fix(lifecycle): release invocation context after platform report to prevent memory leak (#1050)

JIRA: https://datadoghq.atlassian.net/browse/SVLS-8625
Git issue reported:
#1049

## What does this PR do?

Fixes a memory leak in the extension where `ContextBuffer` accumulated
`Context` objects indefinitely across warm Lambda invocations.

**Root cause:** `on_platform_report()` reads the `Context` for runtime
duration and CPU/network enhanced metrics, but never called
`context_buffer.remove()` afterwards. Each warm invocation appended a
new `Context` to the buffer; none were ever freed.

**Fix:** Call `self.context_buffer.remove(request_id)` at the end of
`on_platform_report()`, after all processing for the invocation is
complete.

**Impact:** Most visible when `DD_CAPTURE_LAMBDA_PAYLOAD=true` — each
retained `Context` holds the full captured request/response payload as
span metadata (~500 KB per invocation). Without the fix, a Lambda
function invoked thousands of times would accumulate hundreds of MB of
leaked memory in the extension process.

## Testing

- Unit tests
- A test with 50+ invocations shows


| without the fix | with the fix |
|--------|--------|
| `buffer size after remove: 52`|`buffer size after remove: 1`|

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: litianningdatadog

bottlecap/src/lifecycle/invocation/context.rs

@@ -9,7 +9,7 @@ use std::{
 
 use libdd_trace_protobuf::pb::Span;
 use serde_json::Value;
-use tracing::debug;
+use tracing::{debug, warn};
 
 #[derive(Debug, Clone, PartialEq)]
 pub struct Context {
@@ -199,7 +199,7 @@ impl ContextBuffer {
         {
             return self.buffer.remove(i);
         }
-        debug!("Context for request_id: {:?} not found", request_id);
+        warn!("Context for request_id: {:?} not found", request_id);
 
         None
     }

bottlecap/src/lifecycle/invocation/processor.rs

@@ -9,7 +9,7 @@ use libdd_trace_protobuf::pb::Span;
 use libdd_trace_utils::tracer_header_tags;
 use serde_json::Value;
 use tokio::time::Instant;
-use tracing::{debug, warn};
+use tracing::{debug, trace, warn};
 
 use crate::{
     config::{self, aws::AwsConfig},
@@ -771,6 +771,14 @@ impl Processor {
             self.enhanced_metrics
                 .set_cpu_time_enhanced_metrics(offsets.cpu_offset.clone());
         }
+
+        // Release the context now that all processing for this invocation is complete.
+        // This prevents unbounded memory growth across warm invocations.
+        self.context_buffer.remove(request_id);
+        trace!(
+            "Context released (buffer size after remove: {})",
+            self.context_buffer.size()
+        );
     }
 
     /// Handles Managed Instance mode platform report processing.
@@ -1612,8 +1620,7 @@ mod tests {
                         duration_ms,
                         status,
                         error_type,
-                        should_have_context_after,
-                    ): (&str, bool, f64, Status, Option<String>, bool) = $value;
+                    ): (&str, bool, f64, Status, Option<String>) = $value;
 
                     let mut processor = setup();
 
@@ -1655,6 +1662,17 @@ mod tests {
                         stats_generator: Arc::new(StatsGenerator::new(stats_concentrator_handle)),
                     });
 
+                    // Verify context state before on_platform_report
+                    let request_id_string_for_get = request_id.to_string();
+                    assert_eq!(
+                        processor.context_buffer.get(&request_id_string_for_get).is_some(),
+                        // Use setup_context because it dictates whether the request is handled up front,
+                        // which in turn signals whether the request is valid/processed.
+                        setup_context,
+                        "Context existence mismatch for request_id: {}",
+                        request_id
+                    );
+
                     // Call on_platform_report
                     let request_id_string = request_id.to_string();
                     processor.on_platform_report(
@@ -1672,7 +1690,7 @@ mod tests {
                     let request_id_string_for_get = request_id.to_string();
                     assert_eq!(
                         processor.context_buffer.get(&request_id_string_for_get).is_some(),
-                        should_have_context_after,
+                        false,
                         "Context existence mismatch for request_id: {}",
                         request_id
                     );
@@ -1682,14 +1700,13 @@ mod tests {
     }
 
     platform_report_managed_instance_tests! {
-        // (request_id, setup_context, duration_ms, status, error_type, should_have_context_after)
+        // (request_id, setup_context, duration_ms, status, error_type)
         test_on_platform_report_managed_instance_mode_with_valid_context: (
             "test-request-id",
             true,  // setup context
             123.45,
             Status::Success,
             None,
-            true,  // context should still exist
         ),
 
         test_on_platform_report_managed_instance_mode_without_context: (
@@ -1698,7 +1715,6 @@ mod tests {
             123.45,
             Status::Success,
             None,
-            false, // context should not exist
         ),
 
         test_on_platform_report_managed_instance_mode_with_error_status: (
@@ -1707,7 +1723,6 @@ mod tests {
             200.0,
             Status::Error,
             Some("RuntimeError".to_string()),
-            true,  // context should still exist
         ),
 
         test_on_platform_report_managed_instance_mode_with_timeout: (
@@ -1716,10 +1731,73 @@ mod tests {
             30000.0,
             Status::Timeout,
             None,
-            true,  // context should still exist
         ),
     }
 
+    #[tokio::test]
+    async fn test_context_removed_after_on_platform_report_on_demand() {
+        use libdd_trace_obfuscation::obfuscation_config::ObfuscationConfig;
+
+        let mut p = setup();
+        let request_id = String::from("test-request-id");
+
+        p.on_invoke_event(request_id.clone());
+        let start_time = chrono::Utc::now();
+        p.on_platform_start(request_id.clone(), start_time);
+        assert!(
+            p.context_buffer.get(&request_id).is_some(),
+            "context must exist before report"
+        );
+
+        let config = Arc::new(config::Config {
+            service: Some("test-service".to_string()),
+            ..config::Config::default()
+        });
+        let tags_provider = Arc::new(provider::Provider::new(
+            Arc::clone(&config),
+            LAMBDA_RUNTIME_SLUG.to_string(),
+            &HashMap::from([("function_arn".to_string(), "test-arn".to_string())]),
+        ));
+        let (stats_concentrator_service, stats_concentrator_handle) =
+            StatsConcentratorService::new(Arc::clone(&config));
+        tokio::spawn(stats_concentrator_service.run());
+        let trace_sender = Arc::new(SendingTraceProcessor {
+            appsec: None,
+            processor: Arc::new(trace_processor::ServerlessTraceProcessor {
+                obfuscation_config: Arc::new(
+                    ObfuscationConfig::new().expect("Failed to create ObfuscationConfig"),
+                ),
+            }),
+            trace_tx: tokio::sync::mpsc::channel(1).0,
+            stats_generator: Arc::new(StatsGenerator::new(stats_concentrator_handle)),
+        });
+
+        p.on_platform_report(
+            &request_id,
+            ReportMetrics::OnDemand(OnDemandReportMetrics {
+                duration_ms: 123.45,
+                billed_duration_ms: 124,
+                memory_size_mb: 256,
+                max_memory_used_mb: 128,
+                init_duration_ms: None,
+                restore_duration_ms: None,
+            }),
+            chrono::Utc::now().timestamp(),
+            Status::Success,
+            None,
+            None,
+            tags_provider,
+            trace_sender,
+        )
+        .await;
+
+        assert!(
+            p.context_buffer.get(&request_id).is_none(),
+            "context must be removed after on_platform_report completes"
+        );
+        assert_eq!(p.context_buffer.size(), 0);
+    }
+
     #[tokio::test]
     async fn test_on_platform_init_start_sets_durable_function_tag() {
         let mut processor = setup();