Doesn't link to libzutil on FreeBSD

[asomers]

On FreeBSD, libzfs_core needs to link to libzutil. zfs-core-sys/build.rs does indeed specify that correctly. However, the resulting binaries don't end up linked to libzutil. It seems like rustc ignores the -l zutil option because the Rust code doesn't have direct references to libzutil, and Rust doesn't consider transitive linking dependencies. Probably related to bazelbuild/rules_rust#78 .

To demonstrate the problem:

cargo build --all-targets
> ldd target/debug/deps/basic-6af3a0e39594d1eb
target/debug/deps/basic-6af3a0e39594d1eb:
	libzfs_core.so.2 => /lib/libzfs_core.so.2 (0x801204000)
	libnvpair.so.2 => /lib/libnvpair.so.2 (0x80120d000)
	libthr.so.3 => /lib/libthr.so.3 (0x801227000)
	libgcc_s.so.1 => /lib/libgcc_s.so.1 (0x801254000)
	libc.so.7 => /lib/libc.so.7 (0x80126f000)
	libm.so.5 => /lib/libm.so.5 (0x801690000)
# Notice no libzutil

However, apply this stupid little patch to the source tree, which calls a libzutil function that does literally nothing, and magically the binaries are all correctly linked:

diff --git a/zfs-core-sys/src/lib.rs b/zfs-core-sys/src/lib.rs
index 2d6a893..d1c85d1 100644
--- a/zfs-core-sys/src/lib.rs
+++ b/zfs-core-sys/src/lib.rs
@@ -6,3 +6,11 @@ extern crate nvpair_sys as nvpair;
 use nvpair::*;
 
 include!("bindings.rs");
+
+
+extern "C" {
+    /// This function does literally nothing on FreeBSD, but we must reference
+    /// it so the optimizer doesn't "helpfully" remove the linkage to
+    /// libzutil.so.
+    pub fn zfs_dev_flush(fd: i32) -> i32;
+}
diff --git a/zfs-core/src/lib.rs b/zfs-core/src/lib.rs
index e56222c..29f38c0 100644
--- a/zfs-core/src/lib.rs
+++ b/zfs-core/src/lib.rs
@@ -146,6 +146,7 @@ impl Zfs {
     /// Create a handle to the Zfs subsystem
     #[doc(alias = "libzfs_core_init")]
     pub fn new() -> io::Result<Self> {
+        unsafe { sys::zfs_dev_flush(0); }
         let v = unsafe { sys::libzfs_core_init() };
 
         if v != 0 {

> ldd target/debug/deps/basic-6af3a0e39594d1eb
target/debug/deps/basic-6af3a0e39594d1eb:
	libzutil.so.2 => /lib/libzutil.so.2 (0x801204000)
	libzfs_core.so.2 => /lib/libzfs_core.so.2 (0x801211000)
	libnvpair.so.2 => /lib/libnvpair.so.2 (0x80121a000)
	libthr.so.3 => /lib/libthr.so.3 (0x801234000)
	libgcc_s.so.1 => /lib/libgcc_s.so.1 (0x801261000)
	libc.so.7 => /lib/libc.so.7 (0x80127c000)
	libm.so.5 => /lib/libm.so.5 (0x80169d000)
	libavl.so.2 => /lib/libavl.so.2 (0x8016d4000)
	libtpool.so.2 => /lib/libtpool.so.2 (0x8016d9000)
	libspl.so.2 => /lib/libspl.so.2 (0x8016df000)