https://bugs.gentoo.org/927740
https://github.com/ccache/ccache/issues/1406
https://github.com/ccache/ccache/commit/33d390bb249ddf1a6b3bdecede71b12c70000da3

From 33d390bb249ddf1a6b3bdecede71b12c70000da3 Mon Sep 17 00:00:00 2001
From: Joel Rosdahl <joel@rosdahl.net>
Date: Wed, 21 Feb 2024 20:25:03 +0100
Subject: [PATCH] fix: Allow nonexistent include files in preprocessor mode

After 43c3a44aadcb nonexistent include files found in the preprocessed
output would increment "Could not read or parse input file" and fall
back to just running the compiler. This made ccache not attempt
preprocessor mode for generated code with #line header referencing files
that can't be found, e.g. in an out-of-source build scenario.

Fix this by just disabling direct mode, restoring the pre-43c3a44aadcb
behavior in such cases.

Fixes #1406.

(cherry picked from commit 08da6cd4da41fa213d387f52e3cf1d1f2a42366b)
--- a/src/ccache.cpp
+++ b/src/ccache.cpp
@@ -352,7 +352,12 @@ remember_include_file(Context& ctx,
 
   DirEntry dir_entry(path, DirEntry::LogOnError::yes);
   if (!dir_entry.exists()) {
-    return tl::unexpected(Statistic::bad_input_file);
+    if (ctx.config.direct_mode()) {
+      LOG("Include file {} does not exist, disabling direct mode",
+          dir_entry.path());
+      ctx.config.set_direct_mode(false);
+    }
+    return {};
   }
   if (dir_entry.is_directory()) {
     // Ignore directory, typically $PWD.