Add missing flush functionality to os platforms

Platforms:
- FreeBSD
- Haiku
- Linux
- NetBSD
- OpenBSD

core/os/os_freebsd.odin

@@ -447,8 +447,7 @@ close :: proc(fd: Handle) -> Error {
 }
 
 flush :: proc(fd: Handle) -> Error {
-	// do nothing
-	return nil
+	return cast(_Platform_Error)freebsd.fsync(cast(freebsd.Fd)fd)
 }
 
 // If you read or write more than `INT_MAX` bytes, FreeBSD returns `EINVAL`.

core/os/os_haiku.odin

@@ -142,6 +142,7 @@ foreign libc {
 	@(link_name="unlink")         _unix_unlink         :: proc(path: cstring) -> c.int ---
 	@(link_name="rmdir")          _unix_rmdir          :: proc(path: cstring) -> c.int ---
 	@(link_name="mkdir")          _unix_mkdir          :: proc(path: cstring, mode: mode_t) -> c.int ---
+	@(link_name="fsync")          _unix_fsync          :: proc(fd: Handle) -> c.int ---
 
 	@(link_name="getpagesize")    _unix_getpagesize    :: proc() -> c.int ---
 	@(link_name="sysconf")        _sysconf             :: proc(name: c.int) -> c.long ---
@@ -218,7 +219,10 @@ close :: proc(fd: Handle) -> Error {
 }
 
 flush :: proc(fd: Handle) -> Error {
-	// do nothing
+	result := _unix_fsync(fd)
+	if result == -1 {
+		return get_last_error()
+	}
 	return nil
 }
 

core/os/os_linux.odin

@@ -584,8 +584,7 @@ close :: proc(fd: Handle) -> Error {
 }
 
 flush :: proc(fd: Handle) -> Error {
-	// do nothing
-	return nil
+	return _get_errno(unix.sys_fsync(int(fd)))
 }
 
 // If you read or write more than `SSIZE_MAX` bytes, result is implementation defined (probably an error).

core/os/os_netbsd.odin

@@ -443,6 +443,7 @@ foreign libc {
 	@(link_name="rmdir")            _unix_rmdir         :: proc(path: cstring) -> c.int ---
 	@(link_name="mkdir")            _unix_mkdir         :: proc(path: cstring, mode: mode_t) -> c.int ---
 	@(link_name="fcntl")            _unix_fcntl         :: proc(fd: Handle, cmd: c.int, #c_vararg args: ..any) -> c.int ---
+	@(link_name="fsync")            _unix_fsync         :: proc(fd: Handle) -> c.int ---
 	@(link_name="dup")              _unix_dup           :: proc(fd: Handle) -> Handle ---
 	
 	@(link_name="fdopendir")        _unix_fdopendir     :: proc(fd: Handle) -> Dir ---
@@ -506,7 +507,10 @@ close :: proc(fd: Handle) -> Error {
 }
 
 flush :: proc(fd: Handle) -> Error {
-	// do nothing
+	result := _unix_fsync(fd)
+	if result == -1 {
+		return get_last_error()
+	}
 	return nil
 }
 

core/os/os_openbsd.odin

@@ -366,6 +366,7 @@ foreign libc {
 	@(link_name="unlink")         _unix_unlink         :: proc(path: cstring) -> c.int ---
 	@(link_name="rmdir")          _unix_rmdir          :: proc(path: cstring) -> c.int ---
 	@(link_name="mkdir")          _unix_mkdir          :: proc(path: cstring, mode: mode_t) -> c.int ---
+	@(link_name="fsync")          _unix_fsync          :: proc(fd: Handle) -> c.int ---
 	@(link_name="dup")            _unix_dup            :: proc(fd: Handle) -> Handle ---
 
 	@(link_name="getpagesize")    _unix_getpagesize    :: proc() -> c.int ---
@@ -430,7 +431,10 @@ close :: proc(fd: Handle) -> Error {
 }
 
 flush :: proc(fd: Handle) -> Error {
-	// do nothing
+	result := _unix_fsync(fd)
+	if result == -1 {
+		return get_last_error()
+	}
 	return nil
 }