Fix nix log with CA derivations

Fix #6209

When trying to run `nix log <installable>`, try first to resolve the derivation pointed to
by `<installable>` as it is the resolved one that holds the build log.

This has a couple of shortcomings:
1. It’s expensive as it requires re-reading the derivation
2. It’s brittle because if the derivation doesn’t exist anymore or can’t
   be resolved (which is the case if any one of its build inputs is missing),
   then we can’t access the log anymore

However, I don’t think we can do better (at least not right now).
The alternatives I see are:
1. Copy the build log for the un-resolved derivation. But that means a
   lot of duplication
2. Store the results of the resolving in the db. Which might be the best
   long-term solution, but leads to a whole new class of potential
   issues.

src/libstore/binary-cache-store.cc

@@ -504,18 +504,10 @@ void BinaryCacheStore::addSignatures(const StorePath & storePath, const StringSe
 
 std::optional<std::string> BinaryCacheStore::getBuildLog(const StorePath & path)
 {
-    auto drvPath = path;
-
-    if (!path.isDerivation()) {
-        try {
-            auto info = queryPathInfo(path);
-            // FIXME: add a "Log" field to .narinfo
-            if (!info->deriver) return std::nullopt;
-            drvPath = *info->deriver;
-        } catch (InvalidPath &) {
-            return std::nullopt;
-        }
-    }
+    auto maybePath = getBuildDerivationPath(path);
+    if (!maybePath)
+        return std::nullopt;
+    auto drvPath = maybePath.value();
 
     auto logPath = "log/" + std::string(baseNameOf(printStorePath(drvPath)));
 

src/libstore/local-fs-store.cc

@@ -89,17 +89,10 @@ const std::string LocalFSStore::drvsLogDir = "drvs";
 
 std::optional<std::string> LocalFSStore::getBuildLog(const StorePath & path_)
 {
-    auto path = path_;
-
-    if (!path.isDerivation()) {
-        try {
-            auto info = queryPathInfo(path);
-            if (!info->deriver) return std::nullopt;
-            path = *info->deriver;
-        } catch (InvalidPath &) {
-            return std::nullopt;
-        }
-    }
+    auto maybePath = getBuildDerivationPath(path_);
+    if (!maybePath)
+        return std::nullopt;
+    auto path = maybePath.value();
 
     auto baseName = path.to_string();
 

src/libstore/store-api.cc

@@ -1300,6 +1300,34 @@ Derivation readDerivationCommon(Store& store, const StorePath& drvPath, bool req
     }
 }
 
+std::optional<StorePath> Store::getBuildDerivationPath(const StorePath & path)
+{
+
+    if (!path.isDerivation()) {
+        try {
+            auto info = queryPathInfo(path);
+            if (!info->deriver) return std::nullopt;
+            return *info->deriver;
+        } catch (InvalidPath &) {
+            return std::nullopt;
+        }
+    }
+
+    if (!settings.isExperimentalFeatureEnabled(Xp::CaDerivations) || !isValidPath(path))
+        return path;
+
+    auto drv = readDerivation(path);
+    if (!derivationHasKnownOutputPaths(drv.type())) {
+        // The build log is actually attached to the corresponding
+        // resolved derivation, so we need to get it first
+        auto resolvedDrv = drv.tryResolve(*this);
+        if (resolvedDrv)
+            return writeDerivation(*this, *resolvedDrv, NoRepair, true);
+    }
+
+    return path;
+}
+
 Derivation Store::readDerivation(const StorePath & drvPath)
 { return readDerivationCommon(*this, drvPath, true); }
 

src/libstore/store-api.hh

@@ -618,6 +618,13 @@ public:
      */
     StorePathSet exportReferences(const StorePathSet & storePaths, const StorePathSet & inputPaths);
 
+    /**
+     * Given a store path, return the realisation actually used in the realisation of this path:
+     * - If the path is a content-addressed derivation, try to resolve it
+     * - Otherwise, find one of its derivers
+     */
+    std::optional<StorePath> getBuildDerivationPath(const StorePath &);
+
     /* Hack to allow long-running processes like hydra-queue-runner to
        occasionally flush their path info cache. */
     void clearPathInfoCache()

tests/build-hook-ca-floating.nix

@@ -1,53 +1,6 @@
 { busybox }:
 
-with import ./config.nix;
-
-let
-
-  mkDerivation = args:
-    derivation ({
-      inherit system;
-      builder = busybox;
-      args = ["sh" "-e" args.builder or (builtins.toFile "builder-${args.name}.sh" "if [ -e .attrs.sh ]; then source .attrs.sh; fi; eval \"$buildCommand\"")];
-      outputHashMode = "recursive";
-      outputHashAlgo = "sha256";
-      __contentAddressed = true;
-    } // removeAttrs args ["builder" "meta"])
-    // { meta = args.meta or {}; };
-
-  input1 = mkDerivation {
-    shell = busybox;
-    name = "build-remote-input-1";
-    buildCommand = "echo FOO > $out";
-    requiredSystemFeatures = ["foo"];
-  };
-
-  input2 = mkDerivation {
-    shell = busybox;
-    name = "build-remote-input-2";
-    buildCommand = "echo BAR > $out";
-    requiredSystemFeatures = ["bar"];
-  };
-
-  input3 = mkDerivation {
-    shell = busybox;
-    name = "build-remote-input-3";
-    buildCommand = ''
-      read x < ${input2}
-      echo $x BAZ > $out
-    '';
-    requiredSystemFeatures = ["baz"];
-  };
-
-in
-
-  mkDerivation {
-    shell = busybox;
-    name = "build-remote";
-    buildCommand =
-      ''
-        read x < ${input1}
-        read y < ${input3}
-        echo "$x $y" > $out
-      '';
-  }
+import ./build-hook.nix {
+  inherit busybox;
+  contentAddressed = true;
+}

tests/build-hook.nix

@@ -1,15 +1,22 @@
-{ busybox }:
+{ busybox, contentAddressed ? false }:
 
 with import ./config.nix;
 
 let
 
+  caArgs = if contentAddressed then {
+      outputHashMode = "recursive";
+      outputHashAlgo = "sha256";
+      __contentAddressed = true;
+    } else {};
+
   mkDerivation = args:
     derivation ({
       inherit system;
       builder = busybox;
       args = ["sh" "-e" args.builder or (builtins.toFile "builder-${args.name}.sh" "if [ -e .attrs.sh ]; then source .attrs.sh; fi; eval \"$buildCommand\"")];
-    } // removeAttrs args ["builder" "meta" "passthru"])
+    } // removeAttrs args ["builder" "meta" "passthru"]
+    // caArgs)
     // { meta = args.meta or {}; passthru = args.passthru or {}; };
 
   input1 = mkDerivation {

tests/build-remote.sh

@@ -63,12 +63,9 @@ nix path-info --store $TEST_ROOT/machine3 --all \
   | grep builder-build-remote-input-3.sh
 
 
-# Temporarily disabled because of https://github.com/NixOS/nix/issues/6209
-if [[ -z "$CONTENT_ADDRESSED" ]]; then
-  for i in input1 input3; do
-    nix log --store $TEST_ROOT/machine0 --file "$file" --arg busybox $busybox passthru."$i" | grep hi-$i
-  done
-fi
+for i in input1 input3; do
+nix log --store $TEST_ROOT/machine0 --file "$file" --arg busybox $busybox passthru."$i" | grep hi-$i
+done
 
 # Behavior of keep-failed
 out="$(nix-build 2>&1 failing.nix \