

    const now = Date.now();
    this._performanceStartTime = otperformance.now();
    this._performanceOffset =
      now - (this._performanceStartTime + getTimeOrigin());
    this._startTimeProvided = startTime != null;

    this.startTime = this._getTime(startTime ?? now);

    this.resource = parentTracer.resource;
    this.instrumentationLibrary = parentTracer.instrumentationLibrary;
    this._spanLimits = parentTracer.getSpanLimits();
    this._spanProcessor = parentTracer.getActiveSpanProcessor();
    this._spanProcessor.onStart(this, context);
    this._attributeValueLengthLimit =
      this._spanLimits.attributeValueLengthLimit || 0;
  }

Doesn't seem like the span has optional variables like this till now atleast, but I think it would be simple enough, would not break in the constructor or the wrapped call in the constructor.

Let me know what you think and I can make a PR about it.

[anuraaga]

Thanks for the clarification @ArtAhmetaj - indeed, I was thinking the constructor isn't called but you are right that it may be since it is currently a public API. Actually coincidentally, I found it used in such a way in Prisma instrumentation just yesterday. So the second approach seems better to me too