Fix stale direct-mode cache hits when a source file's own body changes

src/apps/fastcache-cc/DirectManifest.hpp

@@ -136,11 +136,22 @@ enum class DirectError : std::uint8_t
 /// Toolchain headers are dropped (the stamp covers them); project headers are
 /// canonicalized and hashed. Entries are deduplicated and sorted by canonical
 /// path, so the same build state yields byte-identical manifests on any machine.
-/// @param includePaths   Absolute include paths, as emitted by the compiler.
+///
+/// `includePaths` must also contain the translation unit's own source path, not
+/// only the files it `#include`s: neither `/showIncludes` nor a GNU depfile ever
+/// names the primary TU (a depfile rule's target is explicitly excluded from its
+/// own dependency list), so a manifest built from headers alone has no entry
+/// that changes when the TU's own body is edited, and ValidateManifest would
+/// keep validating a stale object forever (issue #49 / issue #51). The caller
+/// (RecordManifest) adds it before calling this function; there is nothing
+/// source-specific about the handling here — it is canonicalized, hashed, and
+/// deduplicated exactly like any header.
+/// @param includePaths   Absolute paths the compile depends on: every `#include`d
+///                       header plus the translation unit's own source path.
 /// @param layout         This build's roots.
 /// @param toolchainStamp Identity standing in for the toolchain headers.
 /// @param objectKey      Key the compiled object is already stored under.
-/// @return The manifest, or DirectError when a project header cannot be canonicalized.
+/// @return The manifest, or DirectError when an entry cannot be canonicalized.
 [[nodiscard]] std::expected<DirectManifest, DirectError> BuildManifest(std::vector<std::string> const& includePaths,
                                                                        PathCanon::Layout const& layout,
                                                                        std::string_view toolchainStamp,

src/apps/fastcache-cc/DirectManifest_test.cpp

@@ -319,6 +319,54 @@ TEST_CASE("Build then validate a manifest against real files on disk")
     std::filesystem::remove_all(root);
 }
 
+TEST_CASE("ValidateManifest catches an edit to the translation unit itself, MSVC-style")
+{
+    // /showIncludes (and therefore ParseIncludePaths) never names the primary
+    // translation unit -- only the headers it pulls in. RecordManifest (main.cpp)
+    // compensates by adding the TU's own source path to the list it hands to
+    // BuildManifest, alongside the headers /showIncludes reported. Without that,
+    // editing a .cpp's own body while leaving every header untouched is invisible
+    // to ValidateManifest, and a direct-mode hit replays a stale object forever
+    // (see issue #49 / issue #51).
+    auto const root = std::filesystem::temp_directory_path() / "fc-direct-source-edit";
+    std::filesystem::remove_all(root);
+    std::filesystem::create_directories(root / "src");
+
+    auto const headerPath = root / "src" / "header.hpp";
+    {
+        std::ofstream out { headerPath, std::ios::binary };
+        out << "#pragma once\nint helper();\n";
+    }
+
+    auto const sourcePath = root / "src" / "a.cpp";
+    {
+        std::ofstream out { sourcePath, std::ios::binary };
+        out << "#include \"header.hpp\"\nint main() { return 0; }\n";
+    }
+
+    FastCache::PathCanon::Layout const layout { .sourceRoot = root.string(), .buildTree = (root / "out").string() };
+    constexpr std::string_view stamp = "cl-test-1";
+
+    // MSVC's /showIncludes lists only the header; RecordManifest appends the
+    // source path itself before calling BuildManifest, which this mirrors.
+    auto built = BuildManifest({ headerPath.string(), sourcePath.string() }, layout, stamp, "objkey-1");
+    REQUIRE(built.has_value());
+    REQUIRE(built->entries.size() == 2);
+    CHECK(ValidateManifest(*built, layout, stamp));
+
+    // Edit the .cpp body itself -- no header touched.
+    {
+        std::ofstream out { sourcePath, std::ios::binary };
+        out << "#include \"header.hpp\"\nint main() { return 1; }\n";
+    }
+
+    // The manifest must no longer validate: the TU itself is part of what a hit
+    // reproduces, so its own content has to be covered too.
+    CHECK_FALSE(ValidateManifest(*built, layout, stamp));
+
+    std::filesystem::remove_all(root);
+}
+
 TEST_CASE("BuildManifest normalizes '..' segments and mixed separators to one token")
 {
     // Real /showIncludes output echoes the resolved-but-unnormalized path, e.g.

src/apps/fastcache-cc/main.cpp

@@ -667,8 +667,16 @@ void RecordManifest(Config const& cfg,
         if (auto const depText = ReadDepFile(cmd))
             includes = Cc::ParseDepFilePaths(*depText);
 
-    if (includes.empty())
-        return;
+    // The TU itself must be part of what a direct hit revalidates, not only the
+    // headers it includes. /showIncludes never names the primary source (it only
+    // emits "Note: including file:" for #include targets), and a GNU depfile's
+    // rule deliberately excludes its own target from its dependency list — so
+    // without this, editing a .cpp's own body while leaving every header
+    // untouched is invisible to ValidateManifest and a stale object is replayed
+    // forever (issue #49 / issue #51). BuildManifest already canonicalizes,
+    // hashes, and dedupes every path handed to it, so adding the source here is
+    // enough; no format change is needed.
+    includes.push_back(cmd.source);
 
     // The manifest points at the object's ordinary key rather than causing a second
     // copy to be stored: L1 keeps values uncompressed, so duplicating objects would