From 6afa32a2504fa90b48f74979bb4061cb397e9270 Mon Sep 17 00:00:00 2001
From: Alex Crichton <alex@alexcrichton.com>
Date: Fri, 12 Feb 2016 10:28:03 -0800
Subject: [PATCH] std: Don't always create stdin for children

For example if `Command::output` or `Command::status` is used then stdin is just
immediately closed. Add an option for this so as an optimization we can avoid
creating pipes entirely.

This should help reduce the number of active file descriptors when spawning
processes on Unix and the number of active handles on Windows.
---
 src/libstd/process.rs             |  7 ++++---
 src/libstd/sys/unix/process.rs    | 13 ++++++++-----
 src/libstd/sys/windows/process.rs |  6 ++++--
 3 files changed, 16 insertions(+), 10 deletions(-)

diff --git a/src/libstd/process.rs b/src/libstd/process.rs
index ec86dd062b54..fe5e49ecb09b 100644
--- a/src/libstd/process.rs
+++ b/src/libstd/process.rs
@@ -295,7 +295,7 @@ impl Command {
     /// By default, stdin, stdout and stderr are inherited from the parent.
     #[stable(feature = "process", since = "1.0.0")]
     pub fn spawn(&mut self) -> io::Result<Child> {
-        self.inner.spawn(imp::Stdio::Inherit).map(Child::from_inner)
+        self.inner.spawn(imp::Stdio::Inherit, true).map(Child::from_inner)
     }
 
     /// Executes the command as a child process, waiting for it to finish and
@@ -318,7 +318,7 @@ impl Command {
     /// ```
     #[stable(feature = "process", since = "1.0.0")]
     pub fn output(&mut self) -> io::Result<Output> {
-        self.inner.spawn(imp::Stdio::MakePipe).map(Child::from_inner)
+        self.inner.spawn(imp::Stdio::MakePipe, false).map(Child::from_inner)
             .and_then(|p| p.wait_with_output())
     }
 
@@ -340,7 +340,8 @@ impl Command {
     /// ```
     #[stable(feature = "process", since = "1.0.0")]
     pub fn status(&mut self) -> io::Result<ExitStatus> {
-        self.spawn().and_then(|mut p| p.wait())
+        self.inner.spawn(imp::Stdio::Inherit, false).map(Child::from_inner)
+                  .and_then(|mut p| p.wait())
     }
 }
 
diff --git a/src/libstd/sys/unix/process.rs b/src/libstd/sys/unix/process.rs
index 28475f50ce63..5696cb2b52f7 100644
--- a/src/libstd/sys/unix/process.rs
+++ b/src/libstd/sys/unix/process.rs
@@ -216,7 +216,7 @@ impl Command {
         self.stderr = Some(stderr);
     }
 
-    pub fn spawn(&mut self, default: Stdio)
+    pub fn spawn(&mut self, default: Stdio, needs_stdin: bool)
                  -> io::Result<(Process, StdioPipes)> {
         const CLOEXEC_MSG_FOOTER: &'static [u8] = b"NOEX";
 
@@ -225,7 +225,7 @@ impl Command {
                                       "nul byte found in provided data"));
         }
 
-        let (ours, theirs) = try!(self.setup_io(default));
+        let (ours, theirs) = try!(self.setup_io(default, needs_stdin));
         let (input, output) = try!(sys::pipe::anon_pipe());
 
         let pid = unsafe {
@@ -298,7 +298,7 @@ impl Command {
                                   "nul byte found in provided data")
         }
 
-        match self.setup_io(default) {
+        match self.setup_io(default, true) {
             Ok((_, theirs)) => unsafe { self.do_exec(theirs) },
             Err(e) => e,
         }
@@ -408,8 +408,11 @@ impl Command {
     }
 
 
-    fn setup_io(&self, default: Stdio) -> io::Result<(StdioPipes, ChildPipes)> {
-        let stdin = self.stdin.as_ref().unwrap_or(&default);
+    fn setup_io(&self, default: Stdio, needs_stdin: bool)
+                -> io::Result<(StdioPipes, ChildPipes)> {
+        let null = Stdio::Null;
+        let default_stdin = if needs_stdin {&default} else {&null};
+        let stdin = self.stdin.as_ref().unwrap_or(default_stdin);
         let stdout = self.stdout.as_ref().unwrap_or(&default);
         let stderr = self.stderr.as_ref().unwrap_or(&default);
         let (their_stdin, our_stdin) = try!(stdin.to_child_stdio(true));
diff --git a/src/libstd/sys/windows/process.rs b/src/libstd/sys/windows/process.rs
index fa118be6fe6b..524c932eed43 100644
--- a/src/libstd/sys/windows/process.rs
+++ b/src/libstd/sys/windows/process.rs
@@ -123,7 +123,7 @@ impl Command {
         self.stderr = Some(stderr);
     }
 
-    pub fn spawn(&mut self, default: Stdio)
+    pub fn spawn(&mut self, default: Stdio, needs_stdin: bool)
                  -> io::Result<(Process, StdioPipes)> {
         // To have the spawning semantics of unix/windows stay the same, we need
         // to read the *child's* PATH if one is provided. See #15149 for more
@@ -181,7 +181,9 @@ impl Command {
             stdout: None,
             stderr: None,
         };
-        let stdin = self.stdin.as_ref().unwrap_or(&default);
+        let null = Stdio::Null;
+        let default_stdin = if needs_stdin {&default} else {&null};
+        let stdin = self.stdin.as_ref().unwrap_or(default_stdin);
         let stdout = self.stdout.as_ref().unwrap_or(&default);
         let stderr = self.stderr.as_ref().unwrap_or(&default);
         let stdin = try!(stdin.to_handle(c::STD_INPUT_HANDLE, &mut pipes.stdin));