Remove extra lock-taking in preopen setup (#491)

Issue [#8392] in Wasmtime highlights how preopen registration can result
in a hang when compiled for a threaded environment. The problem is that
`internal_register_preopened_fd_unlocked` assumes it will not touch the
global lock because its caller, `__wasilibc_populate_preopens`, already
has taken the lock. Unfortunately, a refactoring in #408 (which
introduces `internal_register_preopened_fd_unlocked`) did not catch that
the `resize` function called internally also takes the global lock. This
change removes that locking in `resize` under the assumption that it
will only be called when the lock is already taken.

[#8392]: bytecodealliance/wasmtime#8392

libc-bottom-half/sources/preopens.c

@@ -59,15 +59,13 @@ static void assert_invariants(void) {
 
 /// Allocate space for more preopens. Returns 0 on success and -1 on failure.
 static int resize(void) {
-    LOCK(lock);
     size_t start_capacity = 4;
     size_t old_capacity = preopen_capacity;
     size_t new_capacity = old_capacity == 0 ? start_capacity : old_capacity * 2;
 
     preopen *old_preopens = preopens;
     preopen *new_preopens = calloc(sizeof(preopen), new_capacity);
     if (new_preopens == NULL) {
-        UNLOCK(lock);
         return -1;
     }
 
@@ -77,7 +75,6 @@ static int resize(void) {
     free(old_preopens);
 
     assert_invariants();
-    UNLOCK(lock);
     return 0;
 }
 
@@ -101,8 +98,7 @@ static const char *strip_prefixes(const char *path) {
     return path;
 }
 
-/// Similar to `internal_register_preopened_fd_unlocked` but does not
-/// take a lock.
+/// Similar to `internal_register_preopened_fd` but does not take a lock.
 static int internal_register_preopened_fd_unlocked(__wasi_fd_t fd, const char *relprefix) {
     // Check preconditions.
     assert_invariants();