fix(node-http-handler): shift http1 expect 100-continue requests to isolated http Agents

Author: kuhe

.changeset/chilled-apples-arrive.md

@@ -0,0 +1,5 @@
+---
+"@smithy/node-http-handler": patch
+---
+
+shfit http1 calls with expect 100-continue header to isolated http Agents

packages/node-http-handler/src/node-http-handler.spec.ts

@@ -261,6 +261,34 @@ describe("NodeHttpHandler", () => {
         await nodeHttpHandler.handle(httpRequest as any);
         expect(vi.mocked(hRequest as any).mock.calls[0][0]?.host).toEqual("host");
       });
+
+      it("creates a new http(s) Agent if the request has expect: 100-continue header", async () => {
+        const nodeHttpHandler = new NodeHttpHandler({});
+        {
+          const httpRequest = {
+            protocol: "http:",
+            hostname: "[host]",
+            path: "/some/path",
+            headers: {
+              expect: "100-continue",
+            },
+          };
+          await nodeHttpHandler.handle(httpRequest as any);
+          expect(vi.mocked(hRequest as any).mock.calls[0][0]?.agent).not.toBe(
+            (nodeHttpHandler as any).config.httpAgent
+          );
+        }
+        {
+          const httpRequest = {
+            protocol: "http:",
+            hostname: "[host]",
+            path: "/some/path",
+            headers: {},
+          };
+          await nodeHttpHandler.handle(httpRequest as any);
+          expect(vi.mocked(hRequest as any).mock.calls[1][0]?.agent).toBe((nodeHttpHandler as any).config.httpAgent);
+        }
+      });
     });
   });
 

packages/node-http-handler/src/node-http-handler.ts

@@ -203,7 +203,19 @@ or increase socketAcquisitionWarningTimeout=(millis) in the NodeHttpHandler conf
 
       // determine which http(s) client to use
       const isSSL = request.protocol === "https:";
-      const agent = isSSL ? this.config.httpsAgent : this.config.httpAgent;
+
+      const headers = request.headers ?? {};
+      const expectContinue = (headers.Expect ?? headers.expect) === "100-continue";
+
+      let agent = isSSL ? this.config.httpsAgent : this.config.httpAgent;
+      if (expectContinue) {
+        // Because awaiting 100-continue desynchronizes the request and request body transmission,
+        // such requests must be offloaded to a separate Agent instance.
+        // Additional logic will exist on the client using this handler to determine whether to add the header at all.
+
+        // hAgent also has options, but not in the type system.
+        agent = new (isSSL ? hsAgent : hAgent)((agent as hsAgent).options);
+      }
 
       // If the request is taking a long time, check socket usage and potentially warn.
       // This warning will be cancelled if the request resolves.

packages/node-http-handler/src/write-request-body.ts

@@ -20,7 +20,7 @@ export async function writeRequestBody(
   maxContinueTimeoutMs = MIN_WAIT_TIME
 ): Promise<void> {
   const headers = request.headers ?? {};
-  const expect = headers["Expect"] || headers["expect"];
+  const expect = headers.Expect || headers.expect;
 
   let timeoutId = -1;
   let sendBody = true;