unix: make uv_cwd() report UV_ENOBUFS

Make uv_cwd() do what the documentation says it did when the destination
buffer is too small: report UV_ENOBUFS and set the `size` in/out param
to the size of the path including the trailing nul byte.

Fixes: https://github.com/libuv/libuv/issues/2333
PR-URL: https://github.com/libuv/libuv/pull/2335
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Saúl Ibarra Corretgé <saghul@gmail.com>

src/unix/core.c

@@ -701,16 +701,38 @@ ssize_t uv__recvmsg(int fd, struct msghdr* msg, int flags) {
 
 
 int uv_cwd(char* buffer, size_t* size) {
+  char scratch[1 + UV__PATH_MAX];
+
   if (buffer == NULL || size == NULL)
     return UV_EINVAL;
 
-  if (getcwd(buffer, *size) == NULL)
+  /* Try to read directly into the user's buffer first... */
+  if (getcwd(buffer, *size) != NULL)
+    goto fixup;
+
+  if (errno != ERANGE)
     return UV__ERR(errno);
 
+  /* ...or into scratch space if the user's buffer is too small
+   * so we can report how much space to provide on the next try.
+   */
+  if (getcwd(scratch, sizeof(scratch)) == NULL)
+    return UV__ERR(errno);
+
+  buffer = scratch;
+
+fixup:
+
   *size = strlen(buffer);
+
   if (*size > 1 && buffer[*size - 1] == '/') {
-    buffer[*size-1] = '\0';
-    (*size)--;
+    *size -= 1;
+    buffer[*size] = '\0';
+  }
+
+  if (buffer == scratch) {
+    *size += 1;
+    return UV_ENOBUFS;
   }
 
   return 0;

src/unix/fs.c

@@ -471,22 +471,13 @@ static int uv__fs_closedir(uv_fs_t* req) {
   return 0;
 }
 
-#if defined(_POSIX_PATH_MAX)
-# define UV__FS_PATH_MAX _POSIX_PATH_MAX
-#elif defined(PATH_MAX)
-# define UV__FS_PATH_MAX PATH_MAX
-#else
-# define UV__FS_PATH_MAX_FALLBACK 8192
-# define UV__FS_PATH_MAX UV__FS_PATH_MAX_FALLBACK
-#endif
-
 static ssize_t uv__fs_pathmax_size(const char* path) {
   ssize_t pathmax;
 
   pathmax = pathconf(path, _PC_PATH_MAX);
 
   if (pathmax == -1)
-    pathmax = UV__FS_PATH_MAX;
+    pathmax = UV__PATH_MAX;
 
   return pathmax;
 }
@@ -497,7 +488,9 @@ static ssize_t uv__fs_readlink(uv_fs_t* req) {
   char* buf;
   char* newbuf;
 
-#if defined(UV__FS_PATH_MAX_FALLBACK)
+#if defined(_POSIX_PATH_MAX) || defined(PATH_MAX)
+  maxlen = uv__fs_pathmax_size(req->path);
+#else
   /* We may not have a real PATH_MAX.  Read size of link.  */
   struct stat st;
   int ret;
@@ -515,8 +508,6 @@ static ssize_t uv__fs_readlink(uv_fs_t* req) {
      for some symlinks, such as those in /proc or /sys.  */
   if (maxlen == 0)
     maxlen = uv__fs_pathmax_size(req->path);
-#else
-  maxlen = uv__fs_pathmax_size(req->path);
 #endif
 
   buf = uv__malloc(maxlen);

src/unix/internal.h

@@ -25,6 +25,7 @@
 #include "uv-common.h"
 
 #include <assert.h>
+#include <limits.h> /* _POSIX_PATH_MAX, PATH_MAX */
 #include <stdlib.h> /* abort */
 #include <string.h> /* strrchr */
 #include <fcntl.h>  /* O_CLOEXEC, may be */
@@ -60,6 +61,14 @@
 # include <AvailabilityMacros.h>
 #endif
 
+#if defined(_POSIX_PATH_MAX)
+# define UV__PATH_MAX _POSIX_PATH_MAX
+#elif defined(PATH_MAX)
+# define UV__PATH_MAX PATH_MAX
+#else
+# define UV__PATH_MAX 8192
+#endif
+
 #if defined(__ANDROID__)
 int uv__pthread_sigmask(int how, const sigset_t* set, sigset_t* oset);
 # ifdef pthread_sigmask

test/test-cwd-and-chdir.c

@@ -33,9 +33,16 @@ TEST_IMPL(cwd_and_chdir) {
   size_t size2;
   int err;
 
+  size1 = 1;
+  err = uv_cwd(buffer_orig, &size1);
+  ASSERT(err == UV_ENOBUFS);
+  ASSERT(size1 > 1);
+
   size1 = sizeof buffer_orig;
   err = uv_cwd(buffer_orig, &size1);
   ASSERT(err == 0);
+  ASSERT(size1 > 0);
+  ASSERT(buffer_orig[size1] != '/');
 
   err = uv_chdir(buffer_orig);
   ASSERT(err == 0);