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

runtime/bin/file_linux.cc

Index: runtime/bin/file_linux.cc
diff --git a/runtime/bin/file_linux.cc b/runtime/bin/file_linux.cc
index 9fc4bd15dcb442d07d826dbd1f789671397c23e5..75af828cbffeb1cfd92025d88ba97558a9f7e00d 100644
--- a/runtime/bin/file_linux.cc
+++ b/runtime/bin/file_linux.cc
@@ -368,14 +368,21 @@ char* File::LinkTarget(const char* pathname) {
     errno = ENOENT;
     return NULL;
   }
-  size_t target_size = link_stats.st_size;

[kustermann, 2015/01/22 09:00:19]

One alternative would be to check if "target_size > 0" and if so, trust it,
otherwise fallback to a 1024 buffer.

[Søren Gjesse, 2015/01/22 10:25:52]

I thought about that, but then (unlikely) the link could have changed between
this and the readlink call, so I decided to stay safe.

+  // 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 = 1024;

[kustermann, 2015/01/22 09:00:19]

You could consider using PATH_MAX here.

[Søren Gjesse, 2015/01/22 10:25:52]

Done.

+  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));
-  size_t read_size = NO_RETRY_EXPECTED(
-      readlink(pathname, target_name, target_size + 1));
-  if (read_size != target_size) {
-    free(target_name);
+  if (target_name == NULL) {
     return NULL;
   }
+  memmove(target_name, target, target_size);
   target_name[target_size] = '\0';
   return target_name;
 }