fuse_readdir: atomic initialized flag; bound error string; check msgbuf allocation

- ReadDir::_initialized is set by the first thread through the
  lazy-init path and read by every subsequent caller; make it
  std::atomic<bool> so the read/write pair has a happens-before
  relationship.

- The 'no valid mergerfs branch found' error directory entry used
  strcpy into a fixed-size dirent buffer. The message happens to
  fit today but the buffer is only NAME_MAX wide; switch to a
  length-bounded strncpy driven by the constexpr string size.

- The getdents implementations for seq/cor/cosr allocated a msgbuf
  without checking for allocation failure and then dereferenced it.
  Return -ENOMEM when the allocation fails.

src/fuse_readdir.cpp

@@ -90,15 +90,18 @@ _handle_ENOENT(const fuse_file_info_t *ffi_,
   if(!di->fusepath.empty())
     return -ENOENT;
 
+  constexpr const char msg[] =
+    "error: no valid mergerfs branch found, check config";
+
   de = {};
 
   de.d_ino = 0;
   de.d_off = 0;
   de.d_type = DT_UNKNOWN;
-  strcpy(de.d_name,"error: no valid mergerfs branch found, check config");
+  strncpy(de.d_name,msg,(sizeof(msg) - 1));
   de.d_reclen = sizeof(de);
 
-  fuse_dirents_add(buf_,&de,::strlen(de.d_name));
+  fuse_dirents_add(buf_,&de,(sizeof(msg) - 1));
 
   return 0;
 }

src/fuse_readdir.hpp

@@ -22,6 +22,7 @@
 #include "tofrom_string.hpp"
 #include "fuse_readdir_base.hpp"
 
+#include <atomic>
 #include <memory>
 #include <mutex>
 
@@ -58,7 +59,7 @@ namespace FUSE
     void initialize();
 
   private:
-    bool _initialized = false;
+    std::atomic<bool> _initialized = false;
     std::shared_ptr<std::string>       _str;
     std::shared_ptr<FUSE::ReadDirBase> _impl;
   };

src/fuse_readdir_cor_getdents.icpp

@@ -53,6 +53,9 @@ _readdir(const fs::path &branch_path_,
   DEFER{ fs::close(fd); };
 
   buf = msgbuf_alloc();
+  if(not buf)
+    return -ENOMEM;
+
   DEFER{ msgbuf_free(buf); };
 
   fs::inode::ReaddirCalc inodecalc(branch_path_,rel_dirpath_);

src/fuse_readdir_cosr_getdents.icpp

@@ -87,6 +87,8 @@ _readdir(std::vector<std::future<DirRV>> &dh_futures_,
   fuse_msgbuf_t *buf;
 
   buf = msgbuf_alloc();
+  if(not buf)
+    return -ENOMEM;
   DEFER { msgbuf_free(buf); };
 
   for(auto &dh_future : dh_futures_)

src/fuse_readdir_seq_getdents.icpp

@@ -45,6 +45,8 @@ _readdir(const Branches::Ptr &branches_,
   fuse_dirents_reset(dirents_);
 
   buf = msgbuf_alloc();
+  if(not buf)
+    return -ENOMEM;
   DEFER { msgbuf_free(buf); };
 
   for(const auto &branch : *branches_)