fix: avoid touching staged BMI cache outputs

Author: Sunrisepeak

src/bmi_cache.cppm

@@ -55,8 +55,9 @@ bool is_cached(const CacheKey& key);
 std::expected<DepArtifacts, std::string>
 read_manifest(const CacheKey& key);
 
-// Copy cached files into projectTarget/{gcm.cache,obj}, bumping mtime so
-// ninja sees them as up-to-date relative to the (untouched) source files.
+// Copy missing cached files into projectTarget/{gcm.cache,obj}. Existing
+// project outputs are left untouched: GCC BMIs may differ byte-for-byte between
+// equivalent builds, and overwriting them would dirty downstream modules.
 std::expected<DepArtifacts, std::string>
 stage_into(const CacheKey& key,
            const std::filesystem::path& projectTargetDir);
@@ -150,6 +151,11 @@ stage_into(const CacheKey& key,
     for (auto& g : arts->gcmFiles) {
         auto from = key.gcmDir() / g;
         auto to   = projectGcm   / g;
+        if (std::filesystem::exists(to, ec)) {
+            ec.clear();
+            continue;
+        }
+        ec.clear();
         if (!copy_one(from, to, ec)) {
             return std::unexpected(std::format(
                 "stage gcm '{}': {}", g, ec.message()));
@@ -159,6 +165,11 @@ stage_into(const CacheKey& key,
     for (auto& o : arts->objFiles) {
         auto from = key.objDir() / o;
         auto to   = projectObj   / o;
+        if (std::filesystem::exists(to, ec)) {
+            ec.clear();
+            continue;
+        }
+        ec.clear();
         if (!copy_one(from, to, ec)) {
             return std::unexpected(std::format(
                 "stage obj '{}': {}", o, ec.message()));

tests/unit/test_bmi_cache.cpp

@@ -97,6 +97,78 @@ TEST(BmiCache, PopulateThenStageRoundTrip) {
     EXPECT_EQ(body, "OBJ-A");
 }
 
+TEST(BmiCache, StageIntoDoesNotTouchIdenticalOutputs) {
+    Tmp t;
+    auto home    = t.path / "home";
+    auto project = t.path / "proj" / "target";
+    std::filesystem::create_directories(project / "gcm.cache");
+    std::filesystem::create_directories(project / "obj");
+
+    writeFile(project / "gcm.cache" / "mcpplibs.cmdline.gcm", "GCM-A");
+    writeFile(project / "obj"       / "cmdline.m.o",          "OBJ-A");
+
+    DepArtifacts arts {
+        .gcmFiles = { "mcpplibs.cmdline.gcm" },
+        .objFiles = { "cmdline.m.o" },
+    };
+
+    auto k = makeKey(home);
+    ASSERT_TRUE(populate_from(k, project, arts));
+
+    auto staged = stage_into(k, project);
+    ASSERT_TRUE(staged) << staged.error();
+    auto gcmTime = std::filesystem::last_write_time(project / "gcm.cache" / "mcpplibs.cmdline.gcm");
+    auto objTime = std::filesystem::last_write_time(project / "obj" / "cmdline.m.o");
+
+    auto stagedAgain = stage_into(k, project);
+    ASSERT_TRUE(stagedAgain) << stagedAgain.error();
+    EXPECT_EQ(std::filesystem::last_write_time(project / "gcm.cache" / "mcpplibs.cmdline.gcm"), gcmTime);
+    EXPECT_EQ(std::filesystem::last_write_time(project / "obj" / "cmdline.m.o"), objTime);
+}
+
+TEST(BmiCache, StageIntoDoesNotOverwriteExistingOutputs) {
+    Tmp t;
+    auto home         = t.path / "home";
+    auto cacheProject = t.path / "cache-proj" / "target";
+    auto project      = t.path / "proj" / "target";
+    std::filesystem::create_directories(cacheProject / "gcm.cache");
+    std::filesystem::create_directories(cacheProject / "obj");
+    std::filesystem::create_directories(project / "gcm.cache");
+    std::filesystem::create_directories(project / "obj");
+
+    writeFile(cacheProject / "gcm.cache" / "mcpplibs.cmdline.gcm", "CACHE-GCM");
+    writeFile(cacheProject / "obj"       / "cmdline.m.o",          "CACHE-OBJ");
+
+    DepArtifacts arts {
+        .gcmFiles = { "mcpplibs.cmdline.gcm" },
+        .objFiles = { "cmdline.m.o" },
+    };
+
+    auto k = makeKey(home);
+    ASSERT_TRUE(populate_from(k, cacheProject, arts));
+
+    writeFile(project / "gcm.cache" / "mcpplibs.cmdline.gcm", "PROJECT-GCM");
+    writeFile(project / "obj"       / "cmdline.m.o",          "PROJECT-OBJ");
+    auto gcmTime = std::filesystem::last_write_time(project / "gcm.cache" / "mcpplibs.cmdline.gcm");
+    auto objTime = std::filesystem::last_write_time(project / "obj" / "cmdline.m.o");
+
+    auto staged = stage_into(k, project);
+    ASSERT_TRUE(staged) << staged.error();
+
+    {
+        std::ifstream is(project / "gcm.cache" / "mcpplibs.cmdline.gcm");
+        std::string body((std::istreambuf_iterator<char>(is)), {});
+        EXPECT_EQ(body, "PROJECT-GCM");
+    }
+    {
+        std::ifstream is(project / "obj" / "cmdline.m.o");
+        std::string body((std::istreambuf_iterator<char>(is)), {});
+        EXPECT_EQ(body, "PROJECT-OBJ");
+    }
+    EXPECT_EQ(std::filesystem::last_write_time(project / "gcm.cache" / "mcpplibs.cmdline.gcm"), gcmTime);
+    EXPECT_EQ(std::filesystem::last_write_time(project / "obj" / "cmdline.m.o"), objTime);
+}
+
 TEST(BmiCache, IsCachedFalseWhenSentinelExistsButFileMissing) {
     Tmp t;
     auto home    = t.path / "home";