feat: Make it possible to disable ccache for a certain source code file

5 files changed, 50 insertions, 2 deletions

diff --git a/doc/MANUAL.adoc b/doc/MANUAL.adoc
index 76a5663b..dc7c75f3 100644
--- a/doc/MANUAL.adoc
+++ b/doc/MANUAL.adoc
@@ -687,6 +687,9 @@ _<<Cache debugging>>_.
 
     When true, ccache will just call the real compiler, bypassing the cache
     completely. The default is false.
++
+It is also possible to disable ccache for a specific source code file by adding
+the string `ccache:disable` in a comment in the first 4096 bytes of the file.
 
 [#config_extra_files_to_hash]
 *extra_files_to_hash* (*CCACHE_EXTRAFILES*)::
@@ -1077,6 +1080,15 @@ filesystem as the `CCACHE_DIR` path, but this requirement has been relaxed.
     cache with other users.
 
 
+=== Disabling ccache
+
+To disable ccache completely for all invocations, set <<config_disable,*disable
+= true*>> (`CCACHE_DISABLE=1`). You can also disable ccache for a certain source
+code file by adding the string `ccache:disable` in a comment in the first 4096
+bytes of the file. In the latter case the `Ccache disabled` statistics counter
+will be increased.
+
+
 == Remote storage backends
 
 The <<config_remote_storage,*remote_storage*>> option lets you configure ccache
@@ -1379,6 +1391,9 @@ produce a single object file from a single source file.
 | Called for preprocessing |
 The compiler was called for preprocessing, not compiling.
 
+| Ccache disabled |
+Ccache was disabled by a `ccache:disable` string in the source code file.
+
 | Could not use modules |
 Preconditions for using <<C++ modules>> were not fulfilled.
 
diff --git a/src/ccache.cpp b/src/ccache.cpp
index 9efbd563..9547f538 100644
--- a/src/ccache.cpp
+++ b/src/ccache.cpp
@@ -86,6 +86,14 @@ using core::Statistic;
 // stored in the cache changes in a backwards-incompatible way.
 const char HASH_PREFIX[] = "4";
 
+// Search for k_ccache_disable_token within the first
+// k_ccache_disable_search_limit bytes of the input file.
+const size_t k_ccache_disable_search_limit = 4096;
+
+// String to look for when checking whether to disable ccache for the input
+// file.
+const char k_ccache_disable_token[] = "ccache:disable";
+
 namespace {
 
 // Return nonstd::make_unexpected<Failure> if ccache did not succeed in getting
@@ -136,6 +144,14 @@ Failure::set_exit_code(const int exit_code)
 
 } // namespace
 
+static bool
+should_disable_ccache_for_input_file(const std::string& path)
+{
+  auto content =
+    util::read_file_part<std::string>(path, 0, k_ccache_disable_search_limit);
+  return content && content->find(k_ccache_disable_token) != std::string::npos;
+}
+
 static void
 add_prefix(const Context& ctx, Args& args, const std::string& prefix_command)
 {
@@ -2488,6 +2504,13 @@ do_cache_compilation(Context& ctx)
   Hash common_hash;
   init_hash_debug(ctx, common_hash, 'c', "COMMON", debug_text_file);
 
+  if (should_disable_ccache_for_input_file(ctx.args_info.input_file)) {
+    LOG("{} found in {}, disabling ccache",
+        k_ccache_disable_token,
+        ctx.args_info.input_file);
+    return nonstd::make_unexpected(Statistic::disabled);
+  }
+
   {
     MTR_SCOPE("hash", "common_hash");
     TRY(hash_common_info(
diff --git a/src/core/Statistic.hpp b/src/core/Statistic.hpp
index ba39cfea..fb0e3790 100644
--- a/src/core/Statistic.hpp
+++ b/src/core/Statistic.hpp
@@ -1,4 +1,4 @@
-// Copyright (C) 2021-2022 Joel Rosdahl and other contributors
+// Copyright (C) 2021-2023 Joel Rosdahl and other contributors
 //
 // See doc/AUTHORS.adoc for a complete list of contributors.
 //
@@ -78,7 +78,8 @@ enum class Statistic {
   // 65-80: size (KiB) in level 2 subdirs 0-f
   subdir_size_kibibyte_base = 65,
 
-  END = 81
+  disabled = 81,
+  END = 82
 };
 
 } // namespace core
diff --git a/src/core/Statistics.cpp b/src/core/Statistics.cpp
index aa928a72..f00f40c8 100644
--- a/src/core/Statistics.cpp
+++ b/src/core/Statistics.cpp
@@ -90,6 +90,7 @@ const StatisticsField k_statistics_fields[] = {
         FLAG_UNCACHEABLE),
   FIELD(direct_cache_hit, nullptr),
   FIELD(direct_cache_miss, nullptr),
+  FIELD(disabled, "Ccache disabled", FLAG_UNCACHEABLE),
   FIELD(error_hashing_extra_file, "Error hashing extra file", FLAG_ERROR),
   FIELD(files_in_cache, nullptr, FLAG_NOZERO),
   FIELD(internal_error, "Internal error", FLAG_ERROR),
diff --git a/test/suites/base.bash b/test/suites/base.bash
index ff905942..7c54ec0e 100644
--- a/test/suites/base.bash
+++ b/test/suites/base.bash
@@ -393,6 +393,7 @@ if $RUN_WIN_XFAIL;then
     CCACHE_DEBUG=1 CCACHE_DEBUGDIR=debugdir $CCACHE_COMPILE -c test1.c
     expect_contains debugdir"$(pwd -P)"/test1.o.*.ccache-log "Result: cache_miss"
 fi
+
     # -------------------------------------------------------------------------
     TEST "CCACHE_DISABLE"
 
@@ -402,6 +403,13 @@ fi
     fi
 
     # -------------------------------------------------------------------------
+    TEST "ccache:disable"
+
+    echo '// ccache:disable' >>test1.c
+    $CCACHE_COMPILE -c test1.c 2>/dev/null
+    expect_stat disabled 1
+
+    # -------------------------------------------------------------------------
     TEST "CCACHE_COMMENTS"
 
     $COMPILER -c -o reference_test1.o test1.c