Improve HTTPHeaders description performance (#3063)

### Motivation:

As outlined in this
[issue](#2930) by @weissi, the
current performance of calling `description` on `HTTPHeaders` is
undermined by the dynamism of Array.

### Modifications:

The proposed solution replaces the implementation of `description` to
iterate over the items and print them out manually. This provides a
faster solution since we bypass the cost of calling into `description`
of an Array.


### Result:

A more performant implementation of `HTTPHeaders` description.

---------

Co-authored-by: Johannes Weiss <[email protected]>
Co-authored-by: Cory Benfield <[email protected]>

IntegrationTests/tests_01_http/test_07_headers_work.sh

@@ -19,8 +19,8 @@ source defines.sh
 token=$(create_token)
 start_server "$token"
 do_curl "$token" -H "foo: bar" --http1.0 \
-    "http://foobar.com/dynamic/info" > "${tmp:?"tmp variable not set"}/out"
-if ! grep -q '("foo", "bar")' "$tmp/out"; then
+    "http://foobar.com/dynamic/info" >"${tmp:?"tmp variable not set"}/out"
+if ! grep -q 'foo: bar' "$tmp/out"; then
     fail "couldn't find header in response"
 fi
 stop_server "$token"

Sources/NIOHTTP1/HTTPTypes.swift

@@ -313,7 +313,10 @@ public struct HTTPHeaders: CustomStringConvertible, ExpressibleByDictionaryLiter
     internal var keepAliveState: KeepAliveState = .unknown
 
     public var description: String {
-        self.headers.description
+        self.headers.lazy.map {
+            "\($0.0): \($0.1)"
+        }
+        .joined(separator: "; ")
     }
 
     internal var names: [String] {

Sources/NIOPerformanceTester/main.swift

@@ -276,6 +276,17 @@ measureAndPrint(desc: "http_headers_canonical_form_trimming_whitespace_from_long
     return count
 }
 
+measureAndPrint(desc: "http_headers_description_100k") {
+    let headers = HTTPHeaders(Array(repeating: ("String", "String"), count: 100))
+
+    for _ in 0..<100_000 {
+        let str = headers.description
+        precondition(str.utf8.count > 100)
+    }
+
+    return 0
+}
+
 measureAndPrint(desc: "bytebuffer_write_12MB_short_string_literals") {
     let bufferSize = 12 * 1024 * 1024
     var buffer = ByteBufferAllocator().buffer(capacity: bufferSize)

Tests/NIOHTTP1Tests/HTTPHeadersTest.swift

@@ -400,4 +400,26 @@ class HTTPHeadersTest: XCTestCase {
         headers.reserveCapacity(4)
         XCTAssertEqual(headers.capacity, 4)
     }
+
+    func testHTTPHeadersDescription() {
+        let originalHeaders = [
+            ("User-Agent", "1"),
+            ("host", "2"),
+            ("X-SOMETHING", "3"),
+            ("SET-COOKIE", "foo=bar"),
+            ("Set-Cookie", "buz=cux"),
+        ]
+
+        let headers = HTTPHeaders(originalHeaders)
+
+        let expectedOutput = """
+            User-Agent: 1; \
+            host: 2; \
+            X-SOMETHING: 3; \
+            SET-COOKIE: foo=bar; \
+            Set-Cookie: buz=cux
+            """
+
+        XCTAssertEqual(expectedOutput, headers.description)
+    }
 }