[clangd] Prevent tests from failing when /tmp/compile_flags.txt exists

[adamcz]

Summary

The BuildDir test uses RealFS and would fail when /tmp/compile_flags.txt
or /tmp/compile_commands.json would exist, since it would read it for
files not covered by the CDB it creates and then fail an IsEmpty() test.

With this change, we create a compile_flags.txt file simply to prevent
CDB from looking for it in parent directory.

Long-term, all CDB implementations should use VFS and the test should
use MemoryFS instead, and be fully hermetic, but for now this will do.

Fixes https://github.com/clangd/clangd/issues/354

[hokein]

Thanks, this looks like a reasonable workaround.

clang-tools-extra/clangd/unittests/GlobalCompilationDatabaseTests.cpp

TEST(GlobalCompilationDatabaseTest, BuildDir) {
   RealThreadsafeFS TFS;
   auto Command = [&](llvm::StringRef Relative) {
     DirectoryBasedGlobalCompilationDatabase::Options Opts(TFS);
     return DirectoryBasedGlobalCompilationDatabase(Opts)
         .getCompileCommand(FS.path(Relative))
         .getValueOr(tooling::CompileCommand())
         .CommandLine;
   };
-  EXPECT_THAT(Command("x/foo.cc"), IsEmpty());
+
+  // Since we use real FS and place our files somewhere in /tmp/, an existing
+  // /tmp/compile_flags.txt or /tmp/compile_commands.json could affect the test.
+  // By creating this file we prevent CDB from looking outside of the test
+  // directory.
+  FS.write("compile_flags.txt", "-DFOO");
+
+  EXPECT_THAT(Command("x/foo.cc"), Contains("-DFOO"));
   const char *const CDB =
       R"cdb(
       [
         {
           "file": "{0}/x/foo.cc",
           "command": "clang -DXYZZY {0}/x/foo.cc",
           "directory": "{0}",
         },
         {
           "file": "{0}/bar.cc",
           "command": "clang -DXYZZY {0}/bar.cc",
           "directory": "{0}",
         }
       ]
       )cdb";
   FS.write("x/build/compile_commands.json", CDB);
-  EXPECT_THAT(Command("x/foo.cc"), Contains("-DXYZZY"));
-  EXPECT_THAT(Command("bar.cc"), IsEmpty())
-      << "x/build/compile_flags.json only applicable to x/";
+  EXPECT_THAT(Command("x/foo.cc"),
+              AllOf(Contains("-DXYZZY"), Not(Contains("-DFOO"))));
+  EXPECT_THAT(Command("bar.cc"),
+              AllOf(Contains("-DFOO"), Not(Contains("-DXYZZY"))))
+      << "x/build/compile_commands.json only applicable to x/";
 }
 
 TEST(GlobalCompilationDatabaseTest, NonCanonicalFilenames) {
   OverlayCDB DB(nullptr);
   std::vector<std::string> DiscoveredFiles;
   auto Sub =
       DB.watch([&DiscoveredFiles](const std::vector<std::string> Changes) {
         DiscoveredFiles = Changes;