generic-builder: optimize - reduce execve calls when sourcing

This improves the implementation of `dumpVars` by removing a call to `install`.

This improves performance when sourcing setup.sh by more than 10%
It should also improve the performance when transitioning between build phases significantly as no process executions are issued anymore,

A test is added, which ensures that no extra execve calls are issued while sourcing setup.sh.

pkgs/stdenv/generic/setup.sh

@@ -1181,13 +1181,18 @@ substituteAllInPlace() {
 # the environment used for building.
 dumpVars() {
     if [ "${noDumpEnvVars:-0}" != 1 ]; then
-        # On darwin, install(1) cannot be called with /dev/stdin or fd from process substitution
-        # so first we create the file and then write to it
-        # See https://github.com/NixOS/nixpkgs/issues/335016
-        {
-            install -m 0600 /dev/null "$NIX_BUILD_TOP/env-vars" &&
-            export 2>/dev/null >| "$NIX_BUILD_TOP/env-vars"
-        } || true
+        # Don't use `install` here to prevent executing a process each time.
+
+        # Set umask to create env-vars file with 0600 permissions (owner read/write only)
+        local old_umask
+        old_umask=$(umask)
+        umask 0077
+
+        # Dump all environment variables to the env-vars file
+        export 2>/dev/null > "$NIX_BUILD_TOP/env-vars"
+
+        # Restore original umask
+        umask "$old_umask"
     fi
 }
 

pkgs/test/stdenv/default.nix

@@ -404,6 +404,50 @@ in
     stdenv' = bootStdenv;
   };
 
+  ensure-no-execve-in-setup-sh =
+    derivation {
+      name = "ensure-no-execve-in-setup-sh";
+      system = stdenv.system;
+      builder = "${stdenv.bootstrapTools}/bin/bash";
+      PATH = "${pkgs.strace}/bin:${stdenv.bootstrapTools}/bin";
+      initialPath = [
+        stdenv.bootstrapTools
+        pkgs.strace
+      ];
+      args = [
+        "-c"
+        ''
+          countCall() {
+            echo "$stats" | tr -s ' ' | grep "$1" | cut -d ' ' -f5
+          }
+
+          # prevent setup.sh from running `nproc` when cores=0
+          # (this would mess up the syscall stats)
+          export NIX_BUILD_CORES=1
+
+          echo "Analyzing setup.sh with strace"
+          stats=$(strace -fc bash -c ". ${../../stdenv/generic/setup.sh}" 2>&1)
+          echo "$stats" | head -n15
+
+          # fail if execve calls is > 1
+          stats=$(strace -fc bash -c ". ${../../stdenv/generic/setup.sh}" 2>&1)
+          execveCalls=$(countCall execve)
+          if [ "$execveCalls" -gt 1 ]; then
+            echo "execve calls: $execveCalls; expected: 1"
+            echo "ERROR: setup.sh should not launch additional processes when being sourced"
+            exit 1
+          else
+            echo "setup.sh doesn't launch extra processes when sourcing, as expected"
+          fi
+
+          touch $out
+        ''
+      ];
+    }
+    // {
+      meta = { };
+    };
+
   structuredAttrsByDefault = lib.recurseIntoAttrs {
 
     hooks = lib.recurseIntoAttrs (
@@ -570,6 +614,5 @@ in
           diff $out/json $goldenJson
         '';
       };
-
   };
 }