Respect $XDG_CACHE_HOME if available for Bazel's output root on Linux…

… and BSD

It may well be that $HOME is on a read-only mount. In this case, it is the
convention in the Linux and BSD world that $XDG_CACHE_HOME points to an
appropriate writable location.

Fixes #16937

RELNOTES: Bazel on Linux and BSD now respects the XDG_CACHE_HOME environment
variable instead of assuming that ~/.cache/bazel is writable.
PiperOrigin-RevId: 614772057
Change-Id: I6377d7a90fb929843d18e82f5ed3d0adc55ac5c6

site/en/remote/output-directories.md

@@ -30,9 +30,10 @@ The solution that's currently implemented:
   subdirectory thereof. In other words, Bazel must be invoked from inside a
   [repository](../external/overview#repository). Otherwise, an error is
   reported.
-* The _outputRoot_ directory defaults to `~/.cache/bazel` on Linux,
-  `/private/var/tmp` on macOS, and on Windows it defaults to `%HOME%` if set,
-  else `%USERPROFILE%` if set, else the result of calling
+* The _outputRoot_ directory defaults to `${XDG_CACHE_HOME}/bazel` (or
+  `~/.cache/bazel`, if the `XDG_CACHE_HOME` environment variable is not set) on
+  Linux, `/private/var/tmp` on macOS, and on Windows it defaults to `%HOME%` if
+  set, else `%USERPROFILE%` if set, else the result of calling
   `SHGetKnownFolderPath()` with the `FOLDERID_Profile` flag set. If the
   environment variable `$TEST_TMPDIR` is set, as in a test of Bazel itself,
   then that value overrides the default.

src/main/cpp/blaze_util_bsd.cc

@@ -60,17 +60,24 @@ namespace blaze {
 using blaze_util::GetLastErrorString;
 using std::string;
 
+// ${XDG_CACHE_HOME}/bazel, a.k.a. ~/.cache/bazel by default (which is the
+// fallback when XDG_CACHE_HOME is not set)
 string GetOutputRoot() {
-  char buf[2048];
-  struct passwd pwbuf;
-  struct passwd *pw = nullptr;
-  int uid = getuid();
-  int r = getpwuid_r(uid, &pwbuf, buf, 2048, &pw);
-  if (r == 0 && pw != nullptr) {
-    return blaze_util::JoinPath(pw->pw_dir, ".cache/bazel");
-  } else {
-    return "/tmp";
+  string xdg_cache_home = GetPathEnv("XDG_CACHE_HOME");
+  if (xdg_cache_home.empty()) {
+    char buf[2048];
+    struct passwd pwbuf;
+    struct passwd *pw = nullptr;
+    int uid = getuid();
+    int r = getpwuid_r(uid, &pwbuf, buf, 2048, &pw);
+    if (r == 0 && pw != nullptr) {
+      xdg_cache_home = blaze_util::JoinPath(pw->pw_dir, ".cache");
+    } else {
+      return "/tmp";
+    }
   }
+
+  return blaze_util::JoinPath(xdg_cache_home, "bazel");
 }
 
 void WarnFilesystemType(const blaze_util::Path &output_base) {

src/main/cpp/blaze_util_linux.cc

@@ -43,27 +43,29 @@ using blaze_util::GetLastErrorString;
 using std::string;
 using std::vector;
 
+// ${XDG_CACHE_HOME}/bazel, a.k.a. ~/.cache/bazel by default (which is the
+// fallback when XDG_CACHE_HOME is not set)
 string GetOutputRoot() {
-  string base;
-  string home = GetHomeDir();
-  if (!home.empty()) {
-    base = home;
-  } else {
-    char buf[2048];
-    struct passwd pwbuf;
-    struct passwd *pw = nullptr;
-    int uid = getuid();
-    int r = getpwuid_r(uid, &pwbuf, buf, 2048, &pw);
-    if (r == 0 && pw != nullptr) {
-      base = pw->pw_dir;
+  string xdg_cache_home = GetPathEnv("XDG_CACHE_HOME");
+  if (xdg_cache_home.empty()) {
+    string home = GetHomeDir();  // via $HOME env variable
+    if (home.empty()) {
+      // Fall back to home dir from password database
+      char buf[2048];
+      struct passwd pwbuf;
+      struct passwd *pw = nullptr;
+      int uid = getuid();
+      int r = getpwuid_r(uid, &pwbuf, buf, 2048, &pw);
+      if (r == 0 && pw != nullptr) {
+        home = pw->pw_dir;
+      } else {
+        return "/tmp";
+      }
     }
+    xdg_cache_home = blaze_util::JoinPath(home, ".cache");
   }
 
-  if (!base.empty()) {
-    return blaze_util::JoinPath(base, ".cache/bazel");
-  }
-
-  return "/tmp";
+  return blaze_util::JoinPath(xdg_cache_home, "bazel");
 }
 
 void WarnFilesystemType(const blaze_util::Path &output_base) {

src/test/cpp/startup_options_test.cc

@@ -92,15 +92,27 @@ TEST_F(StartupOptionsTest, JavaLoggingOptions) {
 #ifdef __linux
 TEST_F(StartupOptionsTest, OutputRootPreferTestTmpdirIfSet) {
   SetEnv("HOME", "/nonexistent/home");
+  SetEnv("XDG_CACHE_HOME", "/nonexistent/cache");
   SetEnv("TEST_TMPDIR", "/nonexistent/tmpdir");
   ReinitStartupOptions();
 
   ASSERT_EQ("/nonexistent/tmpdir", startup_options_->output_root);
 }
 
+TEST_F(StartupOptionsTest,
+       OutputRootPreferXdgCacheHomeIfSetAndTestTmpdirUnset) {
+  SetEnv("HOME", "/nonexistent/home");
+  SetEnv("XDG_CACHE_HOME", "/nonexistent/cache");
+  UnsetEnv("TEST_TMPDIR");
+  ReinitStartupOptions();
+
+  ASSERT_EQ("/nonexistent/cache/bazel", startup_options_->output_root);
+}
+
 TEST_F(StartupOptionsTest, OutputRootUseHomeDirectory) {
   SetEnv("HOME", "/nonexistent/home");
   UnsetEnv("TEST_TMPDIR");
+  UnsetEnv("XDG_CACHE_HOME");
   ReinitStartupOptions();
 
   ASSERT_EQ("/nonexistent/home/.cache/bazel", startup_options_->output_root);