libexpr: Add recoverable errors

This provides an explicit API for call-fail-retry-succeed evaluation
flows, such as currently used in NixOps4.

An alternative design would simply reset the `Value` to the original
thunk instead of `tFailed` under the condition of catching a
`RecoverableEvalError`.
That is somewhat simpler, but I believe the presence of `tFailed` is
beneficial for possible use in the repl; being able to show the error
sooner, without re-evaluation.

The hasPos method and isEmpty function are required in order to avoid
an include loop.

src/libexpr-c/nix_api_value.cc

@@ -1,4 +1,5 @@
 #include "nix/expr/attr-set.hh"
+#include "nix/expr/eval-error.hh"
 #include "nix/util/configuration.hh"
 #include "nix/expr/eval.hh"
 #include "nix/store/globals.hh"
@@ -89,8 +90,13 @@ static void nix_c_primop_wrapper(
     f(userdata, &ctx, (EvalState *) &state, (nix_value **) args, (nix_value *) &vTmp);
 
     if (ctx.last_err_code != NIX_OK) {
-        /* TODO: Throw different errors depending on the error code */
-        state.error<nix::EvalError>("Error from custom function: %s", *ctx.last_err).atPos(pos).debugThrow();
+        if (ctx.last_err_code == NIX_ERR_RECOVERABLE) {
+            state.error<nix::RecoverableEvalError>("Recoverable error from custom function: %s", *ctx.last_err)
+                .atPos(pos)
+                .debugThrow();
+        } else {
+            state.error<nix::EvalError>("Error from custom function: %s", *ctx.last_err).atPos(pos).debugThrow();
+        }
     }
 
     if (!vTmp.isValid()) {

src/libexpr-tests/nix_api_expr.cc

@@ -437,4 +437,105 @@ TEST_F(nix_api_expr_test, nix_value_call_multi_no_args)
     assert_ctx_ok();
     ASSERT_EQ(3, rInt);
 }
+
+// The following is a test case for retryable thunks.
+// This is a requirement for the current way in which NixOps4 evaluates its deployment expressions.
+// An alternative strategy could be implemented, but unwinding the stack may be a more efficient way to deal with many
+// suspensions/resumptions, compared to e.g. using a thread or coroutine stack for each suspended dependency. This test
+// models the essential bits of a deployment tool that uses such a strategy.
+
+// State for the retryable primop - simulates deployment resource availability
+struct DeploymentResourceState
+{
+    bool vm_created = false;
+};
+
+static void primop_load_resource_input(
+    void * user_data, nix_c_context * context, EvalState * state, nix_value ** args, nix_value * ret)
+{
+    assert(context);
+    assert(state);
+    auto * resource_state = static_cast<DeploymentResourceState *>(user_data);
+
+    // Get the resource input name argument
+    std::string input_name;
+    if (nix_get_string(context, args[0], OBSERVE_STRING(input_name)) != NIX_OK)
+        return;
+
+    // Only handle "vm_id" input - throw for anything else
+    if (input_name != "vm_id") {
+        std::string error_msg = "unknown resource input: " + input_name;
+        nix_set_err_msg(context, NIX_ERR_NIX_ERROR, error_msg.c_str());
+        return;
+    }
+
+    if (resource_state->vm_created) {
+        // VM has been created, return the ID
+        nix_init_string(context, ret, "vm-12345");
+    } else {
+        // VM not created yet, fail with dependency error
+        nix_set_err_msg(context, NIX_ERR_RECOVERABLE, "VM not yet created");
+    }
+}
+
+TEST_F(nix_api_expr_test, nix_expr_thunk_re_evaluation_after_deployment)
+{
+    // This test demonstrates NixOps4's requirement: a thunk calling a primop should be
+    // re-evaluable when deployment resources become available that were not available initially.
+
+    DeploymentResourceState resource_state;
+
+    PrimOp * primop = nix_alloc_primop(
+        ctx,
+        primop_load_resource_input,
+        1,
+        "loadResourceInput",
+        nullptr,
+        "load a deployment resource input",
+        &resource_state);
+    assert_ctx_ok();
+
+    nix_value * primopValue = nix_alloc_value(ctx, state);
+    assert_ctx_ok();
+    nix_init_primop(ctx, primopValue, primop);
+    assert_ctx_ok();
+
+    nix_value * inputName = nix_alloc_value(ctx, state);
+    assert_ctx_ok();
+    nix_init_string(ctx, inputName, "vm_id");
+    assert_ctx_ok();
+
+    // Create a single thunk by using nix_init_apply instead of nix_value_call
+    // This creates a lazy application that can be forced multiple times
+    nix_value * thunk = nix_alloc_value(ctx, state);
+    assert_ctx_ok();
+    nix_init_apply(ctx, thunk, primopValue, inputName);
+    assert_ctx_ok();
+
+    // First force: VM not created yet, should fail
+    nix_value_force(ctx, state, thunk);
+    ASSERT_EQ(NIX_ERR_NIX_ERROR, nix_err_code(ctx));
+    ASSERT_THAT(nix_err_msg(nullptr, ctx, nullptr), testing::HasSubstr("VM not yet created"));
+
+    // Clear the error context for the next attempt
+    nix_c_context_free(ctx);
+    ctx = nix_c_context_create();
+
+    // Simulate deployment process: VM gets created
+    resource_state.vm_created = true;
+
+    // Second force of the SAME thunk: this is where the "failed" value issue appears
+    // With failed value caching, this should fail because the thunk is marked as permanently failed
+    // Without failed value caching (or with retryable failures), this should succeed
+    nix_value_force(ctx, state, thunk);
+
+    // If we get here without error, the thunk was successfully re-evaluated
+    assert_ctx_ok();
+
+    std::string result;
+    nix_get_string(ctx, thunk, OBSERVE_STRING(result));
+    assert_ctx_ok();
+    ASSERT_STREQ("vm-12345", result.c_str());
+}
+
 } // namespace nixC

src/libexpr/eval-error.cc

@@ -113,5 +113,6 @@ template class EvalErrorBuilder<MissingArgumentError>;
 template class EvalErrorBuilder<InfiniteRecursionError>;
 template class EvalErrorBuilder<InvalidPathError>;
 template class EvalErrorBuilder<IFDError>;
+template class EvalErrorBuilder<RecoverableEvalError>;
 
 } // namespace nix

src/libexpr/eval.cc

@@ -1,4 +1,5 @@
 #include "nix/expr/eval.hh"
+#include "nix/expr/eval-error.hh"
 #include "nix/expr/eval-settings.hh"
 #include "nix/expr/primops.hh"
 #include "nix/expr/print-options.hh"
@@ -27,6 +28,7 @@
 #include "parser-tab.hh"
 
 #include <algorithm>
+#include <exception>
 #include <iostream>
 #include <sstream>
 #include <cstring>
@@ -2062,6 +2064,54 @@ void ExprBlackHole::eval(EvalState & state, [[maybe_unused]] Env & env, Value &
 // always force this to be separate, otherwise forceValue may inline it and take
 // a massive perf hit
 [[gnu::noinline]]
+void EvalState::handleEvalExceptionForThunk(Env * env, Expr * expr, Value & v, const PosIdx pos)
+{
+    v.mkThunk(env, expr);
+    tryFixupBlackHolePos(v, pos);
+
+    auto e = std::current_exception();
+    Value * recovery = nullptr;
+    try {
+        std::rethrow_exception(e);
+    } catch (const RecoverableEvalError & e) {
+        recovery = allocValue();
+    } catch (...) {
+    }
+    if (recovery) {
+        recovery->mkThunk(env, expr);
+    }
+    v.mkFailed(e, recovery);
+}
+
+[[gnu::noinline]]
+void EvalState::handleEvalExceptionForApp(Value & v)
+{
+    auto e = std::current_exception();
+    Value * recovery = nullptr;
+    try {
+        std::rethrow_exception(e);
+    } catch (const RecoverableEvalError & e) {
+        recovery = allocValue();
+    } catch (...) {
+    }
+    if (recovery) {
+        *recovery = v;
+    }
+    v.mkFailed(e, recovery);
+}
+
+[[gnu::noinline]]
+void EvalState::handleEvalFailed(Value & v, const PosIdx pos)
+{
+    assert(v.isFailed());
+    if (auto recoveryValue = v.failed()->recoveryValue) {
+        v = *recoveryValue;
+        forceValue(v, pos);
+    } else {
+        std::rethrow_exception(v.failed()->ex);
+    }
+}
+
 void EvalState::tryFixupBlackHolePos(Value & v, PosIdx pos)
 {
     if (!v.isBlackhole())
@@ -2070,7 +2120,8 @@ void EvalState::tryFixupBlackHolePos(Value & v, PosIdx pos)
     try {
         std::rethrow_exception(e);
     } catch (InfiniteRecursionError & e) {
-        e.atPos(positions[pos]);
+        if (!e.hasPos())
+            e.atPos(positions[pos]);
     } catch (...) {
     }
 }

src/libexpr/include/nix/expr/eval-error.hh

@@ -56,6 +56,14 @@ MakeError(MissingArgumentError, EvalError);
 MakeError(InfiniteRecursionError, EvalError);
 MakeError(IFDError, EvalBaseError);
 
+/**
+ * An evaluation error which should be retried instead of rethrown
+ *
+ * A RecoverableEvalError is not an EvalError, because we shouldn't cache it in the eval cache, as it should be retried
+ * anyway.
+ */
+MakeError(RecoverableEvalError, EvalBaseError);
+
 struct InvalidPathError : public EvalError
 {
 public:

src/libexpr/include/nix/expr/eval-inline.hh

@@ -5,6 +5,7 @@
 #include "nix/expr/eval.hh"
 #include "nix/expr/eval-error.hh"
 #include "nix/expr/eval-settings.hh"
+#include <exception>
 
 namespace nix {
 
@@ -97,19 +98,19 @@ void EvalState::forceValue(Value & v, const PosIdx pos)
             else
                 ExprBlackHole::throwInfiniteRecursionError(*this, v);
         } catch (...) {
-            tryFixupBlackHolePos(v, pos);
-            v.mkFailed();
+            handleEvalExceptionForThunk(env, expr, v, pos);
             throw;
         }
     } else if (v.isApp()) {
         try {
             callFunction(*v.app().left, *v.app().right, v, pos);
         } catch (...) {
-            v.mkFailed();
+            handleEvalExceptionForApp(v);
             throw;
         }
-    } else if (v.isFailed())
-        std::rethrow_exception(v.failed()->ex);
+    } else if (v.isFailed()) {
+        handleEvalFailed(v, pos);
+    }
 }
 
 [[gnu::always_inline]]

src/libexpr/include/nix/expr/eval.hh

@@ -610,8 +610,28 @@ public:
      */
     inline void forceValue(Value & v, const PosIdx pos);
 
+private:
+
+    /**
+     * Internal support function for forceValue
+     *
+     * This code is factored out so that it's not in the heavily inlined hot path.
+     */
+    void handleEvalExceptionForThunk(Env * env, Expr * expr, Value & v, const PosIdx pos);
+
+    /**
+     * Internal support function for forceValue
+     *
+     * This code is factored out so that it's not in the heavily inlined hot path.
+     */
+    void handleEvalExceptionForApp(Value & v);
+
+    void handleEvalFailed(Value & v, PosIdx pos);
+
     void tryFixupBlackHolePos(Value & v, PosIdx pos);
 
+public:
+
     /**
      * Force a value, then recursively force list elements and
      * attributes.

src/libexpr/include/nix/expr/value.hh

@@ -2,6 +2,7 @@
 ///@file
 
 #include <cassert>
+#include <exception>
 #include <span>
 #include <type_traits>
 #include <concepts>
@@ -279,9 +280,15 @@ struct ValueBase
     {
     public:
         std::exception_ptr ex;
+        /**
+         * Optional value for recovering `RecoverableEvalError`
+         * Must be set iff `ex` is an instance of `RecoverableEvalError`.
+         */
+        Value * recoveryValue;
 
-        Failed(std::exception_ptr && ex)
+        Failed(std::exception_ptr ex, Value * recoveryValue)
             : ex(ex)
+            , recoveryValue(recoveryValue)
         {
         }
     };
@@ -1086,9 +1093,9 @@ public:
         setStorage(n);
     }
 
-    inline void mkFailed() noexcept
+    inline void mkFailed(std::exception_ptr e, Value * recovery) noexcept
     {
-        setStorage(new Value::Failed(std::current_exception()));
+        setStorage(new Value::Failed(e, recovery));
     }
 
     bool isList() const noexcept

src/libutil-c/nix_api_util.h

@@ -98,6 +98,13 @@ enum nix_err {
      */
     NIX_ERR_NIX_ERROR = -4,
 
+    /**
+     * @brief A recoverable error occurred.
+     *
+     * This is used primarily by C API *consumers* to communicate that a failed
+     * primop call should be retried on the next evaluation attempt.
+     */
+    NIX_ERR_RECOVERABLE = -5,
 };
 
 typedef enum nix_err nix_err;

src/libutil/include/nix/util/error.hh

@@ -51,6 +51,7 @@ struct LinesOfCode
    FIXME: Untangle this mess. Should there be AbstractPos as there used to be before
    4feb7d9f71? */
 struct Pos;
+bool isEmpty(const Pos & pos);
 
 void printCodeLines(std::ostream & out, const std::string & prefix, const Pos & errPos, const LinesOfCode & loc);
 
@@ -187,6 +188,11 @@ public:
         err.pos = pos;
     }
 
+    bool hasPos()
+    {
+        return err.pos.get() && !isEmpty(*err.pos.get());
+    }
+
     void pushTrace(Trace trace)
     {
         err.traces.push_front(trace);

src/libutil/position.cc

@@ -7,6 +7,11 @@ Pos::operator std::shared_ptr<const Pos>() const
     return std::make_shared<const Pos>(*this);
 }
 
+bool isEmpty(const Pos & pos)
+{
+    return !pos.operator bool();
+}
+
 std::optional<LinesOfCode> Pos::getCodeLines() const
 {
     if (line == 0)