AtomicFile: Operate on the target of symlinks

Currently, if an AtomicFile is opened on a symlink, the AtomicFile would
end up overwriting the link. This change makes AtomicFiles operate on
the targets of the links and not the links themselves.

```
$ test/AtomicFileTest
1..22
ok 1 - AtomicFileTest: Shall not exists before finalize
ok 2 - AtomicFileTest: Shall exists after finalize
ok 3 - AtomicFileTest: Shall have the correct data
ok 4 - AtomicFileTest: Neither shall exist before finalize
ok 5 - AtomicFileTest: Both shall exists after finalize
ok 6 - AtomicFileTest: First file shall contain the correct data
ok 7 - AtomicFileTest: Second file shall contain the correct data
ok 8 - AtomicFileTest: Appending does not update original before finalize
ok 9 - AtomicFileTest: Finalizing updates the appended data
ok 10 - AtomicFileTest: Read from Atomicfile
ok 11 - AtomicFileTest: Read from Atomicfile should read unfinalized data
ok 12 - AtomicFileTest: Two AtomicFiles should access same temp file (part 1)
ok 13 - AtomicFileTest: Two AtomicFiles should access same temp file (part 2)
ok 14 - AtomicFileTest: Two AtomicFiles should access same temp file (part 3)
ok 15 - AtomicFileTest: File not removed before finalize
ok 16 - AtomicFileTest: File is removed after finalize
ok 17 - AtomicFileTest: writes to symlinks end up in target
ok 18 - AtomicFileTest: shall maintain symlink
ok 19 - AtomicFileTest: AtomicFile::write_raw throws on error # skip
ok 20 - AtomicFileTest: AtomicFile::finalize_all() throws on error # skip
ok 21 - AtomicFileTest: AtomicFile::reset clears failure state # skip
ok 22 - AtomicFileTest: AtomicFile::append throws on error # skip
ok 23 - AtomicFileTest: AtomicFile::append did not partially fill the file. # skip
ok 24 - AtomicFileTest: AtomicFile::append failures prevent finalization # skip
```

Fixes #546

Signed-off-by: Shaun Ruffell <sruffell@sruffell.net>

src/AtomicFile.cpp

@@ -91,9 +91,19 @@ AtomicFile::impl::impl (const Path& path)
   static int s_count = 0;
   std::stringstream str; 
 
-  str << path._data << '.' << s_pid << '-' << ++s_count << ".tmp";
+  std::string real_path;
+  if (path.is_link())
+  {
+    real_path = path.realpath();
+  }
+  else
+  {
+    real_path = path._data;
+  }
+
+  str << real_path << '.' << s_pid << '-' << ++s_count << ".tmp";
   temp_file = File (str.str());
-  real_file = File (path);
+  real_file = File (real_path);
 }
 
 ////////////////////////////////////////////////////////////////////////////////

test/AtomicFileTest.cpp

@@ -33,6 +33,7 @@
 #include <sstream>
 #include <string>
 #include <test.h>
+#include <unistd.h>
 
 #ifdef FIU_ENABLE
 
@@ -160,6 +161,37 @@ int fiu_test (UnitTest& t)
 }
 #endif // FIU_ENABLE
 
+//////////////////////////////////////////////////////////////////////////////
+int test_symlink (UnitTest& t)
+{
+  Path link ("link");
+  Path target ("target");
+  File(target).write_raw("empty\n");
+
+  const std::string write_contents = "test file contents\n";
+
+  if (::symlink (target._data.c_str(), link._data.c_str()) != 0)
+  {
+    throw std::runtime_error("Failed to create link in test_symlink");
+  }
+
+  {
+    AtomicFile file(link);
+    file.write_raw(write_contents);
+  }
+  AtomicFile::finalize_all ();
+
+
+  // After finalizing we should be able read the data that we wrote through the
+  // target
+  std::string read_contents;
+  AtomicFile::read (target, read_contents);
+  t.is(read_contents, write_contents, "AtomicFileTest: writes to symlinks end up in target");
+  t.is (link.is_link (), true, "AtomicFileTest: shall maintain symlink");
+
+  return 0;
+}
+
 //////////////////////////////////////////////////////////////////////////////
 int test (UnitTest& t)
 {
@@ -382,6 +414,10 @@ int test (UnitTest& t)
     AtomicFile::finalize_all ();
     t.is (test.exists (), false, "AtomicFileTest: File is removed after finalize");
   }
+
+  tempDir.clear();
+  test_symlink(t);
+
   return 0;
 }