fix: respect user-provided Connection header in fetch() requests (#21049)

Co-authored-by: Claude Bot <claude-bot@bun.sh>
Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: jarred-sumner-bot <220441119+jarred-sumner-bot@users.noreply.github.com>
Co-authored-by: Jarred Sumner <jarred@jarredsumner.com>
Co-authored-by: claude[bot] <209825114+claude[bot]@users.noreply.github.com>
Co-authored-by: Jarred Sumner <Jarred-Sumner@users.noreply.github.com>

src/http.zig

@@ -542,6 +542,7 @@ pub fn buildRequest(this: *HTTPClient, body_len: usize) picohttp.Request {
     var override_accept_encoding = false;
     var override_accept_header = false;
     var override_host_header = false;
+    var override_connection_header = false;
     var override_user_agent = false;
     var add_transfer_encoding = true;
     var original_content_length: ?string = null;
@@ -560,8 +561,10 @@ pub fn buildRequest(this: *HTTPClient, body_len: usize) picohttp.Request {
                 continue;
             },
             hashHeaderConst("Connection") => {
-                if (!this.flags.disable_keepalive) {
-                    continue;
+                override_connection_header = true;
+                const connection_value = this.headerStr(header_values[i]);
+                if (std.ascii.eqlIgnoreCase(connection_value, "close")) {
+                    this.flags.disable_keepalive = true;
                 }
             },
             hashHeaderConst("if-modified-since") => {
@@ -609,7 +612,7 @@ pub fn buildRequest(this: *HTTPClient, body_len: usize) picohttp.Request {
         header_count += 1;
     }
 
-    if (!this.flags.disable_keepalive) {
+    if (!override_connection_header and !this.flags.disable_keepalive) {
         request_headers_buf[header_count] = connection_header;
         header_count += 1;
     }

test/js/web/fetch/fetch-connection-header.test.ts

@@ -0,0 +1,93 @@
+import { serve } from "bun";
+import { describe, expect, it, test } from "bun:test";
+
+describe("fetch Connection header", () => {
+  // Helper function to capture headers from a request
+  const captureHeadersFromRequest = async (fetchOptions: RequestInit): Promise<Record<string, string>> => {
+    return new Promise((resolve, reject) => {
+      // Create a temporary server to capture headers
+      const tempServer = serve({
+        port: 0,
+        fetch(req) {
+          const capturedHeaders: Record<string, string> = {};
+          for (const [name, value] of req.headers.entries()) {
+            capturedHeaders[name.toLowerCase()] = value;
+          }
+          tempServer.stop();
+          resolve(capturedHeaders);
+          return new Response("OK");
+        },
+      });
+
+      const tempPort = tempServer.port;
+      const url = `http://localhost:${tempPort}/test`;
+
+      // Make the request to temp server
+      fetch(url, fetchOptions)
+        .then(response => {
+          if (response.status !== 200) {
+            tempServer.stop();
+            reject(new Error(`Expected status 200, got ${response.status}`));
+          }
+        })
+        .catch(error => {
+          tempServer.stop();
+          reject(error);
+        });
+    });
+  };
+
+  test.each([
+    ["close", "close"],
+    ["keep-alive", "keep-alive"],
+    ["upgrade", "upgrade"],
+    ["Upgrade", "Upgrade"], // Test case preservation
+  ])("should respect Connection: %s header", async (inputValue, expectedValue) => {
+    const headers = await captureHeadersFromRequest({
+      headers: { Connection: inputValue },
+    });
+
+    expect(headers.connection).toBe(expectedValue);
+  });
+
+  test.each([
+    ["connection", "close"],
+    ["Connection", "close"],
+    ["CONNECTION", "close"],
+  ])("should respect case-insensitive header name: %s", async (headerName, expectedValue) => {
+    const headers = await captureHeadersFromRequest({
+      headers: { [headerName]: expectedValue },
+    });
+
+    expect(headers.connection).toBe(expectedValue);
+  });
+
+  it("should respect Connection header in Request object", async () => {
+    const headers = await captureHeadersFromRequest({
+      headers: { Connection: "close" },
+    });
+    expect(headers.connection).toBe("close");
+  });
+
+  it("should default to keep-alive when no Connection header provided", async () => {
+    const headers = await captureHeadersFromRequest({});
+    expect(headers.connection).toBe("keep-alive");
+  });
+
+  it("should handle multiple headers including Connection", async () => {
+    const headers = await captureHeadersFromRequest({
+      headers: {
+        "accept": "application/json",
+        "accept-encoding": "gzip, deflate",
+        "accept-language": "en-US",
+        "connection": "close",
+        "user-agent": "test-agent",
+        "x-test-header": "test-value",
+      },
+    });
+
+    expect(headers.connection).toBe("close");
+    expect(headers.accept).toBe("application/json");
+    expect(headers["x-test-header"]).toBe("test-value");
+  });
+});

test/no-validate-exceptions.txt

@@ -519,6 +519,7 @@ test/js/web/fetch/client-fetch.test.ts
 test/js/web/fetch/content-length.test.js
 test/js/web/fetch/cookies.test.ts
 test/js/web/fetch/fetch-args.test.ts
+test/js/web/fetch/fetch-connection-header.test.ts
 test/js/web/fetch/fetch-gzip.test.ts
 test/js/web/fetch/fetch-preconnect.test.ts
 test/js/web/fetch/fetch-redirect.test.ts