Fix issue with Link.target and Link.targetSync failing on some file systems

runtime/bin/file_linux.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "platform/globals.h"
 #if defined(TARGET_OS_LINUX)
 
 #include "bin/file.h"
 
 #include <errno.h>  // NOLINT
@@ skipping 350 matching lines @@
 }
 
 
 char* File::LinkTarget(const char* pathname) {
   struct stat64 link_stats;
   if (TEMP_FAILURE_RETRY(lstat64(pathname, &link_stats)) != 0) return NULL;
   if (!S_ISLNK(link_stats.st_mode)) {
     errno = ENOENT;
     return NULL;
   }
-  size_t target_size = link_stats.st_size;
-  char* target_name = reinterpret_cast<char*>(malloc(target_size + 1));
-  size_t read_size = NO_RETRY_EXPECTED(
-      readlink(pathname, target_name, target_size + 1));
-  if (read_size != target_size) {
-    free(target_name);
+  // Don't rely on the link_stats.st_size for the size of the link
+  // target. For some filesystems, e.g. procfs, this value is always
+  // 0. Also the link might have changed before the readlink call.
+  const int kBufferSize = PATH_MAX + 1;
+  char target[kBufferSize];
+  size_t target_size = TEMP_FAILURE_RETRY(
+      readlink(pathname, target, kBufferSize));
+  if (target_size <= 0) {
     return NULL;
   }
+  char* target_name = reinterpret_cast<char*>(malloc(target_size + 1));
+  if (target_name == NULL) {
+    return NULL;
+  }
+  memmove(target_name, target, target_size);
   target_name[target_size] = '\0';
   return target_name;
 }
 
 
 bool File::IsAbsolutePath(const char* pathname) {
   return (pathname != NULL && pathname[0] == '/');
 }
 
 
@@ skipping 63 matching lines @@
   return (file_1_info.st_ino == file_2_info.st_ino &&
           file_1_info.st_dev == file_2_info.st_dev) ?
       File::kIdentical :
       File::kDifferent;
 }
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_LINUX)