Handle broken pipe errors on stdout gracefully

Replace `println!` calls on the results paths with `writeln!` on
`io::stdout()` and propagate the `io::Error` to `main`, where
`ErrorKind::BrokenPipe` is turned into a quiet `exit(0)`. This matches
the conventional behavior of command-line tools when their output pipe
is closed early (e.g. `hyperfine ... | head -n 0`), and fixes panics in
all run-output paths: the per-benchmark header and results, the speed
comparison summary, and `-`-style exports to stdout. The warnings and
"Note" outputs to stderr are converted the same way.

The earlier fix for #453 only covered the `-h`/`--help` output. A
regression test runs hyperfine with a pre-closed stdout pipe.

Assisted-by: AI tooling (patch generated and verified automatically)

src/benchmark/mod.rs

@@ -5,6 +5,7 @@ pub mod scheduler;
 pub mod timing_result;
 
 use std::cmp;
+use std::io::{self, Write};
 
 use crate::benchmark::executor::BenchmarkIteration;
 use crate::command::Command;
@@ -140,12 +141,13 @@ impl<'a> Benchmark<'a> {
     /// Run the benchmark for a single command
     pub fn run(&self) -> Result<BenchmarkResult> {
         if self.options.output_style != OutputStyleOption::Disabled {
-            println!(
+            writeln!(
+                io::stdout(),
                 "{}{}: {}",
                 "Benchmark ".bold(),
                 (self.number + 1).to_string().bold(),
                 self.command.get_name_with_unused_parameters(),
-            );
+            )?;
         }
 
         let mut times_real: Vec<Second> = vec![];
@@ -359,18 +361,20 @@ impl<'a> Benchmark<'a> {
 
         if self.options.output_style != OutputStyleOption::Disabled {
             if times_real.len() == 1 {
-                println!(
+                writeln!(
+                    io::stdout(),
                     "  Time ({} ≡):        {:>8}  {:>8}     [User: {}, System: {}]",
                     "abs".green().bold(),
                     mean_str.green().bold(),
                     "        ", // alignment
                     user_str.blue(),
                     system_str.blue()
-                );
+                )?;
             } else {
                 let stddev_str = format_duration(t_stddev.unwrap(), Some(time_unit));
 
-                println!(
+                writeln!(
+                    io::stdout(),
                     "  Time ({} ± {}):     {:>8} ± {:>8}    [User: {}, System: {}]",
                     "mean".green().bold(),
                     "σ".green(),
@@ -378,16 +382,17 @@ impl<'a> Benchmark<'a> {
                     stddev_str.green(),
                     user_str.blue(),
                     system_str.blue()
-                );
+                )?;
 
-                println!(
+                writeln!(
+                    io::stdout(),
                     "  Range ({} … {}):   {:>8} … {:>8}    {}",
                     "min".cyan(),
                     "max".purple(),
                     min_str.cyan(),
                     max_str.purple(),
                     num_str.dimmed()
-                );
+                )?;
             }
         }
 
@@ -430,15 +435,15 @@ impl<'a> Benchmark<'a> {
         }
 
         if !warnings.is_empty() {
-            eprintln!(" ");
+            writeln!(io::stderr(), " ")?;
 
             for warning in &warnings {
-                eprintln!("  {}: {}", "Warning".yellow(), warning);
+                writeln!(io::stderr(), "  {}: {}", "Warning".yellow(), warning)?;
             }
         }
 
         if self.options.output_style != OutputStyleOption::Disabled {
-            println!(" ");
+            writeln!(io::stdout(), " ")?;
         }
 
         self.run_cleanup_command(self.command.get_parameters().iter().cloned(), output_policy)?;

src/benchmark/scheduler.rs

@@ -3,6 +3,7 @@ use super::executor::{Executor, MockExecutor, RawExecutor, ShellExecutor};
 use super::{relative_speed, Benchmark};
 use colored::*;
 use std::cmp::Ordering;
+use std::io::{self, Write};
 
 use crate::command::{Command, Commands};
 use crate::export::ExportManager;
@@ -58,13 +59,13 @@ impl<'a> Scheduler<'a> {
         Ok(())
     }
 
-    pub fn print_relative_speed_comparison(&self) {
+    pub fn print_relative_speed_comparison(&self) -> Result<()> {
         if self.options.output_style == OutputStyleOption::Disabled {
-            return;
+            return Ok(());
         }
 
         if self.results.len() < 2 {
-            return;
+            return Ok(());
         }
 
         let reference = self
@@ -81,15 +82,16 @@ impl<'a> Scheduler<'a> {
         ) {
             match self.options.sort_order_speed_comparison {
                 SortOrder::MeanTime => {
-                    println!("{}", "Summary".bold());
+                    writeln!(io::stdout(), "{}", "Summary".bold())?;
 
                     let reference = annotated_results.iter().find(|r| r.is_reference).unwrap();
                     let others = annotated_results.iter().filter(|r| !r.is_reference);
 
-                    println!(
+                    writeln!(
+                        io::stdout(),
                         "  {} ran",
                         reference.result.command_with_unused_parameters.cyan()
-                    );
+                    )?;
 
                     for item in others {
                         let stddev = if let Some(stddev) = item.relative_speed_stddev {
@@ -114,18 +116,20 @@ impl<'a> Scheduler<'a> {
                                 stddev
                             ),
                         };
-                        println!(
+                        writeln!(
+                            io::stdout(),
                             "{} {}",
                             comparator,
                             item.result.command_with_unused_parameters.magenta()
-                        );
+                        )?;
                     }
                 }
                 SortOrder::Command => {
-                    println!("{}", "Relative speed comparison".bold());
+                    writeln!(io::stdout(), "{}", "Relative speed comparison".bold())?;
 
                     for item in annotated_results {
-                        println!(
+                        writeln!(
+                            io::stdout(),
                             "  {}{}  {}",
                             format!("{:10.2}", item.relative_speed).bold().green(),
                             if item.is_reference {
@@ -136,12 +140,13 @@ impl<'a> Scheduler<'a> {
                                 "        ".into()
                             },
                             item.result.command_with_unused_parameters,
-                        );
+                        )?;
                     }
                 }
             }
         } else {
-            eprintln!(
+            writeln!(
+                io::stderr(),
                 "{}: The benchmark comparison could not be computed as some benchmark times are zero. \
                  This could be caused by background interference during the initial calibration phase \
                  of hyperfine, in combination with very fast commands (faster than a few milliseconds). \
@@ -149,8 +154,10 @@ impl<'a> Scheduler<'a> {
                  --shell=none/-N option. If it does not help either, you command is most likely too fast \
                  to be accurately benchmarked by hyperfine.",
                  "Note".bold().red()
-            );
+            )?;
         }
+
+        Ok(())
     }
 
     pub fn final_export(&self) -> Result<()> {

src/export/mod.rs

@@ -1,5 +1,5 @@
 use std::fs::{File, OpenOptions};
-use std::io::Write;
+use std::io::{self, Write};
 
 mod asciidoc;
 mod csv;
@@ -144,8 +144,8 @@ impl ExportManager {
                 }
                 ExportTarget::Stdout => {
                     if !intermediate {
-                        println!();
-                        println!("{}", String::from_utf8(content()?).unwrap());
+                        writeln!(io::stdout())?;
+                        writeln!(io::stdout(), "{}", String::from_utf8(content()?).unwrap())?;
                     }
                 }
             }

src/main.rs

@@ -4,6 +4,7 @@
 )]
 
 use std::env;
+use std::io::{self, Write};
 
 use benchmark::scheduler::Scheduler;
 use cli::get_cli_arguments;
@@ -44,17 +45,37 @@ fn run() -> Result<()> {
 
     let mut scheduler = Scheduler::new(&commands, &options, &export_manager);
     scheduler.run_benchmarks()?;
-    scheduler.print_relative_speed_comparison();
+    scheduler.print_relative_speed_comparison()?;
     scheduler.final_export()?;
 
     Ok(())
 }
 
+/// Check whether `error` or one of its causes is an `io::Error` of kind
+/// `BrokenPipe`. This happens when the process on the reading end of a
+/// pipe (e.g. `hyperfine … | head -n 1`) terminates before hyperfine is
+/// done writing its results.
+fn caused_by_broken_pipe(error: &anyhow::Error) -> bool {
+    error.chain().any(|cause| {
+        cause
+            .downcast_ref::<io::Error>()
+            .is_some_and(|e| e.kind() == io::ErrorKind::BrokenPipe)
+    })
+}
+
 fn main() {
     match run() {
         Ok(_) => {}
         Err(e) => {
-            eprintln!("{} {:#}", "Error:".red(), e);
+            // When writing to a closed pipe, exit quietly with a zero exit
+            // code — the same behavior that non-Rust CLI tools exhibit when
+            // they are terminated by `SIGPIPE`.
+            if caused_by_broken_pipe(&e) {
+                std::process::exit(0);
+            }
+            // The write to stderr can itself fail if stderr is closed; the
+            // error message cannot be shown in that case anyway.
+            let _ = writeln!(io::stderr(), "{} {:#}", "Error:".red(), e);
             std::process::exit(1);
         }
     }

tests/integration_tests.rs

@@ -1,5 +1,5 @@
 mod common;
-use common::hyperfine;
+use common::{hyperfine, hyperfine_raw_command};
 
 use predicates::prelude::*;
 
@@ -33,6 +33,47 @@ fn one_run_is_supported() {
         .success();
 }
 
+/// Regression test: hyperfine must not panic when writing to a closed
+/// stdout pipe (e.g. `hyperfine ... | head -n 1` or a downstream process
+/// that terminates early). A `BrokenPipe` error should result in a quiet
+/// exit with code 0 instead of a `println!` panic.
+#[test]
+fn exits_quietly_when_stdout_is_closed() {
+    use std::process::Stdio;
+
+    for args in [
+        vec!["--runs=1", "echo dummy benchmark"],
+        vec!["--runs=1", "--export-json=-", "echo dummy benchmark"],
+        vec!["--runs=1", "echo dummy benchmark", "echo second benchmark"],
+    ] {
+        // A pipe whose read end has been closed before hyperfine is even
+        // started: every write to it will fail with `BrokenPipe`.
+        let (reader, writer) = std::io::pipe().expect("failed to create pipe");
+        drop(reader);
+
+        let output = hyperfine_raw_command()
+            .args(&args)
+            .stdout(writer)
+            .stderr(Stdio::piped())
+            .spawn()
+            .expect("failed to spawn hyperfine")
+            .wait_with_output()
+            .expect("failed to wait for hyperfine");
+        let stderr = String::from_utf8_lossy(&output.stderr);
+
+        assert!(
+            !stderr.contains("panicked"),
+            "hyperfine panicked while writing to a closed pipe:\n{}",
+            stderr
+        );
+        assert!(
+            output.status.success(),
+            "hyperfine did not exit cleanly; stderr:\n{}",
+            stderr
+        );
+    }
+}
+
 #[test]
 fn can_run_commands_without_a_shell() {
     hyperfine()