Fix bufused when there are more entries to be read

This commit updates the bufused value to be that of the buffer length
(buf_len) when there are more entries to be read.

Currently what is happening is that if another directory entry cannot
be written to the buffer then the current value of bufused is being
"returned", which indicates that all directory entries have been
written to the buffer which is not the case.

Fixes: #174

src/uvwasi.c

@@ -1384,8 +1384,14 @@ uvwasi_errno_t uvwasi_fd_readdir(uvwasi_t* uvwasi,
       }
 
       /* Write dirent to the buffer if it will fit. */
-      if (UVWASI_SERDES_SIZE_dirent_t + *bufused > buf_len)
+      if (UVWASI_SERDES_SIZE_dirent_t + *bufused > buf_len) {
+        /* If there are more entries to be written to the buffer we set
+         * bufused, which is the return value, to the length of the buffer
+         * which indicates that there are more entries to be read.
+         */
+        *bufused = buf_len;
         break;
+      }
 
       uvwasi_serdes_write_dirent_t(buf, *bufused, &dirent);
       *bufused += UVWASI_SERDES_SIZE_dirent_t;

test/test-fd-readdir-ls.c

@@ -0,0 +1,87 @@
+#include <assert.h>
+#include <stdlib.h>
+#include <string.h>
+#include "uv.h"
+#include "uvwasi.h"
+#include "wasi_serdes.h"
+
+#define TEST_TMP_DIR "./out/tmp"
+#define TEST_PATH_READDIR TEST_TMP_DIR "/test_readdir_ls"
+
+
+#if !defined(_WIN32) && !defined(__ANDROID__)
+static void touch_file(const char* name) {
+  uv_fs_t req;
+  int r;
+
+  r = uv_fs_open(NULL,
+                 &req,
+                 name,
+                 O_WRONLY | O_CREAT | O_TRUNC,
+                 S_IWUSR | S_IRUSR,
+                 NULL);
+  uv_fs_req_cleanup(&req);
+  assert(r >= 0);
+  r = uv_fs_close(NULL, &req, r, NULL);
+  uv_fs_req_cleanup(&req);
+  assert(r == 0);
+}
+#endif /* !defined(_WIN32) && !defined(__ANDROID__) */
+
+/*
+ * This is a test case for https://github.com/nodejs/uvwasi/issues/174.
+ */
+
+int main(void) {
+#if !defined(_WIN32) && !defined(__ANDROID__)
+  uvwasi_t uvwasi;
+  uvwasi_options_t init_options;
+  uvwasi_dircookie_t cookie;
+  uvwasi_size_t buf_size;
+  uvwasi_size_t buf_used;
+  uvwasi_errno_t err;
+  uv_fs_t req;
+  uvwasi_fd_t tmp_fd = 3;
+  char buf[4096];
+  int r;
+
+  r = uv_fs_mkdir(NULL, &req, TEST_TMP_DIR, 0777, NULL);
+  uv_fs_req_cleanup(&req);
+  assert(r == 0 || r == UV_EEXIST);
+
+  r = uv_fs_mkdir(NULL, &req, TEST_PATH_READDIR, 0777, NULL);
+  uv_fs_req_cleanup(&req);
+  assert(r == 0 || r == UV_EEXIST);
+
+  for (int i = 0; i < 300; i++) {
+    const char* format = TEST_PATH_READDIR "/test_file_" "%d";
+    int len = strlen(format) + 3;
+    char file_name[len];
+    snprintf(file_name, len, format, i);
+    touch_file(file_name);
+  }
+
+  uvwasi_options_init(&init_options);
+  init_options.preopenc = 1;
+  init_options.preopens = calloc(1, sizeof(uvwasi_preopen_t));
+  init_options.preopens[0].mapped_path = "/var";
+  init_options.preopens[0].real_path = TEST_PATH_READDIR;
+
+  err = uvwasi_init(&uvwasi, &init_options);
+  assert(err == 0);
+
+  buf_size = 4096;
+  memset(buf, 0, buf_size);
+  buf_used = 0;
+  cookie = UVWASI_DIRCOOKIE_START;
+  err = uvwasi_fd_readdir(&uvwasi, tmp_fd, &buf, buf_size, cookie, &buf_used);
+  assert(err == UVWASI_ESUCCESS);
+
+  assert(buf_used == buf_size);
+
+  uvwasi_destroy(&uvwasi);
+  free(init_options.preopens);
+
+#endif /* !defined(_WIN32) && !defined(__ANDROID__) */
+  return 0;
+}