Add .gitignore file in .cache/clangd/index

[mrexodia]

This solves a common issue where users have to manually add the .cache/clangd/index/ folder to their .gitignore. I got this idea from ruff, which creates .ruff_cache/.gitignore and it would greatly improve the user experience for everyone without requiring per-computer configurations and without any significant cost.

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[llvmbot]

@llvm/pr-subscribers-clang-tools-extra

@llvm/pr-subscribers-clangd

Author: Duncan Ogilvie (mrexodia)

Changes

This solves a common issue where users have to manually add the .cache/clangd/index/ folder to their .gitignore. I got this idea from ruff, which creates .ruff_cache/.gitignore and it would greatly improve the user experience for everyone without requiring per-computer configurations and without any significant cost.

---

Full diff: https://github.com/llvm/llvm-project/pull/170003.diff

1 Files Affected:

• (modified) clang-tools-extra/clangd/index/BackgroundIndexStorage.cpp (+14)

diff --git a/clang-tools-extra/clangd/index/BackgroundIndexStorage.cpp b/clang-tools-extra/clangd/index/BackgroundIndexStorage.cpp
index d887b09482a95..1dff0b55f82cd 100644
--- a/clang-tools-extra/clangd/index/BackgroundIndexStorage.cpp
+++ b/clang-tools-extra/clangd/index/BackgroundIndexStorage.cpp
@@ -45,6 +45,20 @@ class DiskBackedIndexStorage : public BackgroundIndexStorage {
     if (EC != OK) {
       elog("Failed to create directory {0} for index storage: {1}",
            DiskShardRoot, EC.message());
+    } else {
+      // Create a .gitignore file in the directory to ignore all files.
+      llvm::SmallString<128> GitignorePath(DiskShardRoot);
+      llvm::sys::path::append(GitignorePath, ".gitignore");
+      if (!llvm::sys::fs::exists(GitignorePath)) {
+        llvm::raw_fd_ostream GitignoreFile(GitignorePath, EC,
+                                          llvm::sys::fs::OF_None);
+        if (EC == OK) {
+          GitignoreFile << "*\n";
+        } else {
+          elog("Failed to create .gitignore in {0}: {1}", DiskShardRoot,
+              EC.message());
+        }
+      }
     }
   }
 

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[mrexodia]

• Apparently there was a previous patch for this that never got merged: https://reviews.llvm.org/D130863
• Discussion thread that went nowhere: https://discourse.llvm.org/t/clangd-feature-request-automatically-generate-gitignore-in-the-cache-directory/87390
• Cargo is discussing adopting a similar approach: instead of writing a .gitignore with /target -- output a .gitignore with * *inside* target/ rust-lang/cargo#11548 (comment)
• virtualenv does this https://virtualenv.pypa.io/en/latest/changelog.html#bugfixes-20-0-24
• Meson does this too
• Loads of projects add this to their .gitignore already: https://github.com/search?type=code&q=.cache%2Fclangd