feat(tvix/glue): introduce test suite for context strings r/7404

This is an additional test suite on the top of the Nix ones
for context strings matters.

It already smoked out multiple mistakes and potential bugs and non-deterministic result from the evaluator.

It uses a similar technology as the one in the tvix-eval albeit we instantiate a fully fledged evaluator
with in-memory store.

We copy the files instead of symlinking them because crates are built in
isolation, so symlinks cannot work.

Change-Id: I63ae225ce4f83c6e2c8ccd60d779c2f8eb9d08fb
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10619
Autosubmit: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

1 files changed, 72 insertions, 0 deletions

diff --git a/tvix/glue/src/tests/tvix_tests/eval-okay-context-introspection.nix b/tvix/glue/src/tests/tvix_tests/eval-okay-context-introspection.nix
new file mode 100644
index 000000000000..ed78112ab7eb
--- /dev/null
+++ b/tvix/glue/src/tests/tvix_tests/eval-okay-context-introspection.nix
@@ -0,0 +1,72 @@
+# Contrary to the Nix tests, this one does not make any use of `builtins.appendContext`
+# It's a weaker yet interesting test by abusing knowledge on how does our builtins
+# performs propagation.
+let
+  drv = derivation {
+    name = "fail";
+    builder = "/bin/false";
+    system = "x86_64-linux";
+    outputs = [ "out" "foo" ];
+  };
+
+  # `substr` propagates context, we truncate to an empty string and concatenate to the target
+  # to infect it with the context of `copied`.
+  appendContextFrom = copied: target: (builtins.substring 0 0 "${copied}") + "${target}";
+
+  # `split` discards (!!) contexts, we first split by `/` (there's at least one such `/` by
+  # virtue of `target` being a store path, i.e. starting with `$store_root/$derivation_name`)
+  # then, we reassemble the list into a proper string.
+  discardContext = target: builtins.concatStringsSep "" (builtins.split "(.*)" "${target}");
+
+  hasContextInAttrKeys = attrs: builtins.all builtins.hasContext (builtins.attrNames attrs);
+
+  path = "${./eval-okay-context-introspection.nix}";
+
+  # This is a context-less attribute set, which should be exactly the same
+  # as `builtins.getContext combo-path`.
+  desired-context = {
+    "${builtins.unsafeDiscardStringContext path}" = {
+      path = true;
+    };
+    "${builtins.unsafeDiscardStringContext drv.drvPath}" = {
+      outputs = [ "foo" "out" ];
+      allOutputs = true;
+    };
+  };
+
+  combo-path = "${path}${drv.outPath}${drv.foo.outPath}${drv.drvPath}";
+  legit-context = builtins.getContext combo-path;
+
+  reconstructed-path = appendContextFrom combo-path
+    (builtins.unsafeDiscardStringContext combo-path);
+
+  # Eta rule for strings with context.
+  etaRule = str:
+    str == appendContextFrom
+      str
+      (builtins.unsafeDiscardStringContext str);
+
+  etaRule' = str:
+    str == appendContextFrom
+      str
+      (discardContext str);
+
+in
+[
+  (!hasContextInAttrKeys desired-context)
+  (legit-context."${builtins.unsafeDiscardStringContext path}".path)
+  (legit-context."${builtins.unsafeDiscardStringContext drv.drvPath}".outputs == [ "foo" "out" ])
+  # `allOutputs` is present only on DrvClosure-style context string, i.e. the
+  # context string of a drvPath itself, not an outPath.
+  (!builtins.hasAttr "allOutputs" (builtins.getContext drv.outPath)."${builtins.unsafeDiscardStringContext drv.drvPath}")
+  (builtins.hasAttr "allOutputs" legit-context."${builtins.unsafeDiscardStringContext drv.drvPath}")
+  (builtins.hasAttr "allOutputs" (builtins.getContext drv.drvPath)."${builtins.unsafeDiscardStringContext drv.drvPath}")
+  (legit-context == desired-context) # FIXME(raitobezarius): this should not use `builtins.seq`, this is a consequence of excessive laziness of Tvix, I believe.
+  (reconstructed-path == combo-path)
+  # Those are too slow?
+  # (etaRule' "foo")
+  # (etaRule' combo-path)
+  (etaRule "foo")
+  (etaRule drv.drvPath)
+  (etaRule drv.foo.outPath)
+]