[sdk] Provide better concurrency modes

docs/logs/extending-the-sdk/MyExporter.cs

@@ -5,6 +5,7 @@
 using OpenTelemetry;
 using OpenTelemetry.Logs;
 
+[ConcurrencyModes(ConcurrencyModes.Multithreaded | ConcurrencyModes.Reentrant)]
 internal class MyExporter : BaseExporter<LogRecord>
 {
     private readonly string name;

src/OpenTelemetry/.publicApi/Stable/PublicAPI.Unshipped.txt

@@ -1,3 +1,10 @@
+OpenTelemetry.ConcurrencyModes
+OpenTelemetry.ConcurrencyModes.Global = 128 -> OpenTelemetry.ConcurrencyModes
+OpenTelemetry.ConcurrencyModes.Multithreaded = 2 -> OpenTelemetry.ConcurrencyModes
+OpenTelemetry.ConcurrencyModes.Reentrant = 1 -> OpenTelemetry.ConcurrencyModes
+OpenTelemetry.ConcurrencyModesAttribute
+OpenTelemetry.ConcurrencyModesAttribute.ConcurrencyModesAttribute(OpenTelemetry.ConcurrencyModes supported) -> void
+OpenTelemetry.ConcurrencyModesAttribute.Supported.get -> OpenTelemetry.ConcurrencyModes
 OpenTelemetry.Metrics.Exemplar
 OpenTelemetry.Metrics.Exemplar.DoubleValue.get -> double
 OpenTelemetry.Metrics.Exemplar.Exemplar() -> void

src/OpenTelemetry/ConcurrencyModes.cs

@@ -0,0 +1,43 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+namespace OpenTelemetry;
+
+/// <summary>
+/// Describes the concurrency mode of an OpenTelemetry extension.
+/// </summary>
+[Flags]
+public enum ConcurrencyModes : byte
+{
+    /*
+    0 0 0 0 0 0 0 0
+    | | | | | | | |
+    | | | | | | | +--- Reentrant
+    | | | | | | +----- Multithreaded
+    | | | | | +------- (reserved)
+    | | | | +--------- (reserved)
+    | | | +----------- (reserved)
+    | | +------------- (reserved)
+    | +--------------- (reserved)
+    +----------------- Global
+    */
+
+    /// <summary>
+    /// Reentrant, the component can be invoked recursively without resulting
+    /// a deadlock or infinite loop.
+    /// </summary>
+    Reentrant = 0b1,
+
+    /// <summary>
+    /// Multithreaded, the component can be invoked concurrently across
+    /// multiple threads.
+    /// </summary>
+    Multithreaded = 0b10,
+
+    /// <summary>
+    /// Global, when combined with other flags, indicates that a per-instance
+    /// synchronization is insufficient, a global synchronization is required
+    /// across all instances of the component.
+    /// </summary>
+    Global = 0b10000000,
+}

src/OpenTelemetry/ConcurrencyModesAttribute.cs

@@ -0,0 +1,27 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+namespace OpenTelemetry;
+
+/// <summary>
+/// An attribute for declaring the supported <see cref="ConcurrencyModes"/> of an OpenTelemetry extension.
+/// </summary>
+[AttributeUsage(AttributeTargets.Class, AllowMultiple = false, Inherited = true)]
+public sealed class ConcurrencyModesAttribute : Attribute
+{
+    private readonly ConcurrencyModes supportedConcurrencyModes;
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="ConcurrencyModesAttribute"/> class.
+    /// </summary>
+    /// <param name="supported"><see cref="ConcurrencyModes"/>.</param>
+    public ConcurrencyModesAttribute(ConcurrencyModes supported)
+    {
+        this.supportedConcurrencyModes = supported;
+    }
+
+    /// <summary>
+    /// Gets the supported <see cref="ConcurrencyModes"/>.
+    /// </summary>
+    public ConcurrencyModes Supported => this.supportedConcurrencyModes;
+}

src/OpenTelemetry/SimpleExportProcessor.cs

@@ -13,6 +13,7 @@ public abstract class SimpleExportProcessor<T> : BaseExportProcessor<T>
     where T : class
 {
     private readonly object syncObject = new();
+    private readonly ConcurrencyModes supportedConcurrencyModes;
 
     /// <summary>
     /// Initializes a new instance of the <see cref="SimpleExportProcessor{T}"/> class.
@@ -21,11 +22,32 @@ public abstract class SimpleExportProcessor<T> : BaseExportProcessor<T>
     protected SimpleExportProcessor(BaseExporter<T> exporter)
         : base(exporter)
     {
+        var exporterType = exporter.GetType();
+        var attributes = exporterType.GetCustomAttributes(typeof(ConcurrencyModesAttribute), true);
+        if (attributes.Length > 0)
+        {
+            var attr = (ConcurrencyModesAttribute)attributes[attributes.Length - 1];
+            this.supportedConcurrencyModes = attr.Supported;
+        }
     }
 
     /// <inheritdoc />
     protected override void OnExport(T data)
     {
+        if (this.supportedConcurrencyModes.HasFlag(ConcurrencyModes.Multithreaded))
+        {
+            try
+            {
+                this.exporter.Export(new Batch<T>(data));
+            }
+            catch (Exception ex)
+            {
+                OpenTelemetrySdkEventSource.Log.SpanProcessorException(nameof(this.OnExport), ex);
+            }
+
+            return;
+        }
+
         lock (this.syncObject)
         {
             try