add __wasilibc_reset_preopens (#489)

This resets the preopens table to an uninitialized state, forcing it to be
reinitialized next time it is needed.  This is useful when pre-initializing
using e.g. `wizer` or `component-init`.  Such tools are capable of taking a
snapshot of a running application which may later be resumed on an unrelated
runtime (which may have its own, unrelated preopens).

Signed-off-by: Joel Dice <joel.dice@fermyon.com>

expected/wasm32-wasip1-threads/defined-symbols.txt

@@ -395,6 +395,7 @@ __wasilibc_pthread_self
 __wasilibc_register_preopened_fd
 __wasilibc_rename_newat
 __wasilibc_rename_oldat
+__wasilibc_reset_preopens
 __wasilibc_rmdirat
 __wasilibc_stat
 __wasilibc_tell

expected/wasm32-wasip1/defined-symbols.txt

@@ -333,6 +333,7 @@ __wasilibc_populate_preopens
 __wasilibc_register_preopened_fd
 __wasilibc_rename_newat
 __wasilibc_rename_oldat
+__wasilibc_reset_preopens
 __wasilibc_rmdirat
 __wasilibc_stat
 __wasilibc_tell

expected/wasm32-wasip2/defined-symbols.txt

@@ -348,6 +348,7 @@ __wasilibc_populate_preopens
 __wasilibc_register_preopened_fd
 __wasilibc_rename_newat
 __wasilibc_rename_oldat
+__wasilibc_reset_preopens
 __wasilibc_rmdirat
 __wasilibc_stat
 __wasilibc_tell

libc-bottom-half/sources/preopens.c

@@ -260,7 +260,7 @@ void __wasilibc_populate_preopens(void) {
             if (prefix == NULL)
                 goto software;
 
-            // TODO: Remove the cast on `path` once the witx is updated with
+            // TODO: Remove the cast on `prefix` once the witx is updated with
             // char8 support.
             ret = __wasi_fd_prestat_dir_name(fd, (uint8_t *)prefix,
                                              prestat.u.dir.pr_name_len);
@@ -290,3 +290,23 @@ void __wasilibc_populate_preopens(void) {
 software:
     _Exit(EX_SOFTWARE);
 }
+
+void __wasilibc_reset_preopens(void) {
+    LOCK(lock);
+
+    if (num_preopens) {
+        for (int i = 0; i < num_preopens; ++i) {
+            free((void*) preopens[i].prefix);
+        }
+        free(preopens);
+    }
+
+    preopens_populated = false;
+    preopens = NULL;
+    num_preopens = 0;
+    preopen_capacity = 0;
+
+    assert_invariants();
+
+    UNLOCK(lock);
+}