sync: fix a helgrind false positive, for a data race, on PoolProcessor (#24023)

vlib/sync/pool/pool.c.v

@@ -13,7 +13,7 @@ pub struct PoolProcessor {
 mut:
 	njobs           int
 	items           []voidptr
-	results         []voidptr
+	results         shared []voidptr
 	ntask           u32 // reading/writing to this should be atomic
 	waitgroup       sync.WaitGroup
 	shared_context  voidptr
@@ -88,7 +88,9 @@ pub fn (mut pool PoolProcessor) work_on_pointers(items []voidptr) {
 	}
 	unsafe {
 		pool.thread_contexts = []voidptr{len: items.len}
+		lock pool.results {
 			pool.results = []voidptr{len: items.len}
+		}
 		pool.items = []voidptr{cap: items.len}
 		pool.items << items
 		pool.waitgroup.add(njobs)
@@ -115,8 +117,10 @@ fn process_in_thread(mut pool PoolProcessor, task_id int) {
 		if idx >= ilen {
 			break
 		}
+		lock pool.results {
 			pool.results[idx] = cb(mut pool, idx, task_id)
 		}
+	}
 	pool.waitgroup.done()
 }
 
@@ -129,15 +133,19 @@ pub fn (pool &PoolProcessor) get_item[T](idx int) T {
 // get_result - called by the main thread to get a specific result.
 // Retrieves a type safe instance of the produced result.
 pub fn (pool &PoolProcessor) get_result[T](idx int) T {
+	rlock pool.results {
 		return unsafe { *(&T(pool.results[idx])) }
+	}
 }
 
 // get_results - get a list of type safe results in the main thread.
 pub fn (pool &PoolProcessor) get_results[T]() []T {
 	mut res := []T{cap: pool.results.len}
 	for i in 0 .. pool.results.len {
+		rlock pool.results {
 			res << unsafe { *(&T(pool.results[i])) }
 		}
+	}
 	return res
 }
 
@@ -145,8 +153,10 @@ pub fn (pool &PoolProcessor) get_results[T]() []T {
 pub fn (pool &PoolProcessor) get_results_ref[T]() []&T {
 	mut res := []&T{cap: pool.results.len}
 	for i in 0 .. pool.results.len {
+		rlock pool.results {
 			res << unsafe { &T(pool.results[i]) }
 		}
+	}
 	return res
 }
 

vlib/v/gen/c/assign.v

@@ -234,6 +234,9 @@ fn (mut g Gen) assign_stmt(node_ ast.AssignStmt) {
 				// we can't just do `.str` since we need the extra data from the string struct
 				// doing `&string` is also not an option since the stack memory with the data will be overwritten
 				g.expr(left0) // node.left[0])
+				if first_left_type.has_flag(.shared_f) {
+					g.write('->val')
+				}
 				g.writeln('); // free ${type_to_free} on re-assignment2')
 				defer {
 					if af {