add link_cow feature

When enabled if a regular file is opened which has a link count > 1 it will
copy the file to a temporary file and rename over the original. Effectively
breaking the link. This behavior is similar to cow-shell and other LD_PRELOAD
based "CoW" solutions.

src/fs_mktemp.cpp

@@ -0,0 +1,71 @@
+/*
+  ISC License
+
+  Copyright (c) 2018, Antonio SJ Musumeci <trapexit@spawn.link>
+
+  Permission to use, copy, modify, and/or distribute this software for any
+  purpose with or without fee is hereby granted, provided that the above
+  copyright notice and this permission notice appear in all copies.
+
+  THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+  WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+  MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+  ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+  WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+  ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+  OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+
+#include "fs_base_open.hpp"
+
+#include <fcntl.h>
+#include <string.h>
+
+#include <cstdlib>
+
+using std::string;
+
+static
+string
+generate_tmp_path(const string &base_)
+{
+  string tmp;
+
+  tmp = base_;
+  tmp += '_';
+  for(int i = 0; i < 6; i++)
+    tmp += ('A' + (std::rand() % 26));
+
+  return tmp;
+}
+
+namespace fs
+{
+  int
+  mktemp(string    &base_,
+         const int  flags_)
+  {
+    int fd;
+    int count;
+    int flags;
+    string tmppath;
+
+    fd    = -1;
+    count = 10;
+    flags = (flags_ | O_EXCL | O_CREAT);
+    while(count-- > 0)
+      {
+        tmppath = generate_tmp_path(base_);
+
+        fd = fs::open(tmppath,flags,S_IWUSR);
+        if((fd == -1) && (errno == EEXIST))
+          continue;
+        else if(fd != -1)
+          base_ = tmppath;
+
+        return fd;
+      }
+
+    return (errno=EEXIST,-1);
+  }
+}