legacycmd: don't print to stdout from printGraphML

return a string instead, and write that to stdout explicitly.

Change-Id: Ia2d2019064da16fd35630217c445b4bcc4cdf2c2

lix/legacy/graphml.cc

@@ -5,9 +5,7 @@
 #include "lix/libutil/result.hh"
 
 #include <iostream>
-
-
-using std::cout;
+#include <sstream>
 
 namespace nix {
 
@@ -47,14 +45,14 @@ static std::string makeNode(const ValidPathInfo & info)
         (info.path.isDerivation() ? "derivation" : "output-path"));
 }
 
-
-kj::Promise<Result<void>> printGraphML(ref<Store> store, StorePathSet && roots)
+kj::Promise<Result<std::string>> printGraphML(ref<Store> store, StorePathSet && roots)
 try {
     StorePathSet workList(std::move(roots));
     StorePathSet doneSet;
     std::pair<StorePathSet::iterator, bool> ret;
+    std::stringstream result;
 
-    cout << "<?xml version='1.0' encoding='utf-8'?>\n"
+    result << "<?xml version='1.0' encoding='utf-8'?>\n"
            << "<graphml xmlns='http://graphml.graphdrawing.org/xmlns'\n"
            << "    xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance'\n"
            << "    xsi:schemaLocation='http://graphml.graphdrawing.org/xmlns/1.0/graphml.xsd'>\n"
@@ -70,20 +68,20 @@ try {
         if (ret.second == false) continue;
 
         auto info = TRY_AWAIT(store->queryPathInfo(path));
-        cout << makeNode(*info);
+        result << makeNode(*info);
 
         for (auto & p : info->references) {
             if (p != path) {
                 workList.insert(p);
-                cout << makeEdge(path.to_string(), p.to_string());
+                result << makeEdge(path.to_string(), p.to_string());
             }
         }
 
     }
 
-    cout << "</graph>\n";
-    cout << "</graphml>\n";
-    co_return result::success();
+    result << "</graph>\n";
+    result << "</graphml>\n";
+    co_return result.str();
 } catch (...) {
     co_return result::current_exception();
 }

lix/legacy/graphml.hh

@@ -5,6 +5,5 @@
 
 namespace nix {
 
-kj::Promise<Result<void>> printGraphML(ref<Store> store, StorePathSet && roots);
-
+kj::Promise<Result<std::string>> printGraphML(ref<Store> store, StorePathSet && roots);
 }

lix/legacy/nix-store.cc

@@ -503,7 +503,7 @@ opQuery(std::shared_ptr<Store> store, AsyncIoRoot & aio, Strings opFlags, String
                 {
                     roots.insert(j);
                 }
-            aio.blockOn(printGraphML(ref<Store>::unsafeFromPtr(store), std::move(roots)));
+            std::cout << aio.blockOn(printGraphML(ref<Store>::unsafeFromPtr(store), std::move(roots)));
             break;
         }