[Support] Add a new environment variable: LLVM_TEMP_DIR

[vsk]

Summary

The compiler creates unique, temporary files when storing diagnostic
information (e.g when a crash occurs). If an absolute path is not
specified, prefer LLVM_TEMP_DIR for the location of these files.

For example:

$ LLVM_TEMP_DIR=/BuildData clang-borked /tmp/x.c -o /tmp/x

Here, the compiler spits out diagnostic files into /BuildData.

Motivation: This would make it easier for our build infrastructure to extract diagnostic files from crashing bots.

[steven_wu]

Can you just use one of the TEMP environmental variable instead? I don't see any difference between your "LLVM_TEMP_DIR" and the temp dir environmental variable defined for Unix or Windows. That is:

// For Unix
const char *EnvironmentVariables[] = {"TMPDIR", "TMP", "TEMP", "TEMPDIR"};
// For Windows
const char *EnvironmentVariables[] = {"TMP", "TEMP", "USERPROFILE"};

[vsk]

Hm, yeah this is a bit kludgy.

[chapuni]

FYI, I have introduced $LIT_PRESERVES_TMP for it.

'LIT_PRESERVES_TMP': '1',
'TEMP':   WithProperties("%(workdir)s/tmp/TEMP"),
'TMP':    WithProperties("%(workdir)s/tmp/TMP"),
'TMPDIR': WithProperties("%(workdir)s/tmp/TMPDIR"),

factory.addStep(RemoveDirectory(
        dir=WithProperties("tmp"),
        flunkOnFailure=False))
factory.addStep(MakeDirectory(
        dir="tmp/TEMP",
        flunkOnFailure=False))
factory.addStep(MakeDirectory(
        dir="tmp/TMP",
        flunkOnFailure=False))
factory.addStep(MakeDirectory(
        dir="tmp/TMPDIR",
        flunkOnFailure=False))

My builders are capturing them in every build.

[vsk]

That seems cleaner.

Steven brought up another objection off-list which I think is worth mentioning here: using an alternate directory for _all_ temp files could drastically slow down our builds, since we're interesting in storing build data on a network share.

docs/TestingGuide.rst

 of Makefiles. The LNT based approach above is recommended for most
 users, but there are some testing scenarios which are not supported by
 the LNT approach. In addition, LNT currently uses the Makefile setup
 under the covers and so developers who are interested in how LNT works
 under the hood may want to understand the Makefile based setup.
 
 For more information on the ``test-suite`` Makefile setup, please see
 the :doc:`Test Suite Makefile Guide <TestSuiteMakefileGuide>`.
+
+Capturing Diagnostic Information
+--------------------------------
+
+The compiler may crash while running a test. In this situation, it may emit
+useful diagnostic information in the system temp directory. To specify an
+alternate temp directory, set the ``LLVM_TEMP_DIR`` environment variable.

lib/Support/Path.cpp

 } // end unnamed namespace
 
 enum FSEntity {
   FS_Dir,
   FS_File,
   FS_Name
 };
 
+template<unsigned N>
+static void getTempDirectory(bool ErasedOnReboot, SmallString<N> &TDir) {
+  Optional<std::string> TempDir = sys::Process::GetEnv("LLVM_TEMP_DIR");
+  if (TempDir.hasValue()) {
+    std::string &Dir = TempDir.getValue();
+    if (sys::fs::is_directory(Dir)) {
+      TDir.assign(Dir.begin(), Dir.end());
+      return;
+    }
+  }
+
+  sys::path::system_temp_directory(ErasedOnReboot, TDir);
+}
+
 static std::error_code createUniqueEntity(const Twine &Model, int &ResultFD,
                                           SmallVectorImpl<char> &ResultPath,
                                           bool MakeAbsolute, unsigned Mode,
                                           FSEntity Type) {
   SmallString<128> ModelStorage;
   Model.toVector(ModelStorage);
 
   if (MakeAbsolute) {
     // Make model absolute by prepending a temp directory if it's not already.
     if (!sys::path::is_absolute(Twine(ModelStorage))) {
       SmallString<128> TDir;
-      sys::path::system_temp_directory(true, TDir);
+      getTempDirectory(true, TDir);
       sys::path::append(TDir, Twine(ModelStorage));
       ModelStorage.swap(TDir);
     }
   }
 
   // From here on, DO NOT modify model. It may be needed if the randomly chosen
   // path already exists.
   ResultPath = ModelStorage;