[clang] [DependencyScanningFilesystem] Make sure the local/shared cache filename lookups use only absolute paths (PR #66122)

[Argyrios Kyrtzidis via cfe-commits]

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@@ -330,3 +353,20 @@ DependencyScanningWorkerFilesystem::openFileForRead(const Twine &Path) {
     return Result.getError();
   return DepScanFile::create(Result.get());
 }
+
+std::error_code DependencyScanningWorkerFilesystem::setCurrentWorkingDirectory(
+    const Twine &Path) {
+  updateWorkingDirForCacheLookup(Path.str());
+  return ProxyFileSystem::setCurrentWorkingDirectory(Path);
+}
+
+void DependencyScanningWorkerFilesystem::updateWorkingDirForCacheLookup(
+    llvm::ErrorOr<std::string> CWD) {
+  if (CWD && !CWD->empty()) {
+    WorkingDirForCacheLookup = *CWD;
+  } else {
+    // The cache lookup functions will not accept relative paths for safety, so
+    // at least make it absolute from a "root".
+    WorkingDirForCacheLookup = "/";
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akyrtzi wrote:

Oh wait, I see why it is poisoning, I could change it so that it avoids cache lookups if an error occurred?

https://github.com/llvm/llvm-project/pull/66122