clear descriptor table in __wasilibc_reset_preopens (#776)

The original intention of `__wasilibc_reset_preopens` was to clear the
stdio state as the last step of Wizer-style pre-initialization since
none of the handles would be valid when the snapshot is resumed. In
earlier versions of `wasi-libc`, which didn't have native p2 or p3
support for stdio or file I/O, resetting the WASIp1 state was
sufficient; the `wasi_snapshot_preview1` adapter took care of emulating
p1 in terms of p2. However, now that `wasi-libc` has native support for
p2 and p3, we need to also reset the descriptor table.

At this point, the name `__wasilibc_reset_preopens` is a bit misleading
for p2+ since we're clearing more than just the preopens, but changing
the name would break tools which use it, so probably not worth the
trouble.

Author: dicej

expected/wasm32-wasip2/defined-symbols.txt

@@ -522,6 +522,7 @@ ctanhl
 ctanl
 ctime
 ctime_r
+descriptor_table_clear
 descriptor_table_get_ref
 descriptor_table_insert
 descriptor_table_remove
@@ -1569,4 +1570,4 @@ y0f
 y1
 y1f
 yn
-ynf
+ynf

expected/wasm32-wasip3/defined-symbols.txt

@@ -523,6 +523,7 @@ ctanhl
 ctanl
 ctime
 ctime_r
+descriptor_table_clear
 descriptor_table_get_ref
 descriptor_table_insert
 descriptor_table_remove
@@ -1596,4 +1597,4 @@ y0f
 y1
 y1f
 yn
-ynf
+ynf

libc-bottom-half/headers/private/wasi/descriptor_table.h

@@ -239,6 +239,9 @@ int descriptor_table_remove(int fd);
 /// errno on failure.
 int descriptor_table_renumber(int fd, int newfd);
 
+/// Removes all file descriptors from the table, running their destructors.
+void descriptor_table_clear();
+
 #endif // __wasip1__
 
 #endif // DESCRIPTOR_TABLE_H

libc-bottom-half/headers/public/wasi/libc.h

@@ -19,6 +19,13 @@ struct timespec;
 /// afterward, you should call this before doing so.
 void __wasilibc_populate_preopens(void);
 
+/// Clear the set of preopens (and, on p2+, any other open file descriptors).
+///
+/// This is appropriate to call just prior to snapshotting the guest state so
+/// that it can be resumed on another runtime, in which case any
+/// previously-opened handles will no longer be valid.
+void __wasilibc_reset_preopens();
+
 #ifndef __wasip2__
 /// Register the given pre-opened file descriptor under the given path.
 int __wasilibc_register_preopened_fd(int fd, const char *prefix);

libc-bottom-half/sources/descriptor_table.c

@@ -125,6 +125,22 @@ static int remove(descriptor_table_t *table, int fd,
   return 0;
 }
 
+static void clear(descriptor_table_t *table) {
+  for (size_t i = 0; i < table->len; ++i) {
+    descriptor_table_item_t *table_entry = &table->entries[i];
+    if (table_entry->occupied) {
+      descriptor_table_entry_t entry = table_entry->entry;
+      entry.vtable->free(entry.data);
+    }
+  }
+  if (table->entries)
+    free(table->entries);
+  table->entries = NULL;
+  table->next = 0;
+  table->len = 0;
+  table->cap = 0;
+}
+
 static bool stdio_initialized = false;
 
 static int init_stdio() {
@@ -176,3 +192,8 @@ int descriptor_table_remove(int fd) {
   entry.vtable->free(entry.data);
   return 0;
 }
+
+void descriptor_table_clear() {
+  clear(&global_table);
+  stdio_initialized = false;
+}

libc-bottom-half/sources/preopens.c

@@ -418,5 +418,9 @@ void __wasilibc_reset_preopens(void) {
 
   assert_invariants();
 
+#ifndef __wasip1__
+  descriptor_table_clear();
+#endif
+
   UNLOCK(lock);
 }

test/CMakeLists.txt

@@ -330,6 +330,12 @@ add_wasilibc_test(sleep.c)
 add_wasilibc_test(write.c FS)
 add_wasilibc_test(wasi-defines.c)
 
+# This tests the behavior of the WASIp2+ descriptor table and won't succeed on
+# p1:
+if (NOT (WASI STREQUAL "p1"))
+  add_wasilibc_test(clear_fds.c FS)
+endif()
+
 if (CMAKE_C_COMPILER_VERSION VERSION_GREATER 20.0)
   add_wasilibc_test(setjmp.c SETJMP)
   set_tests_properties(setjmp.wasm PROPERTIES LABELS v8fail)

test/src/clear_fds.c

@@ -0,0 +1,33 @@
+#include "test.h"
+#include <errno.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <wasi/libc.h>
+
+#define TEST(c)                                                                \
+  do {                                                                         \
+    errno = 0;                                                                 \
+    if (!(c))                                                                  \
+      t_error("%s failed (errno = %d)\n", #c, errno);                          \
+  } while (0)
+
+int main(void) {
+  char tmp[] = "testsuite-XXXXXX";
+  int fd;
+  TEST((fd = open(tmp, O_RDWR | O_CREAT | O_EXCL, 0600)) > 2);
+  TEST(write(fd, "hello", 6) == 6);
+  TEST(write(2, "test stderr\n", 12) == 12);
+  // This should clear the entire descriptor table, closing all descriptors
+  // (e.g. as the last step of a Wizer-style pre-init function).
+  __wasilibc_reset_preopens();
+  if (!(write(fd, "hello", 6) == -1 && errno == EBADF))
+    t_error("fd should have been closed by `__wasilibc_reset_preopens`");
+  // stdio handles will be lazily reinitialized, so should still work:
+  TEST(write(2, "test stderr\n", 12) == 12);
+  if (fd > 2)
+    TEST(unlink(tmp) != -1);
+  return t_status;
+}