FileSystemManager::CreateFileHardlink: return error code on failure

Instead of returning a plain boolean, return an expected with the same
boolean value that in case of an error indicates the error code. In this
way, an error-specific handling is possible by consumers. While there,
also add proper quoting of the involved file names.

4 files changed, 54 insertions, 38 deletions

diff --git a/src/buildtool/file_system/TARGETS b/src/buildtool/file_system/TARGETS
index 82ddce24..81c7000d 100644
--- a/src/buildtool/file_system/TARGETS
+++ b/src/buildtool/file_system/TARGETS
@@ -40,8 +40,10 @@
     , ["src/buildtool/logging", "log_level"]
     , ["src/buildtool/logging", "logging"]
     , ["src/buildtool/system", "system"]
+    , ["src/utils/cpp", "expected"]
     , ["src/utils/cpp", "path"]
     , ["@", "gsl", "", "gsl"]
+    , ["@", "json", "", "json"]
     ]
   , "stage": ["src", "buildtool", "file_system"]
   }
diff --git a/src/buildtool/file_system/file_system_manager.hpp b/src/buildtool/file_system/file_system_manager.hpp
index 93a911d8..ac23691c 100644
--- a/src/buildtool/file_system/file_system_manager.hpp
+++ b/src/buildtool/file_system/file_system_manager.hpp
@@ -25,7 +25,9 @@
 #include <filesystem>
 #include <fstream>
 #include <optional>
+#include <system_error>
 #include <unordered_set>
+#include <variant>
 
 #ifdef __unix__
 #include <fcntl.h>
@@ -39,10 +41,12 @@
 #endif
 
 #include "gsl/gsl"
+#include "nlohmann/json.hpp"
 #include "src/buildtool/file_system/object_type.hpp"
 #include "src/buildtool/logging/log_level.hpp"
 #include "src/buildtool/logging/logger.hpp"
 #include "src/buildtool/system/system.hpp"
+#include "src/utils/cpp/expected.hpp"
 #include "src/utils/cpp/path.hpp"
 
 namespace detail {
@@ -216,21 +220,28 @@ class FileSystemManager {
         return false;
     }
 
+    /// \brief Try to create a hard link; return unit on success and a
+    /// std::error_condition describing the failure.
     [[nodiscard]] static auto CreateFileHardlink(
         std::filesystem::path const& file_path,
         std::filesystem::path const& link_path,
-        LogLevel log_failure_at = LogLevel::Error) noexcept -> bool {
-        try {
-            std::filesystem::create_hard_link(file_path, link_path);
-            return std::filesystem::is_regular_file(link_path);
-        } catch (std::exception const& e) {
-            Logger::Log(log_failure_at,
-                        "hard linking {} to {}\n{}",
-                        file_path.string(),
-                        link_path.string(),
-                        e.what());
-            return false;
+        LogLevel log_failure_at = LogLevel::Error) noexcept
+        -> expected<std::monostate, std::error_code> {
+        std::error_code ec{};
+        std::filesystem::create_hard_link(file_path, link_path, ec);
+        if (not ec) {
+            if (std::filesystem::is_regular_file(link_path)) {
+                return std::monostate{};
+            }
+            return unexpected(std::error_code{});
         }
+        Logger::Log(log_failure_at,
+                    "failed hard linking {} to {}: {}, {}",
+                    nlohmann::json(file_path.string()).dump(),
+                    nlohmann::json(link_path.string()).dump(),
+                    ec.value(),
+                    ec.message());
+        return unexpected(ec);
     }
 
     template <ObjectType kType, bool kSetEpochTime = false>
diff --git a/src/buildtool/serve_api/serve_service/source_tree.cpp b/src/buildtool/serve_api/serve_service/source_tree.cpp
index 74edaca8..480dfd34 100644
--- a/src/buildtool/serve_api/serve_service/source_tree.cpp
+++ b/src/buildtool/serve_api/serve_service/source_tree.cpp
@@ -895,20 +895,21 @@ auto SourceTreeService::DistdirImportToGit(
     auto const& tmp_path = distdir_tmp_dir->GetPath();
     // link the CAS blobs into the tmp dir
     auto const& cas = storage_.CAS();
-    if (not std::all_of(content_list.begin(),
-                        content_list.end(),
-                        [&cas, tmp_path](auto const& kv) {
-                            auto content_path = cas.BlobPath(
-                                ArtifactDigest(
-                                    kv.second.first, 0, /*is_tree=*/false),
-                                kv.second.second);
-                            if (content_path) {
-                                return FileSystemManager::CreateFileHardlink(
-                                    *content_path,  // from: cas_path/content_id
-                                    tmp_path / kv.first);  // to: tmp_path/name
-                            }
-                            return false;
-                        })) {
+    if (not std::all_of(
+            content_list.begin(),
+            content_list.end(),
+            [&cas, tmp_path](auto const& kv) {
+                auto content_path = cas.BlobPath(
+                    ArtifactDigest(kv.second.first, 0, /*is_tree=*/false),
+                    kv.second.second);
+                if (content_path) {
+                    return FileSystemManager::CreateFileHardlink(
+                               *content_path,  // from: cas_path/content_id
+                               tmp_path / kv.first)  // to: tmp_path/name
+                        .has_value();
+                }
+                return false;
+            })) {
         logger_->Emit(LogLevel::Error,
                       "Failed to create links to CAS content {}",
                       content_id);
diff --git a/src/other_tools/root_maps/distdir_git_map.cpp b/src/other_tools/root_maps/distdir_git_map.cpp
index 54abaae2..9171daae 100644
--- a/src/other_tools/root_maps/distdir_git_map.cpp
+++ b/src/other_tools/root_maps/distdir_git_map.cpp
@@ -42,19 +42,21 @@ namespace {
         content_list,
     std::filesystem::path const& tmp_dir) noexcept -> bool {
     auto const& cas = storage.CAS();
-    return std::all_of(content_list->begin(),
-                       content_list->end(),
-                       [&cas, tmp_dir](auto const& kv) {
-                           auto content_path =
-                               cas.BlobPath(ArtifactDigest(kv.second, 0, false),
-                                            /*is_executable=*/false);
-                           if (content_path) {
-                               return FileSystemManager::CreateFileHardlink(
-                                   *content_path,  // from: cas_path/content_id
-                                   tmp_dir / kv.first);  // to: tmp_dir/name
-                           }
-                           return false;
-                       });
+    return std::all_of(
+        content_list->begin(),
+        content_list->end(),
+        [&cas, tmp_dir](auto const& kv) {
+            auto content_path =
+                cas.BlobPath(ArtifactDigest(kv.second, 0, false),
+                             /*is_executable=*/false);
+            if (content_path) {
+                return FileSystemManager::CreateFileHardlink(
+                           *content_path,       // from: cas_path/content_id
+                           tmp_dir / kv.first)  // to: tmp_dir/name
+                    .has_value();
+            }
+            return false;
+        });
 }
 
 /// \brief Called once we know we have the content blobs in local CAS in order