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
 }
 

core/sys/freebsd/syscalls.odin

@@ -22,6 +22,7 @@ SYS_getpid     : uintptr : 20
 SYS_recvfrom   : uintptr : 29
 SYS_accept     : uintptr : 30
 SYS_fcntl      : uintptr : 92
+SYS_fsync      : uintptr : 95
 SYS_socket     : uintptr : 97
 SYS_connect    : uintptr : 98
 SYS_bind       : uintptr : 104
@@ -200,6 +201,16 @@ accept_nil :: proc "contextless" (s: Fd) -> (Fd, Errno) {
 
 accept :: proc { accept_T, accept_nil }
 
+// Synchronize changes to a file.
+//
+// The fsync() system call appeared in 4.2BSD.
+fsync :: proc "contextless" (fd: Fd) -> Errno {
+	result, _ := intrinsics.syscall_bsd(SYS_fsync,
+		cast(uintptr)fd)
+
+	return cast(Errno)result
+}
+
 // File control.
 //
 // The fcntl() system call appeared in 4.2BSD.