Fix misread of source if path is already valid

When receiving a stream of NARs through the ssh-ng protocol, an already
existing path would cause the NAR archive to not be read in the stream,
resulting in trying to parse the NAR as a ValidPathInfo. This results in
the error message:
    error: not an absolute path: 'nix-archive-1'

Fixes NixOS#6253

Usually this problem is avoided by running QueryValidPaths before
AddMultipleToStore, but can arise when two parallel nix processes gets
the same response from QueryValidPaths. This makes the problem more
prominent when running builds in parallel.

doc/manual/src/release-notes/rl-next.md

@@ -20,3 +20,10 @@
   to the embedded store and not to the host's Nix store.
 
   This requires the `discard-references` experimental feature.
+
+* Fix misread of source if path is already valid.
+  When receiving a stream of NARs through the ssh-ng protocol, an already
+  existing path would cause the NAR archive to not be read in the stream,
+  resulting in trying to parse the NAR as a ValidPathInfo. This results in
+  the error message:
+      error: not an absolute path: 'nix-archive-1'

src/libstore/local-store.cc

@@ -697,6 +697,17 @@ void canonicalisePathMetaData(const Path & path,
 }
 
 
+void discardNar(Source& source, const ValidPathInfo& info) {
+    int count = info.narSize / 4096;
+    char buffer[4096];
+    for (int i=0; i < count; ++i) {
+        source(buffer, 4096);
+    }
+    int extra = info.narSize % 4096;
+    source(buffer, extra);
+}
+
+
 void LocalStore::checkDerivationOutputs(const StorePath & drvPath, const Derivation & drv)
 {
     assert(drvPath.isDerivation());
@@ -1343,9 +1354,15 @@ void LocalStore::addToStore(const ValidPathInfo & info, Source & source,
             optimisePath(realPath, repair); // FIXME: combine with hashPath()
 
             registerValidPath(info);
+        } else {
+            /* Discard the NAR from the source if it has been added already. */
+            discardNar(source, info);
         }
 
         outputLock.setDeletion(true);
+    } else {
+        /* Discard the NAR from the source if it has been added already. */
+        discardNar(source, info);
     }
 }