[clang][extract-api] Enable processing of multiple headers

Before actually executing the ExtractAPIAction, clear the
CompilationInstance's input list and replace it with a single
synthesized file that just includes (or imports in ObjC) all the inputs.

Depends on D122141

Differential Revision: https://reviews.llvm.org/D122175

Author: daniel-grumberg

clang/include/clang/ExtractAPI/FrontendActions.h

@@ -24,9 +24,22 @@ class ExtractAPIAction : public ASTFrontendAction {
   std::unique_ptr<ASTConsumer> CreateASTConsumer(CompilerInstance &CI,
                                                  StringRef InFile) override;
 
+private:
+  /// The synthesized input buffer that contains all the provided input header
+  /// files.
+  std::unique_ptr<llvm::MemoryBuffer> Buffer;
+
 public:
+  /// Prepare to execute the action on the given CompilerInstance.
+  ///
+  /// This is called before executing the action on any inputs. This generates a
+  /// single header that includes all of CI's inputs and replaces CI's input
+  /// list with it before actually executing the action.
+  bool PrepareToExecuteAction(CompilerInstance &CI) override;
+
   static std::unique_ptr<llvm::raw_pwrite_stream>
   CreateOutputFile(CompilerInstance &CI, StringRef InFile);
+  static StringRef getInputBufferName() { return "<extract-api-includes>"; }
 };
 
 } // namespace clang

clang/lib/ExtractAPI/ExtractAPIConsumer.cpp

@@ -27,6 +27,9 @@
 #include "clang/ExtractAPI/Serialization/SymbolGraphSerializer.h"
 #include "clang/Frontend/ASTConsumers.h"
 #include "clang/Frontend/CompilerInstance.h"
+#include "clang/Frontend/FrontendOptions.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/raw_ostream.h"
 
 using namespace clang;
@@ -339,6 +342,35 @@ ExtractAPIAction::CreateASTConsumer(CompilerInstance &CI, StringRef InFile) {
       std::move(OS));
 }
 
+bool ExtractAPIAction::PrepareToExecuteAction(CompilerInstance &CI) {
+  auto &Inputs = CI.getFrontendOpts().Inputs;
+  if (Inputs.empty())
+    return true;
+
+  auto Kind = Inputs[0].getKind();
+
+  // Convert the header file inputs into a single input buffer.
+  SmallString<256> HeaderContents;
+  for (const FrontendInputFile &FIF : Inputs) {
+    if (Kind.isObjectiveC())
+      HeaderContents += "#import";
+    else
+      HeaderContents += "#include";
+    HeaderContents += " \"";
+    HeaderContents += FIF.getFile();
+    HeaderContents += "\"\n";
+  }
+
+  Buffer = llvm::MemoryBuffer::getMemBufferCopy(HeaderContents,
+                                                getInputBufferName());
+
+  // Set that buffer up as our "real" input in the CompilerInstance.
+  Inputs.clear();
+  Inputs.emplace_back(Buffer->getMemBufferRef(), Kind, /*IsSystem*/ false);
+
+  return true;
+}
+
 std::unique_ptr<raw_pwrite_stream>
 ExtractAPIAction::CreateOutputFile(CompilerInstance &CI, StringRef InFile) {
   std::unique_ptr<raw_pwrite_stream> OS =