fix: Disambiguate manifest and result hash for MSVC depend mode

When using depend mode with MSVC and the source file doesn't include any
file, the result hash becomes identical to the manifest hash. Fix this
by adding a "result" hash delimiter, similar to how it's done in the
non-MSVC case (result_key_from_depfile).

This bug has existed since the addition of depend mode support for MSVC
(b1348e5f5ebe10b486b4e86f0114884a04f9531a). It's not a big deal, but the
storage layer will use the same key, so manifest or result parsing might
fail and fall back to just running the compiler.

diff --git a/src/ccache/ccache.cpp b/src/ccache/ccache.cpp
index 39cdd53aad242c00a23edf2e87f7d77e30851b47..d4fe5de4aab57a3a90bbc7a11a1eb290a2fd4cfd 100644 (file)
--- a/src/ccache/ccache.cpp
+++ b/src/ccache/ccache.cpp
@@ -728,9 +728,8 @@ process_preprocessed_file(Context& ctx, Hash& hash, const fs::path& path)
 static tl::expected<Hash::Digest, Failure>
 result_key_from_depfile(Context& ctx, Hash& hash)
 {
-  // Make sure that result hash will always be different from the manifest hash
-  // since there otherwise may a storage key collision (in case the dependency
-  // file is empty).
+  // Ensure the result hash always differs from the manifest hash to avoid
+  // storage key collisions when the dependency file is empty.
   hash.hash_delimiter("result");
 
   const auto file_content =
@@ -804,6 +803,10 @@ struct DoExecuteResult
 static tl::expected<Hash::Digest, Failure>
 result_key_from_includes(Context& ctx, Hash& hash, std::string_view stdout_data)
 {
+  // Ensure the result hash always differs from the manifest hash to avoid
+  // storage key collisions when there are no included files.
+  hash.hash_delimiter("result");
+
   for (std::string_view include :
        compiler::get_includes_from_msvc_show_includes(
          stdout_data, ctx.config.msvc_dep_prefix())) {