Atomically open files with O_CLOEXEC where possible

On Linux the flag is just ignored if it is not supported:
https://lwn.net/Articles/588444/

Touches #24237.

Author: tbu-

src/liblibc/lib.rs

@@ -102,6 +102,7 @@ pub use types::os::arch::extra::*;
 pub use consts::os::c95::*;
 pub use consts::os::posix88::*;
 pub use consts::os::posix01::*;
+pub use consts::os::posix08::*;
 pub use consts::os::bsd44::*;
 pub use consts::os::extra::*;
 
@@ -3608,6 +3609,8 @@ pub mod consts {
             pub const RUSAGE_THREAD: c_int = 1;
         }
         pub mod posix08 {
+            use types::os::arch::c95::c_int;
+            pub const O_CLOEXEC: c_int = 0x80000;
         }
         #[cfg(any(target_arch = "arm",
                   target_arch = "aarch64",
@@ -4267,7 +4270,15 @@ pub mod consts {
             pub const RUSAGE_CHILDREN: c_int = -1;
             pub const RUSAGE_THREAD: c_int = 1;
         }
+        #[cfg(target_os = "freebsd")]
         pub mod posix08 {
+            use types::os::arch::c95::c_int;
+            pub const O_CLOEXEC: c_int = 0x100000;
+        }
+        #[cfg(target_os = "dragonfly")]
+        pub mod posix08 {
+            use types::os::arch::c95::c_int;
+            pub const O_CLOEXEC: c_int = 0x20000;
         }
         pub mod bsd44 {
             use types::os::arch::c95::c_int;
@@ -4710,7 +4721,15 @@ pub mod consts {
             pub const RUSAGE_CHILDREN: c_int = -1;
             pub const RUSAGE_THREAD: c_int = 1;
         }
+        #[cfg(any(target_os = "bitrig", target_os = "openbsd"))]
         pub mod posix08 {
+            use types::os::arch::c95::c_int;
+            pub const O_CLOEXEC: c_int = 0x10000;
+        }
+        #[cfg(target_os = "netbsd")]
+        pub mod posix08 {
+            use types::os::arch::c95::c_int;
+            pub const O_CLOEXEC: c_int = 0x400000;
         }
         pub mod bsd44 {
             use types::os::arch::c95::c_int;
@@ -5148,6 +5167,8 @@ pub mod consts {
             pub const RUSAGE_THREAD: c_int = 1;
         }
         pub mod posix08 {
+            use types::os::arch::c95::c_int;
+            pub const O_CLOEXEC: c_int = 0x1000000;
         }
         pub mod bsd44 {
             use types::os::arch::c95::c_int;

src/libstd/sys/unix/fs.rs

@@ -212,7 +212,7 @@ impl DirEntry {
 impl OpenOptions {
     pub fn new() -> OpenOptions {
         OpenOptions {
-            flags: 0,
+            flags: libc::O_CLOEXEC,
             read: false,
             write: false,
             mode: 0o666,
@@ -269,6 +269,9 @@ impl File {
             libc::open(path.as_ptr(), flags, opts.mode)
         }));
         let fd = FileDesc::new(fd);
+        // Even though we open with the O_CLOEXEC flag, still set CLOEXEC here,
+        // in case the open flag is not supported (it's just ignored by the OS
+        // in that case).
         fd.set_cloexec();
         Ok(File(fd))
     }