guard process creation with a mutex

Fixes an fd leak on all unix systems that don't have pipe2.
As of today, that's all unix systems that are not dragonfly bsd,
freebsd, linux, netbsd, openbsd, or redox -- in particular, macOS.

Fixes evmar#14.

Author: nico

Cargo.lock

@@ -9,6 +9,7 @@ edition = "2018"
 getopts = "0.2"
 anyhow = "1.0"
 libc = "0.2"
+lazy_static = "1.4.0"
 
 [target.'cfg(windows)'.dependencies]
 kernel32-sys = "0.2.2"

Cargo.toml

@@ -13,6 +13,9 @@ mod task;
 pub mod trace;
 pub mod work;
 
+#[macro_use]
+extern crate lazy_static;
+
 #[cfg(not(target_env = "msvc"))]
 use jemallocator::Jemalloc;
 

src/lib.rs

@@ -10,6 +10,7 @@ use crate::graph::{BuildId, RspFile};
 use crate::scanner::Scanner;
 use anyhow::{anyhow, bail};
 use std::sync::mpsc;
+use std::sync::Mutex;
 use std::time::{Duration, Instant};
 
 #[cfg(unix)]
@@ -83,12 +84,21 @@ fn run_task(
     Ok(result)
 }
 
+lazy_static! {
+    static ref TASK_MUTEX: Mutex<i32> = Mutex::new(0);
+}
+
 #[cfg(unix)]
 fn run_command(cmdline: &str) -> anyhow::Result<TaskResult> {
-    let mut cmd = std::process::Command::new("/bin/sh")
+    // Command::spawn() can leak FSs when run concurrently, see #14.
+    let just_one = TASK_MUTEX.lock().unwrap();
+    let p = std::process::Command::new("/bin/sh")
         .arg("-c")
         .arg(cmdline)
-        .output()?;
+        .spawn()?;
+    drop(just_one);
+
+    let mut cmd = p.wait_with_output()?;
     let mut output = Vec::new();
     output.append(&mut cmd.stdout);
     output.append(&mut cmd.stderr);