tests/common/util: Fix #3895 broken pipe error when using pipep input in UCommand

tests/common/util.rs

@@ -17,7 +17,7 @@ use std::env;
 use std::ffi::CString;
 use std::ffi::OsStr;
 use std::fs::{self, hard_link, File, OpenOptions};
-use std::io::{Read, Result, Write};
+use std::io::{BufWriter, Read, Result, Write};
 #[cfg(unix)]
 use std::os::unix::fs::{symlink as symlink_dir, symlink as symlink_file};
 #[cfg(windows)]
@@ -1105,13 +1105,14 @@ impl UCommand {
         }
 
         if let Some(ref input) = self.bytes_into_stdin {
-            let write_result = child
+            let child_stdin = child
                 .stdin
                 .take()
-                .unwrap_or_else(|| panic!("Could not take child process stdin"))
-                .write_all(input);
+                .unwrap_or_else(|| panic!("Could not take child process stdin"));
+            let mut writer = BufWriter::new(child_stdin);
+            let result = writer.write_all(input);
             if !self.ignore_stdin_write_error {
-                if let Err(e) = write_result {
+                if let Err(e) = result {
                     panic!("failed to write to stdin of child: {}", e);
                 }
             }