[M5+] verify_link retries with libc++ stdenv override on link failure

src/codegen/flake.cpp

@@ -18,6 +18,7 @@ struct DepBinding {
     std::string sanitized;     // "nixpkgs_fmt_10_2_1" — input attr,
                                //   let-binding stem, lambda param
     std::optional<std::string> rev; // pinned commit (null → unpinned)
+    bool libcxx_override = false;   // wrap in .override { stdenv = libcxx }
 };
 
 // Replaces every char outside [a-zA-Z0-9_] with '_'. The result is safe
@@ -76,6 +77,7 @@ auto build_bindings(const GenerateInputs& in) -> std::vector<DepBinding> {
             .nixpkgs_attr = std::move(attr),
             .sanitized = sanitize_input_attr(dep.name, dep.version_spec),
             .rev = std::move(ref.rev),
+            .libcxx_override = rec.requires_libcxx_override,
         };
         out.push_back(std::move(b));
     }
@@ -138,20 +140,36 @@ auto emit_let_bindings(const std::vector<const DepBinding*>& pinned)
     return out;
 }
 
+auto base_expr(const DepBinding& b) -> std::string {
+    return b.rev && !b.rev->empty()
+               ? std::format("pkgs_{}.{}", b.sanitized, b.nixpkgs_attr)
+               : std::format("pkgs.{}", b.nixpkgs_attr);
+}
+
 auto emit_build_input_line(const DepBinding& b) -> std::string {
-    if (b.rev && !b.rev->empty()) {
+    auto expr = base_expr(b);
-        return std::format("            pkgs_{}.{}\n", b.sanitized, b.nixpkgs_attr);
+    if (b.libcxx_override) {
+        // Stdenv swap (libc++) plus a `doCheck = false` overrideAttrs
+        // to skip the package's test suite — without it, the rebuilt
+        // dep would re-run its full check phase under the new stdenv,
+        // adding minutes to hours of evaluation/build time.
+        expr = std::format(
+            "(({}.override {{ stdenv = llvmPkgs.libcxxStdenv; }})"
+            ".overrideAttrs (old: {{ doCheck = false; }}))",
+            expr);
     }
-    return std::format("            pkgs.{}\n", b.nixpkgs_attr);
+    return std::format("            {}\n", expr);
 }
 
 auto emit_build_inputs(const std::vector<DepBinding>& bindings) -> std::string {
     std::set<std::string> seen;
     std::string out;
     for (const auto& b : bindings) {
-        auto key = b.rev && !b.rev->empty()
+        // Dedupe by base expression — two deps that resolve to the same
-                       ? std::format("pkgs_{}.{}", b.sanitized, b.nixpkgs_attr)
+        // (set, attr) but differ only in override are considered distinct
-                       : std::format("pkgs.{}", b.nixpkgs_attr);
+        // because the dedup key includes the override flag.
+        auto key = std::format("{}{}", base_expr(b),
+                               b.libcxx_override ? "@libcxx" : "");
         if (seen.insert(key).second) {
             out += emit_build_input_line(b);
         }

src/linkdb/curated.cpp

@@ -185,6 +185,7 @@ auto Database::resolve(const std::string& package, const std::string& version,
                 .find_package = row.find_package,
                 .targets = row.targets,
                 .source = row.source,
+                .requires_libcxx_override = row.requires_libcxx_override,
             };
         }
     }

src/linkdb/linkdb.cppm

@@ -14,6 +14,12 @@ struct Recipe {
     std::string find_package; // post-substitution
     std::vector<std::string> targets; // post-substitution
     std::string source; // 'curated' | 'manual' | etc.
+    // Set when the dep needs `.override { stdenv = llvmPkgs.libcxxStdenv; }`
+    // to link against the user's libc++-built project (e.g. nixpkgs ships
+    // it built against libstdc++ — abseil-cpp, llvm). verify_link toggles
+    // this on after a libstdc++/libc++ link failure. Codegen wraps the
+    // buildInputs entry in the override expression when true.
+    bool requires_libcxx_override = false;
 
     bool operator==(const Recipe&) const = default;
 };
@@ -37,6 +43,7 @@ struct OverlayRow {
     std::vector<std::string> targets;
     std::string source;
     std::int64_t verified_at = 0;
+    bool requires_libcxx_override = false;
 };
 
 // RAII wrapper for an open sqlite3 connection used by the overlay database.
@@ -90,6 +97,13 @@ auto overlay_delete_recipe(OverlayState& state, const std::string& package,
                            const std::string& source)
     -> cargoxx::util::Result<void>;
 
+// Toggle the libcxx-override flag for an existing row. Used by
+// `verify_link` after a libstdc++/libc++ link mismatch.
+auto overlay_set_libcxx_override(OverlayState& state, const std::string& package,
+                                 const std::string& version_range,
+                                 const std::string& source, bool value)
+    -> cargoxx::util::Result<void>;
+
 auto overlay_query(OverlayState& state, const std::string& package)
     -> cargoxx::util::Result<std::vector<OverlayRow>>;
 
@@ -125,6 +139,11 @@ class Database {
                            const std::string& version_range,
                            const std::string& source) -> util::Result<void>;
 
+    auto set_libcxx_override(const std::string& package,
+                             const std::string& version_range,
+                             const std::string& source, bool value)
+        -> util::Result<void>;
+
   private:
     Database() = default;
     std::map<std::string, std::vector<detail::CuratedRecipe>> curated_;

src/linkdb/overlay.cpp

@@ -90,6 +90,25 @@ auto overlay_open(const std::filesystem::path& path)
         });
     }
 
+    // Add the libcxx-override column to legacy overlay files. SQLite's
+    // ADD COLUMN errors when the column already exists; treat that as
+    // success (the column is present, which is all we need).
+    constexpr const char* MIGRATE =
+        "ALTER TABLE recipes ADD COLUMN requires_libcxx_override INTEGER NOT NULL DEFAULT 0";
+    char* mig_err = nullptr;
+    if (sqlite3_exec(state->handle(), MIGRATE, nullptr, nullptr, &mig_err) != SQLITE_OK) {
+        if (mig_err && std::string_view{mig_err}.find("duplicate column") ==
+                           std::string_view::npos) {
+            std::string msg = std::format("cannot migrate overlay schema: {}",
+                                          mig_err ? mig_err : "?");
+            sqlite3_free(mig_err);
+            return std::unexpected(util::Error{
+                util::ErrorCode::LinkdbCorrupt, std::move(msg), "", path, std::nullopt,
+            });
+        }
+        sqlite3_free(mig_err);
+    }
+
     return state;
 }
 
@@ -99,8 +118,8 @@ auto overlay_insert_manual(OverlayState& state, const std::string& package,
     constexpr const char* SQL =
         "INSERT OR REPLACE INTO recipes "
         "(package, version_range, nixpkgs_attr, find_package, targets, components, source, "
-        " verified_at) "
+        " verified_at, requires_libcxx_override) "
-        "VALUES (?, ?, ?, ?, ?, NULL, 'manual', ?)";
+        "VALUES (?, ?, ?, ?, ?, NULL, 'manual', ?, ?)";
 
     sqlite3* db = state.handle();
     sqlite3_stmt* stmt = nullptr;
@@ -119,6 +138,7 @@ auto overlay_insert_manual(OverlayState& state, const std::string& package,
     sqlite3_bind_text(stmt, 4, r.find_package.c_str(), -1, SQLITE_TRANSIENT);
     sqlite3_bind_text(stmt, 5, targets_str.c_str(), -1, SQLITE_TRANSIENT);
     sqlite3_bind_int64(stmt, 6, now);
+    sqlite3_bind_int(stmt, 7, r.requires_libcxx_override ? 1 : 0);
 
     auto rc = sqlite3_step(stmt);
     sqlite3_finalize(stmt);
@@ -137,8 +157,8 @@ auto overlay_insert(OverlayState& state, const std::string& package,
     constexpr const char* SQL =
         "INSERT OR REPLACE INTO recipes "
         "(package, version_range, nixpkgs_attr, find_package, targets, components, source, "
-        " verified_at) "
+        " verified_at, requires_libcxx_override) "
-        "VALUES (?, ?, ?, ?, ?, NULL, ?, ?)";
+        "VALUES (?, ?, ?, ?, ?, NULL, ?, ?, ?)";
 
     sqlite3* db = state.handle();
     sqlite3_stmt* stmt = nullptr;
@@ -155,6 +175,7 @@ auto overlay_insert(OverlayState& state, const std::string& package,
     sqlite3_bind_text(stmt, 5, targets_str.c_str(), -1, SQLITE_TRANSIENT);
     sqlite3_bind_text(stmt, 6, source.c_str(), -1, SQLITE_TRANSIENT);
     sqlite3_bind_int64(stmt, 7, verified_at);
+    sqlite3_bind_int(stmt, 8, r.requires_libcxx_override ? 1 : 0);
 
     auto rc = sqlite3_step(stmt);
     sqlite3_finalize(stmt);
@@ -223,7 +244,8 @@ auto overlay_delete_recipe(OverlayState& state, const std::string& package,
 auto overlay_query(OverlayState& state, const std::string& package)
     -> util::Result<std::vector<OverlayRow>> {
     constexpr const char* SQL =
-        "SELECT version_range, nixpkgs_attr, find_package, targets, source, verified_at "
+        "SELECT version_range, nixpkgs_attr, find_package, targets, source, verified_at, "
+        "       requires_libcxx_override "
         "FROM recipes WHERE package = ?";
 
     sqlite3* db = state.handle();
@@ -254,6 +276,7 @@ auto overlay_query(OverlayState& state, const std::string& package)
         }
         row.source = column_text(stmt, 4);
         row.verified_at = sqlite3_column_int64(stmt, 5);
+        row.requires_libcxx_override = sqlite3_column_int(stmt, 6) != 0;
         out.push_back(std::move(row));
     }
     sqlite3_finalize(stmt);
@@ -278,6 +301,30 @@ auto overlay_is_fresh(const OverlayRow& row, std::int64_t now) -> bool {
     return (now - row.verified_at) < THIRTY_DAYS_SECONDS;
 }
 
+auto overlay_set_libcxx_override(OverlayState& state, const std::string& package,
+                                 const std::string& version_range,
+                                 const std::string& source, bool value)
+    -> util::Result<void> {
+    constexpr const char* SQL =
+        "UPDATE recipes SET requires_libcxx_override = ? "
+        "WHERE package = ? AND version_range = ? AND source = ?";
+    sqlite3* db = state.handle();
+    sqlite3_stmt* stmt = nullptr;
+    if (sqlite3_prepare_v2(db, SQL, -1, &stmt, nullptr) != SQLITE_OK) {
+        return std::unexpected(sqlite_error(db, "prepare update libcxx_override"));
+    }
+    sqlite3_bind_int(stmt, 1, value ? 1 : 0);
+    sqlite3_bind_text(stmt, 2, package.c_str(), -1, SQLITE_TRANSIENT);
+    sqlite3_bind_text(stmt, 3, version_range.c_str(), -1, SQLITE_TRANSIENT);
+    sqlite3_bind_text(stmt, 4, source.c_str(), -1, SQLITE_TRANSIENT);
+    auto rc = sqlite3_step(stmt);
+    sqlite3_finalize(stmt);
+    if (rc != SQLITE_DONE) {
+        return std::unexpected(sqlite_error(db, "step update libcxx_override"));
+    }
+    return {};
+}
+
 } // namespace detail
 
 auto Database::add_manual(const std::string& package, const std::string& version_range,
@@ -333,4 +380,15 @@ auto Database::abort_provisional(const std::string& package,
     return detail::overlay_delete_recipe(*overlay_, package, version_range, source);
 }
 
+auto Database::set_libcxx_override(const std::string& package,
+                                   const std::string& version_range,
+                                   const std::string& source, bool value)
+    -> util::Result<void> {
+    if (auto ok = require_overlay(overlay_); !ok) {
+        return std::unexpected(ok.error());
+    }
+    return detail::overlay_set_libcxx_override(*overlay_, package, version_range,
+                                                source, value);
+}
+
 } // namespace cargoxx::linkdb

src/resolver/verify_link.cpp

@@ -142,12 +142,63 @@ auto verify_link(const VerifyLinkRequest& req, const BuildFn& build_fn)
 
     auto build_result = build_fn(proj_root);
 
-    // Always re-open the db to flip the provisional row's fate.
+    // First-pass success — confirm and we're done.
+    if (build_result) {
+        auto db = linkdb::Database::open(req.overlay_path);
+        if (!db) {
+            return std::unexpected(db.error());
+        }
+        if (auto r = db->confirm_provisional(req.package_name, req.version_spec,
+                                              req.source);
+            !r) {
+            return std::unexpected(r.error());
+        }
+        return {};
+    }
+
+    // First pass failed. Try once more with a libstdc++/libc++ ABI
+    // workaround: flip `requires_libcxx_override` on the overlay row,
+    // which makes flake codegen wrap the dep in
+    // `.override { stdenv = llvmPkgs.libcxxStdenv; }`. cmd_build
+    // re-resolves and regenerates the flake, so the second build_fn
+    // call sees the override.
+    std::cerr << std::format(
+        "verify_link: first build of '{}' failed; retrying with "
+        "libc++/doCheck=false override\n",
+        req.package_name);
+    {
+        auto db = linkdb::Database::open(req.overlay_path);
+        if (!db) {
+            return std::unexpected(db.error());
+        }
+        if (auto r = db->set_libcxx_override(req.package_name, req.version_spec,
+                                              req.source, true);
+            !r) {
+            // Fall through to the abort path below — couldn't even mark.
+            (void)db->abort_provisional(req.package_name, req.version_spec,
+                                         req.source);
+            return std::unexpected(r.error());
+        }
+    }
+
+    // Wipe CMake's per-profile build dirs. The first pass's
+    // CMakeCache.txt has cached `LLVM_DIR` (and other find_package
+    // results) at the default-stdenv path; without clearing it, the
+    // retry's regenerated flake.nix has no effect on what
+    // find_package sees.
+    {
+        std::error_code wipe_ec;
+        for (auto* sub : {"debug", "release"}) {
+            std::filesystem::remove_all(proj_root / "build" / sub, wipe_ec);
+        }
+    }
+
+    auto retry_result = build_fn(proj_root);
     auto db = linkdb::Database::open(req.overlay_path);
     if (!db) {
         return std::unexpected(db.error());
     }
-    if (build_result) {
+    if (retry_result) {
         if (auto r = db->confirm_provisional(req.package_name, req.version_spec,
                                               req.source);
             !r) {
@@ -156,7 +207,7 @@ auto verify_link(const VerifyLinkRequest& req, const BuildFn& build_fn)
         return {};
     }
     (void)db->abort_provisional(req.package_name, req.version_spec, req.source);
-    return std::unexpected(build_result.error());
+    return std::unexpected(retry_result.error());
 }
 
 } // namespace cargoxx::resolver