tests: Suppress environment-dependent warnings

... via _NIX_TEST_NO_ENVIRONMENT_WARNINGS

This environment variable suppresses warnings that depend on the test
environment (such as ulimit warnings in builds on systems with lower
limits, which may well succeed if it weren't for the warning).

This prevents non-deterministic test failures in golden/characterization
tests.

Alternative considered: filtering stderr in test scripts, but that approach
is fragile with binary test output, and potentially multiple call sites.

src/libutil/current-process.cc

@@ -7,6 +7,7 @@
 #include "nix/util/file-system.hh"
 #include "nix/util/processes.hh"
 #include "nix/util/signals.hh"
+#include "nix/util/environment-variables.hh"
 #include <math.h>
 
 #ifdef __APPLE__
@@ -66,14 +67,16 @@ void setStackSize(size_t stackSize)
     if (getrlimit(RLIMIT_STACK, &limit) == 0 && static_cast<size_t>(limit.rlim_cur) < stackSize) {
         savedStackSize = limit.rlim_cur;
         if (limit.rlim_max < static_cast<rlim_t>(stackSize)) {
-            logger->log(
-                lvlWarn,
-                HintFmt(
-                    "Stack size hard limit is %1%, which is less than the desired %2%. If possible, increase the hard limit, e.g. with 'ulimit -Hs %3%'.",
-                    limit.rlim_max,
-                    stackSize,
-                    stackSize / 1024)
-                    .str());
+            if (getEnv("_NIX_TEST_NO_ENVIRONMENT_WARNINGS") != "1") {
+                logger->log(
+                    lvlWarn,
+                    HintFmt(
+                        "Stack size hard limit is %1%, which is less than the desired %2%. If possible, increase the hard limit, e.g. with 'ulimit -Hs %3%'.",
+                        limit.rlim_max,
+                        stackSize,
+                        stackSize / 1024)
+                        .str());
+            }
         }
         auto requestedSize = std::min(static_cast<rlim_t>(stackSize), limit.rlim_max);
         limit.rlim_cur = requestedSize;

tests/functional/common/vars.sh

@@ -49,6 +49,9 @@ if ! isTestOnNixOS; then
   fi
   export _NIX_IN_TEST=$TEST_ROOT/shared
   export _NIX_TEST_NO_LSOF=1
+  # Suppress warnings that depend on the test environment (e.g., ulimit warnings)
+  # to avoid non-deterministic test failures in golden tests
+  export _NIX_TEST_NO_ENVIRONMENT_WARNINGS=1
   export NIX_REMOTE=${NIX_REMOTE_-}
 
 fi # ! isTestOnNixOS