Rollup merge of #38983 - APTy:udp-peek, r=aturon

Add peek APIs to std::net

Adds "peek" APIs to `std::net` sockets, including:
- `UdpSocket.peek()`
- `UdpSocket.peek_from()`
- `TcpStream.peek()`

These methods enable socket reads without side-effects. That is, repeated calls to `peek()` return identical data. This is accomplished by providing the POSIX flag `MSG_PEEK` to the underlying socket read operations.

This also moves the current implementation of `recv_from` out of the platform-independent `sys_common` and into respective `sys/windows` and `sys/unix` implementations. This allows for more platform-dependent implementations where necessary.

Fixes #38980

2 files changed, 43 insertions, 2 deletions

diff --git a/src/libstd/sys/windows/c.rs b/src/libstd/sys/windows/c.rs
index b8679aaa0c5..850d6f49612 100644
--- a/src/libstd/sys/windows/c.rs
+++ b/src/libstd/sys/windows/c.rs
@@ -246,6 +246,7 @@ pub const IP_ADD_MEMBERSHIP: c_int = 12;
 pub const IP_DROP_MEMBERSHIP: c_int = 13;
 pub const IPV6_ADD_MEMBERSHIP: c_int = 12;
 pub const IPV6_DROP_MEMBERSHIP: c_int = 13;
+pub const MSG_PEEK: c_int = 0x2;
 
 #[repr(C)]
 pub struct ip_mreq {
diff --git a/src/libstd/sys/windows/net.rs b/src/libstd/sys/windows/net.rs
index aca6994503f..adf6210d82e 100644
--- a/src/libstd/sys/windows/net.rs
+++ b/src/libstd/sys/windows/net.rs
@@ -147,12 +147,12 @@ impl Socket {
         Ok(socket)
     }
 
-    pub fn read(&self, buf: &mut [u8]) -> io::Result<usize> {
+    fn recv_with_flags(&self, buf: &mut [u8], flags: c_int) -> io::Result<usize> {
         // On unix when a socket is shut down all further reads return 0, so we
         // do the same on windows to map a shut down socket to returning EOF.
         let len = cmp::min(buf.len(), i32::max_value() as usize) as i32;
         unsafe {
-            match c::recv(self.0, buf.as_mut_ptr() as *mut c_void, len, 0) {
+            match c::recv(self.0, buf.as_mut_ptr() as *mut c_void, len, flags) {
                 -1 if c::WSAGetLastError() == c::WSAESHUTDOWN => Ok(0),
                 -1 => Err(last_error()),
                 n => Ok(n as usize)
@@ -160,6 +160,46 @@ impl Socket {
         }
     }
 
+    pub fn read(&self, buf: &mut [u8]) -> io::Result<usize> {
+        self.recv_with_flags(buf, 0)
+    }
+
+    pub fn peek(&self, buf: &mut [u8]) -> io::Result<usize> {
+        self.recv_with_flags(buf, c::MSG_PEEK)
+    }
+
+    fn recv_from_with_flags(&self, buf: &mut [u8], flags: c_int)
+                            -> io::Result<(usize, SocketAddr)> {
+        let mut storage: c::SOCKADDR_STORAGE_LH = unsafe { mem::zeroed() };
+        let mut addrlen = mem::size_of_val(&storage) as c::socklen_t;
+        let len = cmp::min(buf.len(), <wrlen_t>::max_value() as usize) as wrlen_t;
+
+        // On unix when a socket is shut down all further reads return 0, so we
+        // do the same on windows to map a shut down socket to returning EOF.
+        unsafe {
+            match c::recvfrom(self.0,
+                              buf.as_mut_ptr() as *mut c_void,
+                              len,
+                              flags,
+                              &mut storage as *mut _ as *mut _,
+                              &mut addrlen) {
+                -1 if c::WSAGetLastError() == c::WSAESHUTDOWN => {
+                    Ok((0, net::sockaddr_to_addr(&storage, addrlen as usize)?))
+                },
+                -1 => Err(last_error()),
+                n => Ok((n as usize, net::sockaddr_to_addr(&storage, addrlen as usize)?)),
+            }
+        }
+    }
+
+    pub fn recv_from(&self, buf: &mut [u8]) -> io::Result<(usize, SocketAddr)> {
+        self.recv_from_with_flags(buf, 0)
+    }
+
+    pub fn peek_from(&self, buf: &mut [u8]) -> io::Result<(usize, SocketAddr)> {
+        self.recv_from_with_flags(buf, c::MSG_PEEK)
+    }
+
     pub fn read_to_end(&self, buf: &mut Vec<u8>) -> io::Result<usize> {
         let mut me = self;
         (&mut me).read_to_end(buf)