Always use sccache's own jobserver

and close the pipes to the inherited jobserver.

src/commands.rs

@@ -762,7 +762,7 @@ pub fn run_command(cmd: Command) -> Result<i32> {
 
             trace!("Command::PackageToolchain({})", executable.display());
             let runtime = Runtime::new()?;
-            let jobserver = unsafe { Client::new() };
+            let jobserver = Client::new();
             let creator = ProcessCommandCreator::new(&jobserver);
             let args: Vec<_> = env::args_os().collect();
             let env: Vec<_> = env::vars_os().collect();
@@ -788,7 +788,7 @@ pub fn run_command(cmd: Command) -> Result<i32> {
             env_vars,
         } => {
             trace!("Command::Compile {{ {:?}, {:?}, {:?} }}", exe, cmdline, cwd);
-            let jobserver = unsafe { Client::new() };
+            let jobserver = Client::new();
             let conn = connect_or_start_server(&get_addr(), startup_timeout)?;
             let mut runtime = Runtime::new()?;
             let res = do_compile(

src/jobserver.rs

@@ -8,6 +8,68 @@ use futures::StreamExt;
 
 use crate::errors::*;
 
+// The execution model of sccache is that on the first run it spawns a server
+// in the background and detaches it.
+// When normally executing the rust compiler from either cargo or make, it
+// will use cargo/make's jobserver and limit its resource usage accordingly.
+// When executing the rust compiler through the sccache server, that jobserver
+// is not available, and spawning as many rustc as there are CPUs can lead to
+// a quadratic use of the CPU resources (each rustc spawning as many threads
+// as there are CPUs).
+// One way around this issue is to inherit the jobserver from cargo or make
+// when the sccache server is spawned, but that means that in some cases, the
+// cargo or make process can't terminate until the sccache server terminates
+// after its idle timeout (which also never happens if SCCACHE_IDLE_TIMEOUT=0).
+// Also, if the sccache server ends up shared between multiple runs of
+// cargo/make, then which jobserver is used doesn't make sense anymore.
+// Ideally, the sccache client would give a handle to the jobserver it has
+// access to, so that the rust compiler would "just" use the jobserver it
+// would have used if it had run without sccache, but that adds some extra
+// complexity, and requires to use Unix domain sockets.
+// What we do instead is to arbitrary use our own jobserver.
+// Unfortunately, that doesn't absolve us from having to deal with the original
+// jobserver, because make may give us file descriptors to its pipes, and the
+// simple fact of keeping them open can block it.
+// So if it does give us those file descriptors, close the preemptively.
+//
+// unsafe because it can use the wrong fds.
+#[cfg(not(windows))]
+pub unsafe fn discard_inherited_jobserver() {
+    if let Some(value) = ["CARGO_MAKEFLAGS", "MAKEFLAGS", "MFLAGS"]
+        .into_iter()
+        .find_map(|env| std::env::var(env).ok())
+    {
+        if let Some(auth) = value.rsplit(' ').find_map(|arg| {
+            arg.strip_prefix("--jobserver-auth=")
+                .or_else(|| arg.strip_prefix("--jobserver-fds="))
+        }) {
+            if !auth.starts_with("fifo:") {
+                let mut parts = auth.splitn(2, ',');
+                let read = parts.next().unwrap();
+                let write = match parts.next() {
+                    Some(w) => w,
+                    None => return,
+                };
+                let read = read.parse().unwrap();
+                let write = write.parse().unwrap();
+                if read < 0 || write < 0 {
+                    return;
+                }
+                unsafe {
+                    if libc::fcntl(read, libc::F_GETFD) == -1 {
+                        return;
+                    }
+                    if libc::fcntl(write, libc::F_GETFD) == -1 {
+                        return;
+                    }
+                    libc::close(read);
+                    libc::close(write);
+                }
+            }
+        }
+    }
+}
+
 #[derive(Clone)]
 pub struct Client {
     helper: Option<Arc<jobserver::HelperThread>>,
@@ -20,12 +82,8 @@ pub struct Acquired {
 }
 
 impl Client {
-    // unsafe because `from_env` is unsafe (can use the wrong fds)
-    pub unsafe fn new() -> Client {
-        match jobserver::Client::from_env() {
-            Some(c) => Client::_new(c, true),
-            None => Client::new_num(num_cpus::get()),
-        }
+    pub fn new() -> Client {
+        Client::new_num(num_cpus::get())
     }
 
     pub fn new_num(num: usize) -> Client {

src/server.rs

@@ -428,7 +428,7 @@ pub fn start_server(config: &Config, addr: &crate::net::SocketAddr) -> Result<()
         });
         panic_hook(info)
     }));
-    let client = unsafe { Client::new() };
+    let client = Client::new();
     let runtime = tokio::runtime::Builder::new_multi_thread()
         .enable_all()
         .worker_threads(std::cmp::max(20, 2 * num_cpus::get()))

src/test/tests.rs

@@ -88,7 +88,7 @@ where
             CacheMode::ReadWrite,
         ));
 
-        let client = unsafe { Client::new() };
+        let client = Client::new();
         let srv = SccacheServer::new(0, runtime, client, dist_client, storage).unwrap();
         let mut srv: SccacheServer<_, Arc<Mutex<MockCommandCreator>>> = srv;
         let addr = srv.local_addr().unwrap();

src/test/utils.rs

@@ -83,7 +83,7 @@ macro_rules! assert_map_contains {
 }
 
 pub fn new_creator() -> Arc<Mutex<MockCommandCreator>> {
-    let client = unsafe { Client::new() };
+    let client = Client::new();
     Arc::new(Mutex::new(MockCommandCreator::new(&client)))
 }
 

src/util.rs

@@ -842,6 +842,7 @@ impl<'a> Hasher for HashToDigest<'a> {
 /// Pipe `cmd`'s stdio to `/dev/null`, unless a specific env var is set.
 #[cfg(not(windows))]
 pub fn daemonize() -> Result<()> {
+    use crate::jobserver::discard_inherited_jobserver;
     use daemonize::Daemonize;
     use std::env;
     use std::mem;
@@ -853,6 +854,10 @@ pub fn daemonize() -> Result<()> {
         }
     }
 
+    unsafe {
+        discard_inherited_jobserver();
+    }
+
     static mut PREV_SIGSEGV: *mut libc::sigaction = 0 as *mut _;
     static mut PREV_SIGBUS: *mut libc::sigaction = 0 as *mut _;
     static mut PREV_SIGILL: *mut libc::sigaction = 0 as *mut _;