async: add explanatory comment

src/shared/async.h

@@ -6,6 +6,19 @@
 #include "macro.h"
 #include "rm-rf.h"
 
+/* These functions implement various potentially slow operations that are executed asynchronously. They are
+ * carefully written to not use pthreads, but use fork() or clone() (without CLONE_VM) so that the child does
+ * not share any memory with the parent process, and thus cannot possibly interfere with the malloc()
+ * synchronization locks.
+ *
+ * Background: glibc only synchronizes malloc() locks when doing fork(), but not when doing clone()
+ * (regardless if through glibc's own wrapper or ours). This means if another thread in the parent has the
+ * malloc() lock taken while a thread is cloning, the mutex will remain locked in the child (but the other
+ * thread won't exist there), with no chance to ever be unlocked again. This will result in deadlocks. Hence
+ * one has to make the choice: either never use threads in the parent, or never do memory allocation in the
+ * child, or never use clone()/clone3() and stick to fork() only. Because we need clone()/clone3() we opted
+ * for avoiding threads. */
+
 int asynchronous_sync(pid_t *ret_pid);
 int asynchronous_close(int fd);
 int asynchronous_rm_rf(const char *p, RemoveFlags flags);