2 files changed, 45 insertions, 1 deletions
diff --git a/core/01_core.js b/core/01_core.js
index d9a110eea..3a05a0cff 100644
--- a/core/01_core.js
+++ b/core/01_core.js
@@ -22,6 +22,8 @@
     MapPrototypeDelete,
     MapPrototypeSet,
     PromisePrototypeThen,
+    PromisePrototypeFinally,
+    StringPrototypeSlice,
     ObjectAssign,
     SymbolFor,
   } = window.__bootstrap.primordials;
@@ -48,6 +50,13 @@
   // to users. Currently missing bindings.
   const promiseIdSymbol = SymbolFor("Deno.core.internalPromiseId");
 
+  let opCallTracingEnabled = false;
+  const opCallTraces = new Map();
+
+  function enableOpCallTracing() {
+    opCallTracingEnabled = true;
+  }
+
   function setPromise(promiseId) {
     const idx = promiseId % RING_SIZE;
     // Move old promise from ring to map
@@ -139,7 +148,17 @@
     const maybeError = opcallAsync(opsCache[opName], promiseId, arg1, arg2);
     // Handle sync error (e.g: error parsing args)
     if (maybeError) return unwrapOpResult(maybeError);
-    const p = PromisePrototypeThen(setPromise(promiseId), unwrapOpResult);
+    let p = PromisePrototypeThen(setPromise(promiseId), unwrapOpResult);
+    if (opCallTracingEnabled) {
+      // Capture a stack trace by creating a new `Error` object. We remove the
+      // first 6 characters (the `Error\n` prefix) to get just the stack trace.
+      const stack = StringPrototypeSlice(new Error().stack, 6);
+      MapPrototypeSet(opCallTraces, promiseId, { opName, stack });
+      p = PromisePrototypeFinally(
+        p,
+        () => MapPrototypeDelete(opCallTraces, promiseId),
+      );
+    }
     // Save the id on the promise so it can later be ref'ed or unref'ed
     p[promiseIdSymbol] = promiseId;
     return p;
@@ -226,6 +245,8 @@
     BadResourcePrototype,
     Interrupted,
     InterruptedPrototype,
+    enableOpCallTracing,
+    opCallTraces,
   });
 
   ObjectAssign(globalThis.__bootstrap, { core });
diff --git a/core/lib.deno_core.d.ts b/core/lib.deno_core.d.ts
index 9f554f12b..37cd33190 100644
--- a/core/lib.deno_core.d.ts
+++ b/core/lib.deno_core.d.ts
@@ -141,5 +141,28 @@ declare namespace Deno {
     ): undefined | UncaughtExceptionCallback;
 
     export type UncaughtExceptionCallback = (err: any) => void;
+
+    /**
+     * Enables collection of stack traces of all async ops. This allows for
+     * debugging of where a given async op was started. Deno CLI uses this for
+     * improving error message in op sanitizer errors for `deno test`.
+     *
+     * **NOTE:** enabling tracing has a significant negative performance impact.
+     * To get high level metrics on async ops with no added performance cost,
+     * use `Deno.core.metrics()`.
+     */
+    function enableOpCallTracing(): void;
+
+    export interface OpCallTrace {
+      opName: string;
+      stack: string;
+    }
+
+    /**
+     * A map containing traces for all ongoing async ops. The key is the op id.
+     * Tracing only occurs when `Deno.core.enableOpCallTracing()` was previously
+     * enabled.
+     */
+    const opCallTraces: Map<number, OpCallTrace>;
   }
 }