[Backport release-25.05] ci/eval: fix local comparison with baseline (#441831)

.github/workflows/eval.yml

@@ -153,8 +153,8 @@ jobs:
           MATRIX_SYSTEM: ${{ matrix.system }}
         run: |
           nix-build nixpkgs/untrusted/ci --arg nixpkgs ./nixpkgs/untrusted-pinned -A eval.diff \
-            --arg beforeDir "$(readlink ./target)" \
-            --arg afterDir "$(readlink ./merged)" \
+            --arg beforeDir ./target \
+            --arg afterDir ./merged \
             --argstr evalSystem "$MATRIX_SYSTEM" \
             --out-link diff
 
@@ -207,7 +207,7 @@ jobs:
 
           # Use the target branch to get accurate maintainer info
           nix-build nixpkgs/trusted/ci --arg nixpkgs ./nixpkgs/trusted-pinned -A eval.compare \
-            --arg combinedDir "$(realpath ./combined)" \
+            --arg combinedDir ./combined \
             --arg touchedFilesJson ./touched-files.json \
             --argstr githubAuthorId "$AUTHOR_ID" \
             --out-link comparison

ci/eval/README.md

@@ -33,13 +33,13 @@ Note that 16GB memory is the recommended minimum, while with less than 8GB memor
 To compare two commits locally, first run the following on the baseline commit:
 
 ```
-BASELINE=$(nix-build ci -A eval.baseline --no-out-link)
+nix-build ci -A eval.baseline --out-link baseline
 ```
 
 Then, on the commit with your changes:
 
 ```
-nix-build ci -A eval.full --arg baseline $BASELINE
+nix-build ci -A eval.full --arg baseline ./baseline
 ```
 
 Keep in mind to otherwise pass the same set of arguments for both commands (`evalSystems`, `quickTest`, `chunkSize`).

ci/eval/compare/default.nix

@@ -13,6 +13,8 @@
   byName ? false,
 }:
 let
+  combined = builtins.storePath combinedDir;
+
   /*
     Derivation that computes which packages are affected (added, changed or removed) between two revisions of nixpkgs.
     Note: "platforms" are "x86_64-linux", "aarch64-darwin", ...
@@ -75,7 +77,7 @@ let
   # Attrs
   # - keys: "added", "changed", "removed" and "rebuilds"
   # - values: lists of `packagePlatformPath`s
-  diffAttrs = builtins.fromJSON (builtins.readFile "${combinedDir}/combined-diff.json");
+  diffAttrs = builtins.fromJSON (builtins.readFile "${combined}/combined-diff.json");
 
   changedPackagePlatformAttrs = convertToPackagePlatformAttrs diffAttrs.changed;
   rebuildsPackagePlatformAttrs = convertToPackagePlatformAttrs diffAttrs.rebuilds;
@@ -139,8 +141,8 @@ runCommand "compare"
     maintainers = builtins.toJSON maintainers;
     passAsFile = [ "maintainers" ];
     env = {
-      BEFORE_DIR = "${combinedDir}/before";
-      AFTER_DIR = "${combinedDir}/after";
+      BEFORE_DIR = "${combined}/before";
+      AFTER_DIR = "${combined}/after";
     };
   }
   ''

ci/eval/diff.nix

@@ -11,6 +11,9 @@
 }:
 
 let
+  before = builtins.storePath beforeDir;
+  after = builtins.storePath afterDir;
+
   /*
     Computes the key difference between two attrs
 
@@ -64,15 +67,15 @@ let
     in
     builtins.fromJSON data;
 
-  beforeAttrs = getAttrs beforeDir;
-  afterAttrs = getAttrs afterDir;
+  beforeAttrs = getAttrs before;
+  afterAttrs = getAttrs after;
   diffAttrs = diff beforeAttrs afterAttrs;
   diffJson = writeText "diff.json" (builtins.toJSON diffAttrs);
 in
 runCommand "diff" { } ''
   mkdir -p $out/${evalSystem}
 
-  cp -r ${beforeDir} $out/before
-  cp -r ${afterDir} $out/after
+  cp -r ${before} $out/before
+  cp -r ${after} $out/after
   cp ${diffJson} $out/${evalSystem}/diff.json
 ''