Add TCP_NOTSENT_LOWAT socketopt support

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=c9bee3b7fdecb0c1d070c7b54113b3bdfb9a3d36
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=a74f0fa082b76c6a76cba5672f36218518bfdc09
https://blog.cloudflare.com/http-2-prioritization-with-nginx/

Author: the8472

src/sys/unix.rs

@@ -1807,6 +1807,46 @@ impl crate::Socket {
         }
     }
 
+    /// Get the value of the `TCP_NOTSENT_LOWAT` option on this socket.
+    ///
+    /// For more information about this option, see [`set_tcp_notsent_lowat`].
+    ///
+    /// [`set_tcp_notsent_lowat`]: crate::Socket::set_tcp_notsent_lowat
+    #[cfg(all(
+        feature = "all",
+        any(target_os = "android", target_os = "linux")
+    ))]
+    pub fn tcp_notsent_lowat(&self) -> io::Result<u32> {
+        unsafe {
+            getsockopt::<Bool>(
+                self.as_raw(),
+                libc::IPPROTO_TCP,
+                libc::TCP_NOTSENT_LOWAT,
+            )
+            .map(|lowat| lowat as u32)
+        }
+    }
+
+
+    /// Set the value of the `TCP_NOTSENT_LOWAT` option on this socket.
+    ///
+    /// If set the kernel will limit the amount of _unsent_ data in the sendbuffer.
+    /// This differs from `set_send_buffer_size` which limits the sum of unsent and unacknowledged data.
+    #[cfg(all(
+        feature = "all",
+        any(target_os = "android", target_os = "linux")
+    ))]
+    pub fn set_tcp_notsent_lowat(&self, lowat: u32) -> io::Result<()> {
+        unsafe {
+            setsockopt(
+                self.as_raw(),
+                libc::IPPROTO_TCP,
+                libc::TCP_NOTSENT_LOWAT,
+                lowat as c_int,
+            )
+        }
+    }
+
     /// Gets the value for the `SO_BINDTODEVICE` option on this socket.
     ///
     /// This value gets the socket binded device's interface name.

tests/socket.rs

@@ -1458,6 +1458,11 @@ test!(tcp_quickack, set_tcp_quickack(false));
     any(target_os = "android", target_os = "fuchsia", target_os = "linux")
 ))]
 test!(tcp_thin_linear_timeouts, set_tcp_thin_linear_timeouts(true));
+#[cfg(all(
+    feature = "all",
+    any(target_os = "android", target_os = "linux")
+))]
+test!(tcp_notsent_lowat, set_tcp_notsent_lowat(16*1024));
 test!(linger, set_linger(Some(Duration::from_secs(10))));
 test!(
     read_timeout,