Fix temporary directory computation on linux (pr25147)

[labath]

Summary

On linux, the environment variables for temp directories that lldb checks for are generally not
defined, and the temp directory computation failed. This caused expression evaluation to fall
back to creating "/tmp/lldb-*.expr" debugging files instead of the usual
"$TMP/lldb/pid/lldb-*.expr". Crucially, these files were not cleaned up on lldb exit, which
caused clutter in the /tmp folder, especially on long-running machines (e.g. builtbots). This
commit fixes lldb to use llvm::sys::path::system_temp_directory, which does the same environment
variable dance, but (!) also falls back to the P_tmpdir macro, which is how the temp directory is
defined on linux.

Since the linux temp path computation now succeeds, I needed to also modify Android path
computation to check for actual directory existence, rather then checking whether the operation
failed.

lldb/trunk/source/Host/android/HostInfoAndroid.cpp

HostInfoAndroid::ResolveLibraryPath(const std::string& module_path, const ArchSpec& arch)
     }
 
     return FileSpec();
 }
 
 bool
 HostInfoAndroid::ComputeTempFileBaseDirectory(FileSpec &file_spec)
 {
-    if (HostInfoLinux::ComputeTempFileBaseDirectory(file_spec))
-        return true;
+    bool success = HostInfoLinux::ComputeTempFileBaseDirectory(file_spec);
 
-    // If the default mechanism for computing the temp directory failed then
-    // fall back to /data/local/tmp
+    // On Android, there is no path which is guaranteed to be writable. If the user has not
+    // provided a path via an environment variable, the generic algorithm will deduce /tmp, which
+    // is plain wrong. In that case we have an invalid directory, we substitute the path with
+    // /data/local/tmp, which is correct at least in some cases (i.e., when running as shell user).
+    if (!success || !file_spec.Exists())
         file_spec = FileSpec("/data/local/tmp", false);
-    return true;
+
+    return file_spec.Exists();
 }

lldb/trunk/source/Host/common/HostInfoBase.cpp

 #include "lldb/Host/FileSystem.h"
 #include "lldb/Host/Host.h"
 #include "lldb/Host/HostInfo.h"
 #include "lldb/Host/HostInfoBase.h"
 
 #include "llvm/ADT/Triple.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/Support/Host.h"
+#include "llvm/Support/Path.h"
 #include "llvm/Support/raw_ostream.h"
 
 #include <thread>
 #include <mutex> // std::once
 
 using namespace lldb;
 using namespace lldb_private;
 
HostInfoBase::ComputeProcessTempFileDirectory(FileSpec &file_spec)
     ::atexit(CleanupProcessSpecificLLDBTempDir);
     file_spec.GetDirectory().SetCString(temp_file_spec.GetCString());
     return true;
 }
 
 bool
 HostInfoBase::ComputeTempFileBaseDirectory(FileSpec &file_spec)
 {
-    file_spec.Clear();
-
-    const char *tmpdir_cstr = getenv("TMPDIR");
-    if (tmpdir_cstr == nullptr)
-    {
-        tmpdir_cstr = getenv("TMP");
-        if (tmpdir_cstr == nullptr)
-            tmpdir_cstr = getenv("TEMP");
-    }
-    if (!tmpdir_cstr)
-        return false;
-
-    file_spec = FileSpec(tmpdir_cstr, false);
+    llvm::SmallVector<char, 16> tmpdir;
+    llvm::sys::path::system_temp_directory(/*ErasedOnReboot*/ true, tmpdir);
+    file_spec = FileSpec(std::string(tmpdir.data(), tmpdir.size()), true);
     return true;
 }
 
 bool
 HostInfoBase::ComputeGlobalTempFileDirectory(FileSpec &file_spec)
 {
     file_spec.Clear();