primops: Move functions to primops/system.cc

Moved builtins: exec, getEnv

Change-Id: Ie1b64e4ea8c71f5f64a80ab890a467f5194b884c

src/libexpr/primops/system.cc

@@ -0,0 +1,101 @@
+#include "eval-settings.hh"
+#include "primops.hh"
+
+namespace nix {
+
+/**
+ * builtins.exec
+ */
+
+void prim_exec(EvalState & state, const PosIdx pos, Value ** args, Value & v)
+{
+    state.forceList(*args[0], pos, "while evaluating the first argument passed to builtins.exec");
+    auto elems = args[0]->listElems();
+    auto count = args[0]->listSize();
+    if (count == 0) {
+        state.error<EvalError>("at least one argument to 'exec' required").atPos(pos).debugThrow();
+    }
+    NixStringContext context;
+    auto program =
+        state
+            .coerceToString(
+                pos,
+                *elems[0],
+                context,
+                "while evaluating the first element of the argument passed to builtins.exec",
+                false,
+                false
+            )
+            .toOwned();
+    Strings commandArgs;
+    for (unsigned int i = 1; i < args[0]->listSize(); ++i) {
+        commandArgs.push_back(
+            state
+                .coerceToString(
+                    pos,
+                    *elems[i],
+                    context,
+                    "while evaluating an element of the argument passed to builtins.exec",
+                    false,
+                    false
+                )
+                .toOwned()
+        );
+    }
+    try {
+        auto _ = state.realiseContext(context); // FIXME: Handle CA derivations
+    } catch (InvalidPathError & e) {
+        state
+            .error<EvalError>(
+                "cannot execute '%1%', since path '%2%' is not valid", program, e.path
+            )
+            .atPos(pos)
+            .debugThrow();
+    }
+
+    auto output = runProgram(program, true, commandArgs);
+    Expr * parsed;
+    try {
+        parsed = state.parseExprFromString(std::move(output), state.rootPath(CanonPath::root));
+    } catch (Error & e) {
+        e.addTrace(state.positions[pos], "while parsing the output from '%1%'", program);
+        throw;
+    }
+    try {
+        state.eval(parsed, v);
+    } catch (Error & e) {
+        e.addTrace(state.positions[pos], "while evaluating the output from '%1%'", program);
+        throw;
+    }
+}
+
+/**
+ * builtins.getEnv
+ */
+
+static void prim_getEnv(EvalState & state, const PosIdx pos, Value ** args, Value & v)
+{
+    std::string name(state.forceStringNoCtx(
+        *args[0], pos, "while evaluating the first argument passed to builtins.getEnv"
+    ));
+    v.mkString(evalSettings.restrictEval || evalSettings.pureEval ? "" : getEnv(name).value_or(""));
+}
+
+static RegisterPrimOp primop_getEnv({
+    .name = "__getEnv",
+    .args = {"s"},
+    .doc = R"(
+      `getEnv` returns the value of the environment variable *s*, or an
+      empty string if the variable doesn’t exist. This function should be
+      used with care, as it can introduce all sorts of nasty environment
+      dependencies in your Nix expression.
+
+      `getEnv` is used in Nix Packages to locate the file
+      `~/.nixpkgs/config.nix`, which contains user-local settings for Nix
+      Packages. (That is, it does a `getEnv "HOME"` to locate the user’s
+      home directory.)
+    )",
+    .fun = prim_getEnv,
+});
+
+}