Add a facility to get system cache directory and use it in clangd

Summary:
This patch adds a function that is similar to `llvm::sys::path::home_directory`, but provides access to the system cache directory.

For Windows, that is %LOCALAPPDATA%, and applications should put their files under %LOCALAPPDATA%\Organization\Product\.

For *nixes, it adheres to the XDG Base Directory Specification, so it first looks at the XDG_CACHE_HOME environment variable and falls back to ~/.cache/.

Subsequently, the Clangd Index storage leverages this new API to put index files somewhere else than the users home directory.

Fixes https://github.com/clangd/clangd/issues/341

Reviewers: sammccall, chandlerc, Bigcheese

Reviewed By: sammccall

Subscribers: hiraditya, ilya-biryukov, MaskRay, jkorous, dexonsmith, arphaman, kadircet, ormris, usaxena95, cfe-commits, llvm-commits

Tags: #clang-tools-extra, #clang, #llvm

Differential Revision: https://reviews.llvm.org/D78501

include/llvm/Support/Path.h

@@ -368,6 +368,13 @@ void system_temp_directory(bool erasedOnReboot, SmallVectorImpl<char> &result);
 /// @result True if a home directory is set, false otherwise.
 bool home_directory(SmallVectorImpl<char> &result);
 
+/// Get the directory where installed packages should put their
+/// machine-local cache, e.g. $XDG_CACHE_HOME.
+///
+/// @param result Holds the resulting path name.
+/// @result True if the appropriate path was determined, it need not exist.
+bool cache_directory(SmallVectorImpl<char> &result);
+
 /// Has root name?
 ///
 /// root_name != ""

lib/Support/Unix/Path.inc

@@ -1138,6 +1138,19 @@ bool home_directory(SmallVectorImpl<char> &result) {
   return true;
 }
 
+bool cache_directory(SmallVectorImpl<char> &result) {
+  if (const char *RequestedDir = getenv("XDG_CACHE_HOME")) {
+    result.clear();
+    result.append(RequestedDir, RequestedDir + strlen(RequestedDir));
+    return true;
+  }
+  if (!home_directory(result)) {
+    return false;
+  }
+  append(result, ".cache");
+  return true;
+}
+
 static bool getDarwinConfDir(bool TempDir, SmallVectorImpl<char> &Result) {
   #if defined(_CS_DARWIN_USER_TEMP_DIR) && defined(_CS_DARWIN_USER_CACHE_DIR)
   // On Darwin, use DARWIN_USER_TEMP_DIR or DARWIN_USER_CACHE_DIR.

lib/Support/Windows/Path.inc

@@ -1372,6 +1372,10 @@ bool home_directory(SmallVectorImpl<char> &result) {
   return getKnownFolderPath(FOLDERID_Profile, result);
 }
 
+bool cache_directory(SmallVectorImpl<char> &result) {
+  return getKnownFolderPath(FOLDERID_LocalAppData, result);
+}
+
 static bool getTempDirEnvVar(const wchar_t *Var, SmallVectorImpl<char> &Res) {
   SmallVector<wchar_t, 1024> Buf;
   size_t Size = 1024;

unittests/Support/Path.cpp

@@ -13,6 +13,7 @@
 #include "llvm/ADT/Triple.h"
 #include "llvm/BinaryFormat/Magic.h"
 #include "llvm/Config/llvm-config.h"
+#include "llvm/Support/Compiler.h"
 #include "llvm/Support/ConvertUTF.h"
 #include "llvm/Support/Errc.h"
 #include "llvm/Support/ErrorHandling.h"
@@ -305,15 +306,46 @@ TEST(Support, AbsolutePathIteratorEnd) {
   }
 }
 
-TEST(Support, HomeDirectory) {
-  std::string expected;
 #ifdef _WIN32
-  if (wchar_t const *path = ::_wgetenv(L"USERPROFILE")) {
+std::string getEnvWin(const wchar_t *Var) {
+  std::string expected;
+  if (wchar_t const *path = ::_wgetenv(Var)) {
     auto pathLen = ::wcslen(path);
     ArrayRef<char> ref{reinterpret_cast<char const *>(path),
                        pathLen * sizeof(wchar_t)};
     convertUTF16ToUTF8String(ref, expected);
   }
+  return expected;
+}
+#else
+// RAII helper to set and restore an environment variable.
+class WithEnv {
+  const char *Var;
+  llvm::Optional<std::string> OriginalValue;
+
+public:
+  WithEnv(const char *Var, const char *Value) : Var(Var) {
+    if (const char *V = ::getenv(Var))
+      OriginalValue.emplace(V);
+    if (Value)
+      ::setenv(Var, Value, 1);
+    else
+      ::unsetenv(Var);
+    set(Value);
+  }
+  ~WithEnv() {
+    if (OriginalValue)
+      ::setenv(Var, OriginalValue->c_str(), 1);
+    else
+      ::unsetenv(Var);
+  }
+};
+#endif
+
+TEST(Support, HomeDirectory) {
+  std::string expected;
+#ifdef _WIN32
+  expected = getEnvWin(L"USERPROFILE");
 #else
   if (char const *path = ::getenv("HOME"))
     expected = path;
@@ -330,31 +362,48 @@ TEST(Support, HomeDirectory) {
 
 #ifdef LLVM_ON_UNIX
 TEST(Support, HomeDirectoryWithNoEnv) {
-  std::string OriginalStorage;
-  char const *OriginalEnv = ::getenv("HOME");
-  if (OriginalEnv) {
-    // We're going to unset it, so make a copy and save a pointer to the copy
-    // so that we can reset it at the end of the test.
-    OriginalStorage = OriginalEnv;
-    OriginalEnv = OriginalStorage.c_str();
-  }
+  WithEnv Env("HOME", nullptr);
 
   // Don't run the test if we have nothing to compare against.
   struct passwd *pw = getpwuid(getuid());
   if (!pw || !pw->pw_dir) return;
-
-  ::unsetenv("HOME");
-  EXPECT_EQ(nullptr, ::getenv("HOME"));
   std::string PwDir = pw->pw_dir;
 
   SmallString<128> HomeDir;
-  auto status = path::home_directory(HomeDir);
-  EXPECT_TRUE(status);
+  EXPECT_TRUE(path::home_directory(HomeDir));
   EXPECT_EQ(PwDir, HomeDir);
+}
 
-  // Now put the environment back to its original state (meaning that if it was
-  // unset before, we don't reset it).
-  if (OriginalEnv) ::setenv("HOME", OriginalEnv, 1);
+TEST(Support, CacheDirectoryWithEnv) {
+  WithEnv Env("XDG_CACHE_HOME", "/xdg/cache");
+
+  SmallString<128> CacheDir;
+  EXPECT_TRUE(path::cache_directory(CacheDir));
+  EXPECT_EQ("/xdg/cache", CacheDir);
+}
+
+TEST(Support, CacheDirectoryNoEnv) {
+  WithEnv Env("XDG_CACHE_HOME", nullptr);
+
+  SmallString<128> Fallback;
+  ASSERT_TRUE(path::home_directory(Fallback));
+  path::append(Fallback, ".cache");
+
+  SmallString<128> CacheDir;
+  EXPECT_TRUE(path::cache_directory(CacheDir));
+  EXPECT_EQ(Fallback, CacheDir);
+}
+#endif
+
+#ifdef _WIN32
+TEST(Support, CacheDirectory) {
+  std::string Expected = getEnvWin(L"LOCALAPPDATA");
+  // Do not try to test it if we don't know what to expect.
+  if (!Expected.empty()) {
+    SmallString<128> CacheDir;
+    EXPECT_TRUE(path::cache_directory(CacheDir));
+    EXPECT_EQ(Expected, CacheDir);
+  }
 }
 #endif