heroku · Malax · Dec 15, 2022 · Dec 7, 2022 · Dec 15, 2022 · Dec 15, 2022
@@ -15,6 +15,10 @@ separate changelogs for each crate were used. If you need to refer to these old
 - libcnb: Drop the use of the `stacker` crate when recursively removing layer directories. ([#517](https://github.com/heroku/libcnb.rs/pull/517))
 - libcnb-cargo: Updated to Clap v4. ([#511](https://github.com/heroku/libcnb.rs/pull/511))
 
+## Added
+
+- libherokubuildpack: Add `command` and `write` modules for working with `std::process::Command` output streams. ([#535](https://github.com/heroku/libcnb.rs/pull/535))
+
 ## [0.11.1] 2022-09-29
 
 ### Fixed

@@ -15,16 +15,19 @@ include = ["src/**/*", "LICENSE", "README.md"]
 all-features = true
 
 [features]
-default = ["download", "digest", "error", "log", "tar", "toml", "fs"]
+default = ["command", "download", "digest", "error", "log", "tar", "toml", "fs", "write"]
 download = ["dep:ureq", "dep:thiserror"]
 digest = ["dep:sha2"]
 error = ["log", "dep:libcnb"]
 log = ["dep:termcolor"]
 tar = ["dep:tar", "dep:flate2"]
 toml = ["dep:toml"]
 fs = ["dep:pathdiff"]
+command = ["write", "dep:crossbeam-utils"]
+write = []
 
 [dependencies]
+crossbeam-utils = { version = "0.8.2", optional = true }
 flate2 = { version = "1.0.24", optional = true }
 libcnb = { workspace = true, optional = true }
 pathdiff = { version = "0.2.1", optional = true }

@@ -14,6 +14,8 @@ uses Cargo features to allow opt-out of certain modules if they're not needed.
 
 The feature names line up with the modules in this crate. All features are enabled by default.
 
+* **command** -
+  Enabled helpers to work with `std::process::Command`.
 * **download** -
   Enables helpers to download files over HTTP.
 * **digest** -
@@ -28,6 +30,8 @@ The feature names line up with the modules in this crate. All features are enabl
   Enables helpers for working with TOML data.
 * **fs** -
   Enables helpers for filesystem related tasks.
+* **write** -
+  Enables `std::io::Write` proxy implementations.
 
 [Docs]: https://img.shields.io/docsrs/libherokubuildpack
 [docs.rs]: https://docs.rs/libherokubuildpack/latest/libherokubuildpack/

@@ -0,0 +1,185 @@
+use crate::write::tee;
+use crossbeam_utils::thread::ScopedJoinHandle;
+use std::io::Write;
+use std::{io, process, thread};
+use std::{mem, panic};
+
+/// Extension trait for [`process::Command`] that adds functions for use within buildpacks.
+pub trait CommandExt {
+    /// Spawns the command process and sends the output of stdout and stderr to the given writers.
+    ///
+    /// This allows for additional flexibility when dealing with these output streams compared to
+    /// functionality that the stock [`process::Command`] provides. See the [`write`](crate::write)
+    /// module for [`std::io::Write`] implementations designed for common buildpack tasks.
+    ///
+    /// This function will redirect the output unbuffered and in parallel for both streams. This
+    /// means that it can be used to output data from these streams while the command is running,
+    /// providing a live view into the process' output. This function will block until both streams
+    /// have been closed.
+    ///
+    /// # Example:
+    /// ```no_run
+    /// use libherokubuildpack::command::CommandExt;
+    /// use libherokubuildpack::write::tee;
+    /// use std::fs;
+    /// use std::process::Command;
+    ///
+    /// let logfile = fs::File::open("log.txt").unwrap();
+    /// let exit_status = Command::new("date")
+    ///     .spawn_and_write_streams(tee(std::io::stdout(), &logfile), std::io::stderr())
+    ///     .and_then(|mut child| child.wait())
+    ///     .unwrap();
+    /// ```
+    fn spawn_and_write_streams<OW: Write + Send, EW: Write + Send>(
+        &mut self,
+        stdout_write: OW,
+        stderr_write: EW,
+    ) -> io::Result<process::Child>;
+
+    /// Spawns the command process and sends the output of stdout and stderr to the given writers.
+    ///
+    /// In addition to what [`spawn_and_write_streams`](Self::spawn_and_write_streams) does, this
+    /// function captures stdout and stderr as `Vec<u8>` and returns them after waiting for the
+    /// process to finish. This function is meant as a drop-in replacement for existing
+    /// `Command:output` calls.
+    ///
+    /// # Example:
+    /// ```no_run
+    /// use libherokubuildpack::command::CommandExt;
+    /// use libherokubuildpack::write::tee;
+    /// use std::fs;
+    /// use std::process::Command;
+    ///
+    /// let logfile = fs::File::open("log.txt").unwrap();
+    /// let output = Command::new("date")
+    ///     .output_and_write_streams(tee(std::io::stdout(), &logfile), std::io::stderr())
+    ///     .unwrap();
+    ///
+    /// // Return value can be used as with Command::output, but the streams will also be written to
+    /// // the given writers.
+    /// println!(
+    ///     "Process exited with {}, stdout: {:?}, stderr: {:?}",
+    ///     output.status, output.stdout, output.stderr
+    /// );
+    /// ```
+    fn output_and_write_streams<OW: Write + Send, EW: Write + Send>(
+        &mut self,
+        stdout_write: OW,
+        stderr_write: EW,
+    ) -> io::Result<process::Output>;
+}
+
+impl CommandExt for process::Command {
+    fn spawn_and_write_streams<OW: Write + Send, EW: Write + Send>(
+        &mut self,
+        stdout_write: OW,
+        stderr_write: EW,
+    ) -> io::Result<process::Child> {
+        self.stdout(process::Stdio::piped())
+            .stderr(process::Stdio::piped())
+            .spawn()
+            .and_then(|child| write_child_process_output(child, stdout_write, stderr_write))
+    }
+
+    fn output_and_write_streams<OW: Write + Send, EW: Write + Send>(
+        &mut self,
+        stdout_write: OW,
+        stderr_write: EW,
+    ) -> io::Result<process::Output> {
+        let mut stdout_buffer = vec![];
+        let mut stderr_buffer = vec![];
+
+        self.spawn_and_write_streams(
+            tee(&mut stdout_buffer, stdout_write),
+            tee(&mut stderr_buffer, stderr_write),
+        )
+        .and_then(|mut child| child.wait())
+        .map(|status| process::Output {
+            status,
+            stdout: stdout_buffer,
+            stderr: stderr_buffer,
+        })
+    }
+}
+
+fn write_child_process_output<OW: Write + Send, EW: Write + Send>(
+    mut child: process::Child,
+    mut stdout_writer: OW,
+    mut stderr_writer: EW,
+) -> io::Result<process::Child> {
+    // Copying the data to the writers happens in separate threads for stdout and stderr to ensure
+    // they're processed in parallel. Example: imagine the caller uses io::stdout() and io::stderr()
+    // as the writers so that the user can follow along with the command's output. If we copy stdout
+    // first and then stderr afterwards, interleaved stdout and stderr messages will no longer be
+    // interleaved (stderr output is always printed after stdout has been closed).
+    //
+    // The rust compiler currently cannot figure out how long a thread will run (doesn't take the
+    // almost immediate join calls into account) and therefore requires that data used in a thread
+    // lives forever. To avoid requiring 'static lifetimes for the writers, we use crossbeam's
+    // scoped threads here. This enables writers that write, for example, to a mutable buffer.
+    unwind_panic(crossbeam_utils::thread::scope(|scope| {
+        let stdout_copy_thread = mem::take(&mut child.stdout)
+            .map(|mut stdout| scope.spawn(move |_| std::io::copy(&mut stdout, &mut stdout_writer)));
+
+        let stderr_copy_thread = mem::take(&mut child.stderr)
+            .map(|mut stderr| scope.spawn(move |_| std::io::copy(&mut stderr, &mut stderr_writer)));
+
+        let stdout_copy_result = stdout_copy_thread.map_or_else(|| Ok(0), join_and_unwind_panic);
+        let stderr_copy_result = stderr_copy_thread.map_or_else(|| Ok(0), join_and_unwind_panic);
+
+        // Return the first error from either Result, or the child process value
+        stdout_copy_result.and(stderr_copy_result).map(|_| child)
+    }))
+}
+
+fn join_and_unwind_panic<T>(h: ScopedJoinHandle<T>) -> T {
+    unwind_panic(h.join())
+}
+
+fn unwind_panic<T>(t: thread::Result<T>) -> T {
+    match t {
+        Ok(value) => value,
+        Err(err) => panic::resume_unwind(err),
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use crate::command::CommandExt;
+    use std::process::Command;
+
+    #[test]
+    #[cfg(unix)]
+    fn test_spawn_and_write_streams() {
+        let mut stdout_buf = vec![];
+        let mut stderr_buf = vec![];
+
+        Command::new("echo")
+            .args(["-n", "Hello World!"])
+            .spawn_and_write_streams(&mut stdout_buf, &mut stderr_buf)
+            .and_then(|mut child| child.wait())
+            .unwrap();
+
+        assert_eq!(stdout_buf, "Hello World!".as_bytes());
+        assert_eq!(stderr_buf, Vec::<u8>::new());
+    }
+
+    #[test]
+    #[cfg(unix)]
+    fn test_output_and_write_streams() {
+        let mut stdout_buf = vec![];
+        let mut stderr_buf = vec![];
+
+        let output = Command::new("echo")
+            .args(["-n", "Hello World!"])
+            .output_and_write_streams(&mut stdout_buf, &mut stderr_buf)
+            .unwrap();
+
+        assert_eq!(stdout_buf, "Hello World!".as_bytes());
+        assert_eq!(stderr_buf, Vec::<u8>::new());
+
+        assert_eq!(output.status.code(), Some(0));
+        assert_eq!(output.stdout, "Hello World!".as_bytes());
+        assert_eq!(output.stderr, Vec::<u8>::new());
+    }
+}
@@ -7,6 +7,8 @@
 // This lint is too noisy and enforces a style that reduces readability in many cases.
 #![allow(clippy::module_name_repetitions)]
 
+#[cfg(feature = "command")]
+pub mod command;
 #[cfg(feature = "digest")]
 pub mod digest;
 #[cfg(feature = "download")]
@@ -21,3 +23,5 @@ pub mod log;
 pub mod tar;
 #[cfg(feature = "toml")]
 pub mod toml;
+#[cfg(feature = "write")]
+pub mod write;