3 files changed, 79 insertions, 17 deletions
diff --git a/src/libutil/lazy.hh b/src/libutil/lazy.hh
new file mode 100644
index 000000000000..d073e486c2eb
--- /dev/null
+++ b/src/libutil/lazy.hh
@@ -0,0 +1,48 @@
+#include <exception>
+#include <functional>
+#include <mutex>
+
+namespace nix {
+
+/* A helper class for lazily-initialized variables.
+
+     Lazy<T> var([]() { return value; });
+
+   declares a variable of type T that is initialized to 'value' (in a
+   thread-safe way) on first use, that is, when var() is first
+   called. If the initialiser code throws an exception, then all
+   subsequent calls to var() will rethrow that exception. */
+template<typename T>
+class Lazy
+{
+
+    typedef std::function<T()> Init;
+
+    Init init;
+
+    std::once_flag done;
+
+    T value;
+
+    std::exception_ptr ex;
+
+public:
+
+    Lazy(Init init) : init(init)
+    { }
+
+    const T & operator () ()
+    {
+        std::call_once(done, [&]() {
+            try {
+                value = init();
+            } catch (...) {
+                ex = std::current_exception();
+            }
+        });
+        if (ex) std::rethrow_exception(ex);
+        return value;
+    }
+};
+
+}
diff --git a/src/libutil/util.cc b/src/libutil/util.cc
index 98c0aff1e722..1d1f68fc8452 100644
--- a/src/libutil/util.cc
+++ b/src/libutil/util.cc
@@ -1,3 +1,4 @@
+#include "lazy.hh"
 #include "util.hh"
 #include "affinity.hh"
 #include "sync.hh"
@@ -13,10 +14,12 @@
 #include <thread>
 #include <future>
 
-#include <sys/wait.h>
-#include <unistd.h>
 #include <fcntl.h>
 #include <limits.h>
+#include <pwd.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <unistd.h>
 
 #ifdef __APPLE__
 #include <sys/syscall.h>
@@ -417,14 +420,28 @@ Path createTempDir(const Path & tmpRoot, const Path & prefix,
 }
 
 
+static Lazy<Path> getHome2([]() {
+    Path homeDir = getEnv("HOME");
+    if (homeDir.empty()) {
+        char buf[16384];
+        struct passwd pwbuf;
+        struct passwd * pw;
+        if (getpwuid_r(getuid(), &pwbuf, buf, sizeof(buf), &pw) != 0
+            || !pw || !pw->pw_dir || !pw->pw_dir[0])
+            throw Error("cannot determine user's home directory");
+        homeDir = pw->pw_dir;
+    }
+    return homeDir;
+});
+
+Path getHome() { return getHome2(); }
+
+
 Path getCacheDir()
 {
     Path cacheDir = getEnv("XDG_CACHE_HOME");
-    if (cacheDir.empty()) {
-        Path homeDir = getEnv("HOME");
-        if (homeDir.empty()) throw Error("$XDG_CACHE_HOME and $HOME are not set");
-        cacheDir = homeDir + "/.cache";
-    }
+    if (cacheDir.empty())
+        cacheDir = getHome() + "/.cache";
     return cacheDir;
 }
 
@@ -432,11 +449,8 @@ Path getCacheDir()
 Path getConfigDir()
 {
     Path configDir = getEnv("XDG_CONFIG_HOME");
-    if (configDir.empty()) {
-        Path homeDir = getEnv("HOME");
-        if (homeDir.empty()) throw Error("$XDG_CONFIG_HOME and $HOME are not set");
-        configDir = homeDir + "/.config";
-    }
+    if (configDir.empty())
+        configDir = getHome() + "/.config";
     return configDir;
 }
 
@@ -444,11 +458,8 @@ Path getConfigDir()
 Path getDataDir()
 {
     Path dataDir = getEnv("XDG_DATA_HOME");
-    if (dataDir.empty()) {
-        Path homeDir = getEnv("HOME");
-        if (homeDir.empty()) throw Error("$XDG_DATA_HOME and $HOME are not set");
-        dataDir = homeDir + "/.local/share";
-    }
+    if (dataDir.empty())
+        dataDir = getHome() + "/.local/share";
     return dataDir;
 }
 
diff --git a/src/libutil/util.hh b/src/libutil/util.hh
index a9950f830c50..5a9c9513fd5c 100644
--- a/src/libutil/util.hh
+++ b/src/libutil/util.hh
@@ -110,6 +110,9 @@ void deletePath(const Path & path, unsigned long long & bytesFreed);
 Path createTempDir(const Path & tmpRoot = "", const Path & prefix = "nix",
     bool includePid = true, bool useGlobalCounter = true, mode_t mode = 0755);
 
+/* Return $HOME or the user's home directory from /etc/passwd. */
+Path getHome();
+
 /* Return $XDG_CACHE_HOME or $HOME/.cache. */
 Path getCacheDir();