[CMake] Don't exclude llvm-debuginfod if httplib isn't available

[phosek]

Summary

This will result in a tool that's not usable, but it'll still link
correctly rather than failing with a CMake error. This matches what
we do for other tools that have dependencies such as llvm-mt.

[mysterymath]

Could we include a similarly descriptive error message in llvm-debuginfod as in llvm-mt as part of this? As is, this will produce a binary with undefined behavior, as it will hit a llvm_unreachable when attempting to use HTTPServer. Seems like HTTPServer is the right place for this; it should probably report_fatal_error instead of llvm_unreachable, and mention the specific library dep.

[phosek]

In D155323#4501920, @mysterymath wrote:

Could we include a similarly descriptive error message in llvm-debuginfod as in llvm-mt as part of this? As is, this will produce a binary with undefined behavior, as it will hit a llvm_unreachable when attempting to use HTTPServer. Seems like HTTPServer is the right place for this; it should probably report_fatal_error instead of llvm_unreachable, and mention the specific library dep.

Done, this now prints no httplib akin to llvm-mt.

llvm/include/llvm/Debuginfod/HTTPServer.h

 #endif
 
 namespace llvm {
 
 struct HTTPResponse;
 struct StreamingHTTPResponse;
 class HTTPServer;
 
+class HTTPServerError : public ErrorInfo<HTTPServerError, ECError> {
+public:
+  static char ID;
+  HTTPServerError(const Twine &Msg);
+  void log(raw_ostream &OS) const override;
+
+private:
+  std::string Msg;
+};
+
 class HTTPServerRequest {
   friend HTTPServer;
 
 #ifdef LLVM_ENABLE_HTTPLIB
 private:
   HTTPServerRequest(const httplib::Request &HTTPLibRequest,
                     httplib::Response &HTTPLibResponse);
   httplib::Response &HTTPLibResponse;

llvm/lib/Debuginfod/HTTPServer.cpp

 #include "llvm/Support/Regex.h"
 
 #ifdef LLVM_ENABLE_HTTPLIB
 #include "httplib.h"
 #endif
 
 using namespace llvm;
 
+char HTTPServerError::ID = 0;
+
+HTTPServerError::HTTPServerError(const Twine &Msg) : Msg(Msg.str()) {}
+
+void HTTPServerError::log(raw_ostream &OS) const { OS << Msg; }
+
 bool llvm::streamFile(HTTPServerRequest &Request, StringRef FilePath) {
   Expected<sys::fs::file_t> FDOrErr = sys::fs::openNativeFileForRead(FilePath);
   if (Error Err = FDOrErr.takeError()) {
     consumeError(std::move(Err));
     Request.setResponse({404u, "text/plain", "Could not open file to read.\n"});
     return false;
   }
   ErrorOr<std::unique_ptr<MemoryBuffer>> MBOrErr =

 // TODO: Implement barebones standalone HTTP server implementation.
 bool HTTPServer::isAvailable() { return false; }
 
 HTTPServer::HTTPServer() = default;
 
 HTTPServer::~HTTPServer() = default;
 
 void HTTPServerRequest::setResponse(HTTPResponse Response) {
-  llvm_unreachable("No HTTP server implementation available");
+  llvm_unreachable("no httplib");
 }
 
 void HTTPServerRequest::setResponse(StreamingHTTPResponse Response) {
-  llvm_unreachable("No HTTP server implementation available");
+  llvm_unreachable("no httplib");
 }
 
 Error HTTPServer::get(StringRef UrlPathPattern, HTTPRequestHandler Handler) {
-  llvm_unreachable("No HTTP server implementation available");
+  return Error::success();
 }
 
 Error HTTPServer::bind(unsigned ListenPort, const char *HostInterface) {
-  llvm_unreachable("No HTTP server implementation available");
+  return make_error<HTTPServerError>("no httplib");
 }
 
 Expected<unsigned> HTTPServer::bind(const char *HostInterface) {
-  llvm_unreachable("No HTTP server implementation available");
+  return make_error<HTTPServerError>("no httplib");
 }
 
 Error HTTPServer::listen() {
-  llvm_unreachable("No HTTP server implementation available");
+  return make_error<HTTPServerError>("no httplib");
 }
 
 void HTTPServer::stop() {
-  llvm_unreachable("No HTTP server implementation available");
+  llvm_unreachable("no httplib");
 }
 
 #endif // LLVM_ENABLE_HTTPLIB

llvm/tools/CMakeLists.txt

 if(NOT LLVM_USE_INTEL_JITEVENTS )
   set(LLVM_TOOL_LLVM_JITLISTENER_BUILD Off)
 endif()
 
 if(CYGWIN OR NOT LLVM_ENABLE_PIC)
   set(LLVM_TOOL_LTO_BUILD Off)
 endif()
 
-if(NOT LLVM_ENABLE_HTTPLIB)
-  set(LLVM_TOOL_LLVM_DEBUGINFOD_BUILD Off)
-endif()
-
 if (LLVM_TOOL_LLVM_DRIVER_BUILD)
   add_llvm_tool(llvm-driver)
 endif()
 
 # Add LTO, llvm-ar, llvm-config, and llvm-profdata before clang, ExternalProject
 # requires targets specified in DEPENDS to exist before the call to
 # ExternalProject_Add.
 add_llvm_tool_subdirectory(lto)