feat: allow redirection of write requests

This can be useful e.g. for Xcache proxy to allow it to redirect
incoming write requests to the origin.

src/XrdXrootd/XrdXrootdConfig.cc

@@ -1568,7 +1568,8 @@ int XrdXrootdProtocol::xred(XrdOucStream &Config)
         {"rm",       RD_rm},
         {"rmdir",    RD_rmdir},
         {"stat",     RD_stat},
-        {"trunc",    RD_trunc}
+        {"trunc",    RD_trunc},
+        {"write",    RD_open2}
        };
     static const int rHLen = 264;
     char rHost[2][rHLen], *hP[2], *val;

src/XrdXrootd/XrdXrootdXeq.cc

@@ -1392,6 +1392,7 @@ int XrdXrootdProtocol::do_Open()
    mode = (int)ntohs(Request.open.mode);
    opts = (int)ntohs(Request.open.options);
 
+
 // Map the mode and options
 //
    mode = mapMode(mode) | S_IRUSR | S_IWUSR; usage = 'r';
@@ -1458,15 +1459,28 @@ int XrdXrootdProtocol::do_Open()
 //
    doDig = (digFS && SFS_LCLPATH(fn));
 
-// Validate the path and then check if static redirection applies
+// Validate the path/req type and then check if static redirection applies
 //
-   if (doDig) {popt = XROOTDXP_NOLK; opC = 0;}
-      else {int ropt;
-            if (!(popt = Squash(fn))) return vpEmsg("Opening", fn);
-            if (Route[RD_open1].Host[rdType] && (ropt = RPList.Validate(fn)))
-               return Response.Send(kXR_redirect, Route[ropt].Port[rdType],
-                                                  Route[ropt].Host[rdType]);
-           }
+   if (doDig)
+     {
+     popt = XROOTDXP_NOLK; opC = 0;
+     }
+   else
+     {
+       int ropt = -1;
+       if (!(popt = Squash(fn))) return vpEmsg("Opening", fn);
+
+       if (Route[RD_open1].Host[rdType])
+          ropt = RPList.Validate(fn);
+       else
+         if (Route[RD_open2].Host[rdType] && ('w' == usage || strchr(op, 'd')))
+           ropt = RD_open2; 
+       if (ropt >= 0)
+         return Response.Send(
+             kXR_redirect, Route[ropt].Port[rdType],
+             Route[ropt].Host[rdType]
+           );
+     }
 
 // Add the multi-write option if this path supports it
 //