Fix current_exe() on DragonFly

The method using procfs did only work in case procfs is mounted,
which we cannot expect, especially as procfs is deprecated on
DragonFly. Starting with DragonFly 4.6.1 the sysctl method works
correctly, so try sysctl first, and if it returns a path including
a ":" (which is an invalid path), try to read from procfs before
giving up.

Author: mneumann

src/libstd/sys/unix/os.rs

@@ -218,8 +218,44 @@ pub fn current_exe() -> io::Result<PathBuf> {
     }
 }
 
+// Since DragonFly 4.6.1 (commit [1]), the "kern.proc.pathname" sysctl works "correctly", i.e. it
+// does not return paths including a ":" (see [2]). The sysctl has now the same output as reading
+// from "/proc/curproc/file". As procfs is deprecated on DragonFly and not guaranteed to be
+// mounted, the sysctl is the preferred method. For older versions of DragonFly, we fall back to
+// the procfs method. We determine an older version by inspecting the returned path. If it contains
+// an ":", we try the procfs method.
+//
+// [1]: https://gitweb.dragonflybsd.org/dragonfly.git/commit/726f7ca07e193db73635e9c4e24e40c96087d6d9
+// [2]: https://gist.github.com/mneumann/a2f6b6a0a03935b561d6185872a4b222
 #[cfg(target_os = "dragonfly")]
 pub fn current_exe() -> io::Result<PathBuf> {
+    unsafe {
+        let mut mib = [libc::CTL_KERN as c_int,
+                       libc::KERN_PROC as c_int,
+                       libc::KERN_PROC_PATHNAME as c_int,
+                       -1 as c_int];
+        let mut sz: libc::size_t = 0;
+        cvt(libc::sysctl(mib.as_mut_ptr(), mib.len() as ::libc::c_uint,
+                         ptr::null_mut(), &mut sz, ptr::null_mut(),
+                         0 as libc::size_t))?;
+        if sz == 0 {
+            return Err(io::Error::last_os_error())
+        }
+        let mut v: Vec<u8> = Vec::with_capacity(sz as usize);
+        cvt(libc::sysctl(mib.as_mut_ptr(), mib.len() as ::libc::c_uint,
+                         v.as_mut_ptr() as *mut libc::c_void, &mut sz,
+                         ptr::null_mut(), 0 as libc::size_t))?;
+        if sz == 0 {
+            return Err(io::Error::last_os_error());
+        }
+        v.set_len(sz as usize - 1); // chop off trailing NUL
+        if !v.contains(&b':') {
+            // This is a valid path without a ":"
+            return Ok(PathBuf::from(OsString::from_vec(v)));
+        }
+    }
+
+    // Fall back to procfs
     ::fs::read_link("/proc/curproc/file")
 }