[clang] [clang][deps] Keep module cache in memory (PR #192347)

[Michael Spencer via cfe-commits]

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@@ -134,22 +139,39 @@ class InProcessModuleCache : public ModuleCache {
 
   std::error_code write(StringRef Path, llvm::MemoryBufferRef Buffer,
                         off_t &Size, time_t &ModTime) override {
-    // This is a compiler-internal input/output, let's bypass the sandbox.
-    auto BypassSandbox = llvm::sys::sandbox::scopedDisable();
-
-    // FIXME: This could use an in-memory cache to avoid IO, and only write to
-    // disk at the end of the scan.
-    return writeImpl(Path, Buffer, Size, ModTime);
+    ModuleCacheEntry &Entry = getOrCreateEntry(Path);
+    std::lock_guard<std::mutex> Lock(Entry.Mutex);
+    if (Entry.State == ModuleCacheEntry::S_Written) {
+      assert(Entry.Buffer && *Entry.Buffer == Buffer &&
+             "Wrote the same PCM with different contents");
+      return {};
+    }
+    Entry.Buffer =
+        llvm::MemoryBuffer::getMemBufferCopy(Buffer.getBuffer(), Path);
+    Entry.ModTime = llvm::sys::toTimeT(std::chrono::system_clock::now());
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Bigcheese wrote:

Ah, I forgot that only applies to explicitly built pcms. I think that's fine then as this only impacts IBM. May be good to have a comment there.

https://github.com/llvm/llvm-project/pull/192347