Avoid unnecessary allocation by way of tuple conversion. (#994)

Motivation:

Due to https://bugs.swift.org/browse/SR-10614, assigning an array containing a tuple type
to a variable that expects an array containing a different-but-compatible tuple type will cause
an allocation and copy of that array storage.

In some cases this is necessary, but we were doing it in the HTTPHeaders constructors, which meant
that the swift-nio-http2 code was hitting a hilariously over the top number of allocations.

Modifications:

- Change the internal storage type of HTTPHeaders to match the public constructor.

Result:

Fewer allocations when constructing HTTPHeaders

IntegrationTests/tests_01_http/test_07_headers_work.sh

@@ -19,7 +19,7 @@ token=$(create_token)
 start_server "$token"
 do_curl "$token" -H "foo: bar" --http1.0 \
     "http://foobar.com/dynamic/info" > "$tmp/out"
-if ! grep -q '(name: "foo", value: "bar")' "$tmp/out"; then
+if ! grep -q '("foo", "bar")' "$tmp/out"; then
     fail "couldn't find header in response"
 fi
 stop_server "$token"

IntegrationTests/tests_04_performance/test_01_allocation_counts_for_http1.sh

@@ -58,7 +58,7 @@ cd ..
 "$swift_bin" run -c release | tee "$tmp/output"
 )
 
-for test in 1000_reqs_1_conn 1_reqs_1000_conn ping_pong_1000_reqs_1_conn bytebuffer_lots_of_rw future_lots_of_callbacks scheduling_10000_executions; do
+for test in 1000_reqs_1_conn 1_reqs_1000_conn ping_pong_1000_reqs_1_conn bytebuffer_lots_of_rw future_lots_of_callbacks scheduling_10000_executions creating_10000_headers; do
     cat "$tmp/output"  # helps debugging
     total_allocations=$(grep "^$test.total_allocations:" "$tmp/output" | cut -d: -f2 | sed 's/ //g')
     not_freed_allocations=$(grep "^$test.remaining_allocations:" "$tmp/output" | cut -d: -f2 | sed 's/ //g')

IntegrationTests/tests_04_performance/test_01_resources/template/Sources/SwiftBootstrap/SwiftMain.swift

@@ -452,5 +452,16 @@ public func swiftMain() -> Int {
         return 10_000
     }
 
+    measureAndPrint(desc: "creating_10000_headers") {
+        var count = 0
+        
+        for i in 0..<10_000 {
+            let baseHeaders: [(String, String)] = [("Host", "example.com"), ("Content-Length", "4")]
+            count += HTTPHeaders(baseHeaders).count
+        }
+
+        return count
+    }
+
     return 0
 }

Sources/NIOHTTP1/HTTPTypes.swift

@@ -278,7 +278,7 @@ extension HTTPHeaders {
 /// or split representation, such that header fields that are able to be repeated
 /// can be represented appropriately.
 public struct HTTPHeaders: CustomStringConvertible, ExpressibleByDictionaryLiteral {
-    internal var headers: [(name: String, value: String)]
+    internal var headers: [(String, String)]
     internal var keepAliveState: KeepAliveState = .unknown
 
     public var description: String {
@@ -289,7 +289,7 @@ public struct HTTPHeaders: CustomStringConvertible, ExpressibleByDictionaryLiter
         return self.headers.map { $0.0 }
     }
 
-    internal init(_ headers: [Element], keepAliveState: KeepAliveState) {
+    internal init(_ headers: [(String, String)], keepAliveState: KeepAliveState) {
         self.headers = headers
         self.keepAliveState = keepAliveState
     }
@@ -452,7 +452,7 @@ extension HTTPHeaders: RandomAccessCollection {
     public typealias Element = (name: String, value: String)
     
     public struct Index: Comparable {
-        fileprivate let base: Array<HTTPHeaders>.Index
+        fileprivate let base: Array<(String, String)>.Index
         public static func < (lhs: Index, rhs: Index) -> Bool {
             return lhs.base < rhs.base
         }

docker/docker-compose.1604.51.yaml

@@ -24,6 +24,7 @@ services:
       - MAX_ALLOCS_ALLOWED_ping_pong_1000_reqs_1_conn=4600
       - MAX_ALLOCS_ALLOWED_bytebuffer_lots_of_rw=2100
       - MAX_ALLOCS_ALLOWED_future_lots_of_callbacks=99100
+      - MAX_ALLOCS_ALLOWED_creating_10000_headers=10100
 
   test:
     image: swift-nio:16.04-5.1
@@ -34,6 +35,7 @@ services:
       - MAX_ALLOCS_ALLOWED_ping_pong_1000_reqs_1_conn=4600
       - MAX_ALLOCS_ALLOWED_bytebuffer_lots_of_rw=2100
       - MAX_ALLOCS_ALLOWED_future_lots_of_callbacks=99100
+      - MAX_ALLOCS_ALLOWED_creating_10000_headers=10100
 
   echo:
     image: swift-nio:16.04-5.1

docker/docker-compose.1804.50.yaml

@@ -23,6 +23,7 @@ services:
       - MAX_ALLOCS_ALLOWED_bytebuffer_lots_of_rw=2100
       - MAX_ALLOCS_ALLOWED_future_lots_of_callbacks=99100
       - MAX_ALLOCS_ALLOWED_scheduling_10000_executions=20150
+      - MAX_ALLOCS_ALLOWED_creating_10000_headers=10100
 
   test:
     image: swift-nio:18.04-5.0
@@ -34,6 +35,7 @@ services:
       - MAX_ALLOCS_ALLOWED_bytebuffer_lots_of_rw=2100
       - MAX_ALLOCS_ALLOWED_future_lots_of_callbacks=99100
       - MAX_ALLOCS_ALLOWED_scheduling_10000_executions=20150
+      - MAX_ALLOCS_ALLOWED_creating_10000_headers=10100
 
   echo:
     image: swift-nio:18.04-5.0