Release unused capacity after more short reads (#9089)

Call shrink_to_fit on the result buffer in os.pread, os.getrandom,
WindowsConsoleIO.read, _winapi.PeekNamedPipe, _winapi.ReadFile,
_multiprocessing.recv and the openssl MemoryBIO.read.

Assisted-by: Claude:claude-opus-5-5
Assisted-by: Grok:grok-4.7

crates/stdlib/src/multiprocessing.rs

@@ -408,8 +408,11 @@ mod _multiprocessing {
 
     #[pyfunction]
     fn recv(socket: usize, size: usize, vm: &VirtualMachine) -> PyResult<Vec<u8>> {
-        host_multiprocessing::recv_socket(socket as host_multiprocessing::RawSocket, size)
-            .map_err(|_| vm.new_last_os_error())
+        let mut buf =
+            host_multiprocessing::recv_socket(socket as host_multiprocessing::RawSocket, size)
+                .map_err(|_| vm.new_last_os_error())?;
+        buf.shrink_to_fit();
+        Ok(buf)
     }
 
     #[pyfunction]

crates/stdlib/src/openssl.rs

@@ -3706,6 +3706,7 @@ mod _ssl {
                 }
 
                 buf.truncate(nbytes as usize);
+                buf.shrink_to_fit();
                 Ok(buf)
             }
         }

crates/vm/src/stdlib/_io.rs

@@ -6949,6 +6949,7 @@ mod winconsoleio {
             }
 
             buf.truncate(read_len);
+            buf.shrink_to_fit();
             Ok(vm.ctx.new_bytes(buf).into())
         }
 

crates/vm/src/stdlib/_winapi.rs

@@ -768,7 +768,8 @@ mod _winapi {
         if size > 0 {
             let result = host_winapi::peek_named_pipe(handle.0, Some(size as u32))
                 .map_err(|e| e.to_pyexception(vm))?;
-            let buf = result.data.unwrap_or_default();
+            let mut buf = result.data.unwrap_or_default();
+            buf.shrink_to_fit();
             let bytes: PyObjectRef = vm.ctx.new_bytes(buf).into();
             Ok(vm
                 .ctx
@@ -886,9 +887,10 @@ mod _winapi {
             return Ok(result.into());
         }
 
-        let result = vm
+        let mut result = vm
             .allow_threads(|| host_winapi::read_file(handle.0, size))
             .map_err(|e| e.to_pyexception(vm))?;
+        result.data.shrink_to_fit();
         Ok(vm
             .ctx
             .new_tuple(vec![

crates/vm/src/stdlib/posix.rs

@@ -1214,6 +1214,7 @@ pub mod module {
             match vm.allow_threads(|| rustpython_host_env::posix::pread(fd, &mut buffer, offset)) {
                 Ok(n) => {
                     buffer.truncate(n);
+                    buffer.shrink_to_fit();
                     return Ok(vm.ctx.new_bytes(buffer));
                 }
                 Err(err) if err.raw_os_error() == Some(libc::EINTR) => {
@@ -2976,6 +2977,7 @@ pub mod module {
             .map_err(|_| vm.new_last_os_error())?;
             buf.set_len(len);
         }
+        buf.shrink_to_fit();
         Ok(buf)
     }