Clarify spans export concurrency (open-telemetry#4205)

Same as
open-telemetry#4173
for trace SDK.

Fixes
open-telemetry#4134

CHANGELOG.md

@@ -13,6 +13,8 @@ release.
 
 - Make all fields as identifying for Tracer. Previously attributes were omitted from being identifying.
   ([#4161](https://github.com/open-telemetry/opentelemetry-specification/pull/4161))
+- Clarify that `Export` MUST NOT be called by simple and batching processors concurrently.
+  ([#4205](https://github.com/open-telemetry/opentelemetry-specification/pull/4205))
 
 ### Metrics
 

specification/trace/sdk.md

@@ -693,6 +693,9 @@ This is an implementation of `SpanProcessor` which passes finished spans
 and passes the export-friendly span data representation to the configured
 `SpanExporter`, as soon as they are finished.
 
+The processor MUST synchronize calls to `Span Exporter`'s `Export`
+to make sure that they are not invoked concurrently.
+
 **Configurable parameters:**
 
 * `exporter` - the exporter where the spans are pushed.
@@ -703,9 +706,11 @@ This is an implementation of the `SpanProcessor` which create batches of finishe
 spans and passes the export-friendly span data representations to the
 configured `SpanExporter`.
 
+The processor MUST synchronize calls to `Span Exporter`'s `Export`
+to make sure that they are not invoked concurrently.
+
 The processor SHOULD export a batch when any of the following happens AND the
-previous export call has returned (for additional concurrency details see
-the [Export() specification](#exportbatch)):
+previous export call has returned:
 
 - `scheduledDelayMillis` after the processor is constructed OR the first span
   is received by the span processor.
@@ -742,6 +747,9 @@ The goal of the interface is to minimize burden of implementation for
 protocol-dependent telemetry exporters. The protocol exporter is expected to be
 primarily a simple telemetry data encoder and transmitter.
 
+Each implementation MUST document the concurrency characteristics the SDK
+requires of the exporter.
+
 ### Interface Definition
 
 The exporter MUST support three functions: **Export**, **Shutdown**, and **ForceFlush**.
@@ -755,15 +763,15 @@ Protocol exporters that will implement this
 function are typically expected to serialize and transmit the data to the
 destination.
 
-Export() will never be called concurrently for the same exporter instance.
+Export() should not be be called concurrently with other `Export` calls for the
+same exporter instance.
+
 Depending on the implementation the result of the export may be returned to the
 Processor not in the return value of the call to Export() but in a language
 specific way for signaling completion of an asynchronous task. This means that
-while an instance of an exporter will never have its Export() called
+while an instance of an exporter should never have its Export() called
 concurrently it does not mean that the task of exporting can not be done
-concurrently. How this is done is outside the scope of this specification. Each
-implementation MUST document the concurrency characteristics the SDK requires of
-the exporter.
+concurrently. How this is done is outside the scope of this specification.
 
 Export() MUST NOT block indefinitely, there MUST be a reasonable upper limit
 after which the call must time out with an error result (`Failure`).