Fix unspecified behaviour in readStorePathCAMap

When deploying a Hydra instance with current Nix master, most builds
would not run because of errors like this:

  queue monitor: error: --- Error --- hydra-queue-runner
  error: --- UsageError --- nix-daemon
  not a content address because it is not in the form '<prefix>:<rest>': /nix/store/...-somedrv

The last error message is from parseContentAddress, which expects a
colon-separated string, however what we got here is a store path.

Looking at the worker protocol, the following message sent to the Nix
daemon caused the error above:

  0x1E -> wopQuerySubstitutablePathInfos
  0x01 -> Number of paths
  0x16 -> Length of string
  "/nix/store/...-somedrv"
  0x00 -> Length of string
  ""

Looking at writeStorePathCAMap, the store path is indeed the first field
that's transmitted. However, readStorePathCAMap expects it to be the
*second* field *on my machine*, since expression evaluation order is a
classic form of unspecified behaviour[1] in C++.

This has been introduced in https://github.com/NixOS/nix/pull/3689,
specifically in commit 66a62b3189c8c9b0965850e6b3c9b0fda0b50fd8.

[1]: https://en.wikipedia.org/wiki/Unspecified_behavior#Order_of_evaluation_of_subexpressions

Signed-off-by: aszlig <aszlig@nix.build>

1 files changed, 5 insertions, 2 deletions

diff --git a/src/libstore/remote-store.cc b/src/libstore/remote-store.cc
index e4a4ef5af..8bcf92301 100644
--- a/src/libstore/remote-store.cc
+++ b/src/libstore/remote-store.cc
@@ -43,8 +43,11 @@ StorePathCAMap readStorePathCAMap(const Store & store, Source & from)
 {
     StorePathCAMap paths;
     auto count = readNum<size_t>(from);
-    while (count--)
-        paths.insert_or_assign(store.parseStorePath(readString(from)), parseContentAddressOpt(readString(from)));
+    while (count--) {
+        auto path = store.parseStorePath(readString(from));
+        auto ca = parseContentAddressOpt(readString(from));
+        paths.insert_or_assign(path, ca);
+    }
     return paths;
 }