FST: Return EAGAIN instead of EBUSY in case a file is opened for writing via HTTP

This is a hack against the hack in XRootD in the Emsg() function.
We initially wanted to use EBUSY. However, if EBUSY errno is passed in Emsg(), the Emsg() function in XRootD
will return the integer 5 (Hack for proxy support).
This is going to explose when this hack is removed in XRootD 6 hence the
decision to use EAGAIN

Author: ccaffy

common/http/HttpServer.cc

@@ -373,7 +373,7 @@ HttpServer::HttpError(const char* errorText, int errorCode)
     response->SetResponseCode(response->UNPROCESSABLE_ENTITY);
   } else if (errorCode == EPERM || errorCode == EACCES) {
     response->SetResponseCode(response->FORBIDDEN);
-  } else if (errorCode == EBUSY) {
+  } else if (errorCode == EAGAIN) {
     response->SetResponseCode(response->CONFLICT);
   } else {
     response->SetResponseCode(response->INTERNAL_SERVER_ERROR);

fst/XrdFstOfsFile.cc

@@ -278,7 +278,9 @@ XrdFstOfsFile::open(const char* path, XrdSfsFileOpenMode open_mode,
     if (gOFS.openedForWriting.isOpen(mFsId, mFileId)) {
       eos_err("msg=\"forbid replica open for synchronization, file %s "
               "opened in RW mode\"", mNsPath.c_str());
-      return gOFS.Emsg(epname, error, EBUSY, "open - cannot synchronize "
+      // Return resource temporarily unavailable as EBUSY cannot be used due to XRootD hack
+      // in Emsg() that returns the integer "5" (proxy hack)
+      return gOFS.Emsg(epname, error, EAGAIN, "open - cannot synchronize "
                        "file opened in RW mode", mNsPath.c_str());
     }
   }