Release unused capacity after short reads (#9028)

Assisted-by: Codex:gpt-6

crates/stdlib/src/socket.rs

@@ -1931,6 +1931,7 @@ mod _socket {
                     .recv_with_flags(buffer.spare_capacity_mut(), flags)
             })?;
             unsafe { buffer.set_len(n) };
+            buffer.shrink_to_fit();
             Ok(buffer)
         }
 
@@ -1984,6 +1985,7 @@ mod _socket {
                     .recv_from_with_flags(buffer.spare_capacity_mut(), flags)
             })?;
             unsafe { buffer.set_len(n) };
+            buffer.shrink_to_fit();
             Ok((
                 buffer,
                 get_addr_tuple(&addr, zelf.proto.load(Ordering::Relaxed), vm),
@@ -2239,7 +2241,7 @@ mod _socket {
             let ancbufsize = ancbufsize as usize;
             let flags = flags.unwrap_or(0);
 
-            let msg = zelf
+            let mut msg = zelf
                 .sock_op(vm, SockWaitKind::Read, || {
                     let sock = zelf.sock_snapshot()?;
                     let fd = unsafe { std::os::fd::BorrowedFd::borrow_raw(sock_fileno(&sock)) };
@@ -2247,6 +2249,8 @@ mod _socket {
                 })
                 .map_err(|e| e.into_pyexception(vm))?;
 
+            msg.data.shrink_to_fit();
+
             // Build ancdata list
             let ancdata = Self::parse_ancillary_data(&msg.ancdata, vm);
 

crates/stdlib/src/ssl.rs

@@ -3198,7 +3198,7 @@ mod _ssl {
             }
 
             // Helper function to handle return value based on buffer presence
-            let return_data = |data: Vec<u8>,
+            let return_data = |mut data: Vec<u8>,
                                buffer_arg: &OptionalArg<ArgMemoryBuffer>,
                                vm: &VirtualMachine|
              -> PyResult<PyObjectRef> {
@@ -3216,6 +3216,7 @@ mod _ssl {
                     }
                     OptionalArg::Missing => {
                         // Return bytes object
+                        data.shrink_to_fit();
                         Ok(vm.ctx.new_bytes(data).into())
                     }
                 }

crates/vm/src/stdlib/_io.rs

@@ -1439,6 +1439,7 @@ mod _io {
                     match ($x, written > 0) {
                         (Some(0), _) | (None, true) => {
                             out.truncate(written);
+                            out.shrink_to_fit();
                             return Ok(Some(out));
                         }
                         (Some(r), _) => r,
@@ -6250,6 +6251,7 @@ mod fileio {
                     }
                 };
                 bytes.truncate(n);
+                bytes.shrink_to_fit();
                 bytes
             } else {
                 let mut bytes = vec![];

crates/vm/src/stdlib/os.rs

@@ -403,6 +403,7 @@ pub(super) mod _os {
             match vm.allow_threads(|| crt_fd::read(fd, &mut buffer)) {
                 Ok(n) => {
                     buffer.truncate(n);
+                    buffer.shrink_to_fit();
                     return Ok(vm.ctx.new_bytes(buffer));
                 }
                 Err(e) if e.raw_os_error() == Some(libc::EINTR) => {