[clang-tools-extra] [clangd] Respect background index priority (PR #212700)

[Michael Owens via cfe-commits]

https://github.com/why-does-ie-still-exist created https://github.com/llvm/llvm-project/pull/212700

The --background-index-priority CLI option was not wired correctly, leading to it being always 'low' even when otherwise specified.

>From 2f12b4c95e0387019c49265084cad825cedd266c Mon Sep 17 00:00:00 2001
From: Michael <33632547+why-does-ie-still-exist at users.noreply.github.com>
Date: Wed, 29 Jul 2026 00:34:06 -0700
Subject: [PATCH] [clangd] Respect background index priority

The --background-index-priority CLI option was not wired correctly, leading to it being always 'low' even when otherwise specified.
---
 clang-tools-extra/clangd/ClangdServer.cpp          | 14 ++++++++++----
 clang-tools-extra/clangd/ClangdServer.h            |  1 +
 clang-tools-extra/clangd/unittests/ClangdTests.cpp | 10 ++++++++++
 3 files changed, 21 insertions(+), 4 deletions(-)

diff --git a/clang-tools-extra/clangd/ClangdServer.cpp b/clang-tools-extra/clangd/ClangdServer.cpp
index 37eb82116f3a9..a176d8b2022e1 100644
--- a/clang-tools-extra/clangd/ClangdServer.cpp
+++ b/clang-tools-extra/clangd/ClangdServer.cpp
@@ -211,6 +211,15 @@ ClangdServer::Options::operator TUScheduler::Options() const {
   return Opts;
 }
 
+ClangdServer::Options::operator BackgroundIndex::Options() const {
+  BackgroundIndex::Options Opts;
+  Opts.ThreadPoolSize = std::max(AsyncThreadsCount, 1u);
+  Opts.IndexingPriority = BackgroundIndexPriority;
+  Opts.ContextProvider = ContextProvider;
+  Opts.SupportContainedRefs = EnableOutgoingCalls;
+  return Opts;
+}
+
 ClangdServer::ClangdServer(const GlobalCompilationDatabase &CDB,
                            const ThreadsafeFS &TFS, const Options &Opts,
                            Callbacks *Callbacks)
@@ -253,14 +262,11 @@ ClangdServer::ClangdServer(const GlobalCompilationDatabase &CDB,
   if (Opts.StaticIndex)
     AddIndex(Opts.StaticIndex);
   if (Opts.BackgroundIndex) {
-    BackgroundIndex::Options BGOpts;
-    BGOpts.ThreadPoolSize = std::max(Opts.AsyncThreadsCount, 1u);
+    BackgroundIndex::Options BGOpts(Opts);
     BGOpts.OnProgress = [Callbacks](BackgroundQueue::Stats S) {
       if (Callbacks)
         Callbacks->onBackgroundIndexProgress(S);
     };
-    BGOpts.ContextProvider = Opts.ContextProvider;
-    BGOpts.SupportContainedRefs = Opts.EnableOutgoingCalls;
     BackgroundIdx = std::make_unique<BackgroundIndex>(
         TFS, CDB,
         BackgroundIndexStorage::createDiskBackedStorageFactory(
diff --git a/clang-tools-extra/clangd/ClangdServer.h b/clang-tools-extra/clangd/ClangdServer.h
index 264ab7437c248..125aa761619bb 100644
--- a/clang-tools-extra/clangd/ClangdServer.h
+++ b/clang-tools-extra/clangd/ClangdServer.h
@@ -203,6 +203,7 @@ class ClangdServer {
     bool PublishInactiveRegions = false;
 
     explicit operator TUScheduler::Options() const;
+    explicit operator BackgroundIndex::Options() const;
   };
   // Sensible default options for use in tests.
   // Features like indexing must be enabled if desired.
diff --git a/clang-tools-extra/clangd/unittests/ClangdTests.cpp b/clang-tools-extra/clangd/unittests/ClangdTests.cpp
index 9ea7c3e02411d..6e99d7f2583b6 100644
--- a/clang-tools-extra/clangd/unittests/ClangdTests.cpp
+++ b/clang-tools-extra/clangd/unittests/ClangdTests.cpp
@@ -1361,6 +1361,16 @@ int endPreamble;
                   Source.range("inactive3"), Source.range("inactive4"))));
 }
 
+TEST(ClangdServer, BackgroundIndexPriorityPropagatesToIndexingThreads) {
+  auto Opts = ClangdServer::optsForTest();
+  for (auto Priority :
+       {llvm::ThreadPriority::Background, llvm::ThreadPriority::Low,
+        llvm::ThreadPriority::Default}) {
+    Opts.BackgroundIndexPriority = Priority;
+    EXPECT_EQ(BackgroundIndex::Options(Opts).IndexingPriority, Priority);
+  }
+}
+
 } // namespace
 } // namespace clangd
 } // namespace clang