cgen, sync: implement separate read/write locks for rwshared types (#5687)

vlib/sync/sync_nix.c.v

@@ -11,12 +11,32 @@ pub struct Mutex {
 	mutex C.pthread_mutex_t
 }
 
+[ref_only]
+pub struct RwMutex {
+	mutex C.pthread_rwlock_t
+}
+
+[ref_only]
+struct RwMutexAttr {
+	attr C.pthread_rwlockattr_t
+}
+
 pub fn new_mutex() &Mutex {
 	m := &Mutex{}
 	C.pthread_mutex_init(&m.mutex, C.NULL)
 	return m
 }
 
+pub fn new_rwmutex() &RwMutex {
+	m := &RwMutex{}
+	a := &RwMutexAttr{}
+	C.pthread_rwlockattr_init(&a.attr)
+	// Give writer priority over readers
+	C.pthread_rwlockattr_setkind_np(&a.attr, C.PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP)
+	C.pthread_rwlock_init(&m.mutex, &a.attr)
+	return m
+}
+
 // m_lock(), for *manual* mutex handling, since `lock` is a keyword
 pub fn (mut m Mutex) m_lock() {
 	C.pthread_mutex_lock(&m.mutex)
@@ -25,3 +45,22 @@ pub fn (mut m Mutex) m_lock() {
 pub fn (mut m Mutex) unlock() {
 	C.pthread_mutex_unlock(&m.mutex)
 }
+
+// RwMutex has separate read- and write locks
+pub fn (mut m RwMutex) r_lock() {
+	C.pthread_rwlock_rdlock(&m.mutex)
+}
+
+pub fn (mut m RwMutex) w_lock() {
+	C.pthread_rwlock_wrlock(&m.mutex)
+}
+
+// Windows SRWLocks have different function to unlock
+// So provide two functions here, too, to have a common interface
+pub fn (mut m RwMutex) r_unlock() {
+	C.pthread_rwlock_unlock(&m.mutex)
+}
+
+pub fn (mut m RwMutex) w_unlock() {
+	C.pthread_rwlock_unlock(&m.mutex)
+}