feat(lib): Setting http1_writev(true) will now force writev queue usage

Previously, calling `http1_writev(true)` would just keep the default behavior, which was to auto detect if writev was optimal. Now, the auto-detection is still default, but explicitly calling `http1_writev(true)` will skip the auto-detection, and always use writev queue strategy.

Closes #2282

src/client/conn.rs

@@ -75,7 +75,7 @@ where
 #[derive(Clone, Debug)]
 pub struct Builder {
     pub(super) exec: Exec,
-    h1_writev: bool,
+    h1_writev: Option<bool>,
     h1_title_case_headers: bool,
     h1_read_buf_exact_size: Option<usize>,
     h1_max_buf_size: Option<usize>,
@@ -424,7 +424,7 @@ impl Builder {
     pub fn new() -> Builder {
         Builder {
             exec: Exec::Default,
-            h1_writev: true,
+            h1_writev: None,
             h1_read_buf_exact_size: None,
             h1_title_case_headers: false,
             h1_max_buf_size: None,
@@ -443,7 +443,7 @@ impl Builder {
     }
 
     pub(super) fn h1_writev(&mut self, enabled: bool) -> &mut Builder {
-        self.h1_writev = enabled;
+        self.h1_writev = Some(enabled);
         self
     }
 
@@ -609,8 +609,12 @@ impl Builder {
             let (tx, rx) = dispatch::channel();
             let proto = if !opts.http2 {
                 let mut conn = proto::Conn::new(io);
-                if !opts.h1_writev {
-                    conn.set_write_strategy_flatten();
+                if let Some(writev) = opts.h1_writev {
+                    if writev {
+                        conn.set_write_strategy_queue();
+                    } else {
+                        conn.set_write_strategy_flatten();
+                    }
                 }
                 if opts.h1_title_case_headers {
                     conn.set_title_case_headers();

src/client/mod.rs

@@ -964,7 +964,11 @@ impl Builder {
     /// but may also improve performance when an IO transport doesn't
     /// support vectored writes well, such as most TLS implementations.
     ///
-    /// Default is `true`.
+    /// Setting this to true will force hyper to use queued strategy
+    /// which may eliminate unnecessary cloning on some TLS backends
+    ///
+    /// Default is `auto`. In this mode hyper will try to guess which
+    /// mode to use
     pub fn http1_writev(&mut self, val: bool) -> &mut Self {
         self.conn_builder.h1_writev(val);
         self