Figure out the user's home directory if $HOME is not set

misc/launchd/org.nixos.nix-daemon.plist.in

@@ -16,8 +16,6 @@
     <dict>
       <key>NIX_SSL_CERT_FILE</key>
       <string>/nix/var/nix/profiles/default/etc/ssl/certs/ca-bundle.crt</string>
-      <key>XDG_CACHE_HOME</key>
-      <string>/root/.cache</string>
     </dict>
   </dict>
 </plist>

misc/systemd/nix-daemon.service.in

@@ -7,5 +7,3 @@ ConditionPathIsReadWrite=@localstatedir@/nix/daemon-socket
 [Service]
 ExecStart=@@bindir@/nix-daemon nix-daemon --daemon
 KillMode=process
-Environment=XDG_CACHE_HOME=/root/.cache
-Environment=XDG_CONFIG_HOME=/root/.config

src/libexpr/parser.y

@@ -376,7 +376,7 @@ expr_simple
       $$ = stripIndentation(CUR_POS, data->symbols, *$2);
   }
   | PATH { $$ = new ExprPath(absPath($1, data->basePath)); }
-  | HPATH { $$ = new ExprPath(getEnv("HOME", "") + string{$1 + 1}); }
+  | HPATH { $$ = new ExprPath(getHome() + string{$1 + 1}); }
   | SPATH {
       string path($1 + 1, strlen($1) - 2);
       $$ = new ExprApp(CUR_POS,

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;
+    }
+};
+
+}

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;
 }
 

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();
 

src/nix-channel/nix-channel.cc

@@ -169,9 +169,7 @@ int main(int argc, char ** argv)
         setenv("NIX_DOWNLOAD_CACHE", channelCache.c_str(), 1);
 
         // Figure out the name of the `.nix-channels' file to use
-        auto home = getEnv("HOME");
-        if (home.empty())
-            throw Error("$HOME not set");
+        auto home = getHome();
         channelsList = home + "/.nix-channels";
         nixDefExpr = home + "/.nix-defexpr";
 

src/nix-env/nix-env.cc

@@ -192,17 +192,9 @@ static void loadDerivations(EvalState & state, Path nixExprPath,
 }
 
 
-static Path getHomeDir()
-{
-    Path homeDir(getEnv("HOME", ""));
-    if (homeDir == "") throw Error("HOME environment variable not set");
-    return homeDir;
-}
-
-
 static Path getDefNixExprPath()
 {
-    return getHomeDir() + "/.nix-defexpr";
+    return getHome() + "/.nix-defexpr";
 }
 
 
@@ -1188,7 +1180,7 @@ static void opSwitchProfile(Globals & globals, Strings opFlags, Strings opArgs)
         throw UsageError(format("exactly one argument expected"));
 
     Path profile = absPath(opArgs.front());
-    Path profileLink = getHomeDir() + "/.nix-profile";
+    Path profileLink = getHome() + "/.nix-profile";
 
     switchLink(profileLink, profile);
 }
@@ -1413,7 +1405,7 @@ int main(int argc, char * * argv)
             globals.profile = getEnv("NIX_PROFILE", "");
 
         if (globals.profile == "") {
-            Path profileLink = getHomeDir() + "/.nix-profile";
+            Path profileLink = getHome() + "/.nix-profile";
             globals.profile = pathExists(profileLink)
                 ? absPath(readLink(profileLink), dirOf(profileLink))
                 : canonPath(settings.nixStateDir + "/profiles/default");