default FDs to CLOEXEC

Note: this is a behavior change.

Currently, this crate opens FDs without CLOEXEC, which means they
persist across exec. This is not the convention throughout the Rust
ecosystem, which is to do the opposite:

rust-lang/rust#24237

This patch setting CLOEXEC :)

Note that doing that after-the-fact on the resulting FD isn't quite good
enough: if your process is forking a lot, that's racy.

perf-event/src/lib.rs

@@ -205,6 +205,7 @@ pub struct Builder<'a> {
     who: EventPid<'a>,
     cpu: Option<usize>,
     group: Option<&'a mut Group>,
+    cloexec: bool,
 }
 
 #[derive(Debug)]
@@ -470,6 +471,7 @@ impl<'a> Default for Builder<'a> {
             who: EventPid::ThisProcess,
             cpu: None,
             group: None,
+            cloexec: true,
         }
     }
 }
@@ -618,6 +620,12 @@ impl<'a> Builder<'a> {
         self
     }
 
+    /// By default, perf event FDs are open with PERF_FLAG_FD_CLOEXEC, so the FD you get back has
+    /// CLOEXEC set. Call this to not do so.
+    pub fn unset_cloexec(mut self) {
+        self.cloexec = false;
+    }
+
     /// Place the counter in the given [`Group`]. Groups allow a set of counters
     /// to be enabled, disabled, or read as a single atomic operation, so that
     /// the counts can be usefully compared.
@@ -656,7 +664,7 @@ impl<'a> Builder<'a> {
             Some(cpu) => cpu as c_int,
             None => -1,
         };
-        let (pid, flags) = self.who.as_args();
+        let (pid, mut flags) = self.who.as_args();
         let group_fd = match self.group {
             Some(ref mut g) => {
                 g.max_members += 1;
@@ -665,6 +673,10 @@ impl<'a> Builder<'a> {
             None => -1,
         };
 
+        if self.cloexec {
+            flags |= perf_event_open_sys::bindings::PERF_FLAG_FD_CLOEXEC;
+        }
+
         let file = unsafe {
             File::from_raw_fd(check_errno_syscall(|| {
                 sys::perf_event_open(&mut self.attrs, pid, cpu, group_fd, flags as c_ulong)