getEnv(): Return std::optional

This allows distinguishing between an empty value and no value.

src/libexpr/eval.cc

@@ -217,7 +217,7 @@ void initGC()
        that GC_expand_hp() causes a lot of virtual, but not physical
        (resident) memory to be allocated.  This might be a problem on
        systems that don't overcommit. */
-    if (!getenv("GC_INITIAL_HEAP_SIZE")) {
+    if (!getEnv("GC_INITIAL_HEAP_SIZE")) {
         size_t size = 32 * 1024 * 1024;
 #if HAVE_SYSCONF && defined(_SC_PAGESIZE) && defined(_SC_PHYS_PAGES)
         size_t maxSize = 384 * 1024 * 1024;
@@ -306,7 +306,7 @@ EvalState::EvalState(const Strings & _searchPath, ref<Store> store)
     , baseEnv(allocEnv(128))
     , staticBaseEnv(false, 0)
 {
-    countCalls = getEnv("NIX_COUNT_CALLS", "0") != "0";
+    countCalls = getEnv("NIX_COUNT_CALLS").value_or("0") != "0";
 
     assert(gcInitialised);
 
@@ -314,7 +314,7 @@ EvalState::EvalState(const Strings & _searchPath, ref<Store> store)
 
     /* Initialise the Nix expression search path. */
     if (!evalSettings.pureEval) {
-        Strings paths = parseNixPath(getEnv("NIX_PATH", ""));
+        Strings paths = parseNixPath(getEnv("NIX_PATH").value_or(""));
         for (auto & i : _searchPath) addToSearchPath(i);
         for (auto & i : paths) addToSearchPath(i);
     }
@@ -1753,7 +1753,7 @@ bool EvalState::eqValues(Value & v1, Value & v2)
 
 void EvalState::printStats()
 {
-    bool showStats = getEnv("NIX_SHOW_STATS", "0") != "0";
+    bool showStats = getEnv("NIX_SHOW_STATS").value_or("0") != "0";
 
     struct rusage buf;
     getrusage(RUSAGE_SELF, &buf);
@@ -1769,7 +1769,7 @@ void EvalState::printStats()
     GC_get_heap_usage_safe(&heapSize, 0, 0, 0, &totalBytes);
 #endif
     if (showStats) {
-        auto outPath = getEnv("NIX_SHOW_STATS_PATH","-");
+        auto outPath = getEnv("NIX_SHOW_STATS_PATH").value_or("-");
         std::fstream fs;
         if (outPath != "-")
             fs.open(outPath, std::fstream::out);
@@ -1861,7 +1861,7 @@ void EvalState::printStats()
             }
         }
 
-        if (getEnv("NIX_SHOW_SYMBOLS", "0") != "0") {
+        if (getEnv("NIX_SHOW_SYMBOLS").value_or("0") != "0") {
             auto list = topObj.list("symbols");
             symbols.dump([&](const std::string & s) { list.elem(s); });
         }

src/libexpr/primops.cc

@@ -469,7 +469,7 @@ static void prim_tryEval(EvalState & state, const Pos & pos, Value * * args, Val
 static void prim_getEnv(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
     string name = state.forceStringNoCtx(*args[0], pos);
-    mkString(v, evalSettings.restrictEval || evalSettings.pureEval ? "" : getEnv(name));
+    mkString(v, evalSettings.restrictEval || evalSettings.pureEval ? "" : getEnv(name).value_or(""));
 }
 
 

src/libstore/build.cc

@@ -2503,7 +2503,7 @@ void DerivationGoal::initEnv()
        already know the cryptographic hash of the output). */
     if (fixedOutput) {
         for (auto & i : parsedDrv->getStringsAttr("impureEnvVars").value_or(Strings()))
-            env[i] = getEnv(i);
+            env[i] = getEnv(i).value_or("");
     }
 
     /* Currently structured log messages piggyback on stderr, but we
@@ -3075,7 +3075,7 @@ void DerivationGoal::runChild()
 
             /* The tmpDir in scope points at the temporary build directory for our derivation. Some packages try different mechanisms
                to find temporary directories, so we want to open up a broader place for them to dump their files, if needed. */
-            Path globalTmpDir = canonPath(getEnv("TMPDIR", "/tmp"), true);
+            Path globalTmpDir = canonPath(getEnv("TMPDIR").value_or("/tmp"), true);
 
             /* They don't like trailing slashes on subpath directives */
             if (globalTmpDir.back() == '/') globalTmpDir.pop_back();

src/libstore/gc.cc

@@ -870,11 +870,11 @@ void LocalStore::collectGarbage(const GCOptions & options, GCResults & results)
 
 void LocalStore::autoGC(bool sync)
 {
-    static auto fakeFreeSpaceFile = getEnv("_NIX_TEST_FREE_SPACE_FILE", "");
+    static auto fakeFreeSpaceFile = getEnv("_NIX_TEST_FREE_SPACE_FILE");
 
     auto getAvail = [this]() -> uint64_t {
-        if (!fakeFreeSpaceFile.empty())
+        if (fakeFreeSpaceFile)
-            return std::stoll(readFile(fakeFreeSpaceFile));
+            return std::stoll(readFile(*fakeFreeSpaceFile));
 
         struct statvfs st;
         if (statvfs(realStoreDir.c_str(), &st))

src/libstore/globals.cc

@@ -32,20 +32,20 @@ static GlobalConfig::Register r1(&settings);
 
 Settings::Settings()
     : nixPrefix(NIX_PREFIX)
-    , nixStore(canonPath(getEnv("NIX_STORE_DIR", getEnv("NIX_STORE", NIX_STORE_DIR))))
+    , nixStore(canonPath(getEnv("NIX_STORE_DIR").value_or(getEnv("NIX_STORE").value_or(NIX_STORE_DIR))))
-    , nixDataDir(canonPath(getEnv("NIX_DATA_DIR", NIX_DATA_DIR)))
+    , nixDataDir(canonPath(getEnv("NIX_DATA_DIR").value_or(NIX_DATA_DIR)))
-    , nixLogDir(canonPath(getEnv("NIX_LOG_DIR", NIX_LOG_DIR)))
+    , nixLogDir(canonPath(getEnv("NIX_LOG_DIR").value_or(NIX_LOG_DIR)))
-    , nixStateDir(canonPath(getEnv("NIX_STATE_DIR", NIX_STATE_DIR)))
+    , nixStateDir(canonPath(getEnv("NIX_STATE_DIR").value_or(NIX_STATE_DIR)))
-    , nixConfDir(canonPath(getEnv("NIX_CONF_DIR", NIX_CONF_DIR)))
+    , nixConfDir(canonPath(getEnv("NIX_CONF_DIR").value_or(NIX_CONF_DIR)))
-    , nixLibexecDir(canonPath(getEnv("NIX_LIBEXEC_DIR", NIX_LIBEXEC_DIR)))
+    , nixLibexecDir(canonPath(getEnv("NIX_LIBEXEC_DIR").value_or(NIX_LIBEXEC_DIR)))
-    , nixBinDir(canonPath(getEnv("NIX_BIN_DIR", NIX_BIN_DIR)))
+    , nixBinDir(canonPath(getEnv("NIX_BIN_DIR").value_or(NIX_BIN_DIR)))
     , nixManDir(canonPath(NIX_MAN_DIR))
     , nixDaemonSocketFile(canonPath(nixStateDir + DEFAULT_SOCKET_PATH))
 {
     buildUsersGroup = getuid() == 0 ? "nixbld" : "";
-    lockCPU = getEnv("NIX_AFFINITY_HACK", "1") == "1";
+    lockCPU = getEnv("NIX_AFFINITY_HACK") == "1";
 
-    caFile = getEnv("NIX_SSL_CERT_FILE", getEnv("SSL_CERT_FILE", ""));
+    caFile = getEnv("NIX_SSL_CERT_FILE").value_or(getEnv("SSL_CERT_FILE").value_or(""));
     if (caFile == "") {
         for (auto & fn : {"/etc/ssl/certs/ca-certificates.crt", "/nix/var/nix/profiles/default/etc/ssl/certs/ca-bundle.crt"})
             if (pathExists(fn)) {
@@ -56,9 +56,9 @@ Settings::Settings()
 
     /* Backwards compatibility. */
     auto s = getEnv("NIX_REMOTE_SYSTEMS");
-    if (s != "") {
+    if (s) {
         Strings ss;
-        for (auto & p : tokenizeString<Strings>(s, ":"))
+        for (auto & p : tokenizeString<Strings>(*s, ":"))
             ss.push_back("@" + p);
         builders = concatStringsSep(" ", ss);
     }

src/libstore/globals.hh

@@ -66,7 +66,7 @@ public:
     /* File name of the socket the daemon listens to.  */
     Path nixDaemonSocketFile;
 
-    Setting<std::string> storeUri{this, getEnv("NIX_REMOTE", "auto"), "store",
+    Setting<std::string> storeUri{this, getEnv("NIX_REMOTE").value_or("auto"), "store",
         "The default Nix store to use."};
 
     Setting<bool> keepFailed{this, false, "keep-failed",

src/libstore/local-store.hh

@@ -107,7 +107,7 @@ private:
 public:
 
     // Hack for build-remote.cc.
-    PathSet locksHeld = tokenizeString<PathSet>(getEnv("NIX_HELD_LOCKS"));
+    PathSet locksHeld;
 
     /* Initialise the local store, upgrading the schema if
        necessary. */

src/libstore/ssh.cc

@@ -16,7 +16,7 @@ SSHMaster::SSHMaster(const std::string & host, const std::string & keyFile, bool
 
 void SSHMaster::addCommonSSHOpts(Strings & args)
 {
-    for (auto & i : tokenizeString<Strings>(getEnv("NIX_SSHOPTS")))
+    for (auto & i : tokenizeString<Strings>(getEnv("NIX_SSHOPTS").value_or("")))
         args.push_back(i);
     if (!keyFile.empty())
         args.insert(args.end(), {"-i", keyFile});

src/libutil/util.cc

@@ -58,10 +58,11 @@ std::string SysError::addErrno(const std::string & s)
 }
 
 
-string getEnv(const string & key, const string & def)
+std::optional<std::string> getEnv(const std::string & key)
 {
     char * value = getenv(key.c_str());
-    return value ? string(value) : def;
+    if (!value) return {};
+    return std::string(value);
 }
 
 
@@ -438,7 +439,7 @@ void deletePath(const Path & path, unsigned long long & bytesFreed)
 static Path tempName(Path tmpRoot, const Path & prefix, bool includePid,
     int & counter)
 {
-    tmpRoot = canonPath(tmpRoot.empty() ? getEnv("TMPDIR", "/tmp") : tmpRoot, true);
+    tmpRoot = canonPath(tmpRoot.empty() ? getEnv("TMPDIR").value_or("/tmp") : tmpRoot, true);
     if (includePid)
         return (format("%1%/%2%-%3%-%4%") % tmpRoot % prefix % getpid() % counter++).str();
     else
@@ -480,7 +481,7 @@ Path createTempDir(const Path & tmpRoot, const Path & prefix,
 std::string getUserName()
 {
     auto pw = getpwuid(geteuid());
-    std::string name = pw ? pw->pw_name : getEnv("USER", "");
+    std::string name = pw ? pw->pw_name : getEnv("USER").value_or("");
     if (name.empty())
         throw Error("cannot figure out user name");
     return name;
@@ -488,8 +489,8 @@ std::string getUserName()
 
 
 static Lazy<Path> getHome2([]() {
-    Path homeDir = getEnv("HOME");
+    auto homeDir = getEnv("HOME");
-    if (homeDir.empty()) {
+    if (!homeDir) {
         std::vector<char> buf(16384);
         struct passwd pwbuf;
         struct passwd * pw;
@@ -498,7 +499,7 @@ static Lazy<Path> getHome2([]() {
             throw Error("cannot determine user's home directory");
         homeDir = pw->pw_dir;
     }
-    return homeDir;
+    return *homeDir;
 });
 
 Path getHome() { return getHome2(); }
@@ -506,25 +507,21 @@ Path getHome() { return getHome2(); }
 
 Path getCacheDir()
 {
-    Path cacheDir = getEnv("XDG_CACHE_HOME");
+    auto cacheDir = getEnv("XDG_CACHE_HOME");
-    if (cacheDir.empty())
+    return cacheDir ? *cacheDir : getHome() + "/.cache";
-        cacheDir = getHome() + "/.cache";
-    return cacheDir;
 }
 
 
 Path getConfigDir()
 {
-    Path configDir = getEnv("XDG_CONFIG_HOME");
+    auto configDir = getEnv("XDG_CONFIG_HOME");
-    if (configDir.empty())
+    return configDir ? *configDir : getHome() + "/.config";
-        configDir = getHome() + "/.config";
-    return configDir;
 }
 
 std::vector<Path> getConfigDirs()
 {
     Path configHome = getConfigDir();
-    string configDirs = getEnv("XDG_CONFIG_DIRS");
+    string configDirs = getEnv("XDG_CONFIG_DIRS").value_or("");
     std::vector<Path> result = tokenizeString<std::vector<string>>(configDirs, ":");
     result.insert(result.begin(), configHome);
     return result;
@@ -533,10 +530,8 @@ std::vector<Path> getConfigDirs()
 
 Path getDataDir()
 {
-    Path dataDir = getEnv("XDG_DATA_HOME");
+    auto dataDir = getEnv("XDG_DATA_HOME");
-    if (dataDir.empty())
+    return dataDir ? *dataDir : getHome() + "/.local/share";
-        dataDir = getHome() + "/.local/share";
-    return dataDir;
 }
 
 

src/libutil/util.hh

@@ -35,7 +35,7 @@ extern const std::string nativeSystem;
 
 
 /* Return an environment variable. */
-string getEnv(const string & key, const string & def = "");
+std::optional<std::string> getEnv(const std::string & key);
 
 /* Get the entire environment. */
 std::map<std::string, std::string> getEnv();

src/nix-build/nix-build.cc

@@ -344,9 +344,9 @@ static void _main(int argc, char * * argv)
         /* Figure out what bash shell to use. If $NIX_BUILD_SHELL
            is not set, then build bashInteractive from
            <nixpkgs>. */
-        auto shell = getEnv("NIX_BUILD_SHELL", "");
+        auto shell = getEnv("NIX_BUILD_SHELL");
 
-        if (shell == "") {
+        if (!shell) {
 
             try {
                 auto expr = state->parseExprFromString("(import <nixpkgs> {}).bashInteractive", absPath("."));
@@ -382,7 +382,8 @@ static void _main(int argc, char * * argv)
         // Set the environment.
         auto env = getEnv();
 
-        auto tmp = getEnv("TMPDIR", getEnv("XDG_RUNTIME_DIR", "/tmp"));
+        auto tmp = getEnv("TMPDIR");
+        if (!tmp) tmp = getEnv("XDG_RUNTIME_DIR").value_or("/tmp");
 
         if (pure) {
             decltype(env) newEnv;
@@ -394,7 +395,7 @@ static void _main(int argc, char * * argv)
             env["__ETC_PROFILE_SOURCED"] = "1";
         }
 
-        env["NIX_BUILD_TOP"] = env["TMPDIR"] = env["TEMPDIR"] = env["TMP"] = env["TEMP"] = tmp;
+        env["NIX_BUILD_TOP"] = env["TMPDIR"] = env["TEMPDIR"] = env["TMP"] = env["TEMP"] = *tmp;
         env["NIX_STORE"] = store->storeDir;
         env["NIX_BUILD_CORES"] = std::to_string(settings.buildCores);
 
@@ -439,8 +440,8 @@ static void _main(int argc, char * * argv)
                 (Path) tmpDir,
                 (pure ? "" : "p=$PATH; "),
                 (pure ? "" : "PATH=$PATH:$p; unset p; "),
-                dirOf(shell),
+                dirOf(*shell),
-                shell,
+                *shell,
                 (getenv("TZ") ? (string("export TZ='") + getenv("TZ") + "'; ") : ""),
                 envCommand));
 
@@ -460,9 +461,9 @@ static void _main(int argc, char * * argv)
 
         restoreSignals();
 
-        execvp(shell.c_str(), argPtrs.data());
+        execvp(shell->c_str(), argPtrs.data());
 
-        throw SysError("executing shell '%s'", shell);
+        throw SysError("executing shell '%s'", *shell);
     }
 
     else {

src/nix-daemon/nix-daemon.cc

@@ -161,8 +161,9 @@ static void daemonLoop(char * * argv)
     AutoCloseFD fdSocket;
 
     /* Handle socket-based activation by systemd. */
-    if (getEnv("LISTEN_FDS") != "") {
+    auto listenFds = getEnv("LISTEN_FDS");
-        if (getEnv("LISTEN_PID") != std::to_string(getpid()) || getEnv("LISTEN_FDS") != "1")
+    if (listenFds) {
+        if (getEnv("LISTEN_PID") != std::to_string(getpid()) || listenFds != "1")
             throw Error("unexpected systemd environment variables");
         fdSocket = SD_LISTEN_FDS_START;
         closeOnExec(fdSocket.get());

src/nix-env/nix-env.cc

@@ -1425,7 +1425,7 @@ static int _main(int argc, char * * argv)
         globals.instSource.autoArgs = myArgs.getAutoArgs(*globals.state);
 
         if (globals.profile == "")
-            globals.profile = getEnv("NIX_PROFILE", "");
+            globals.profile = getEnv("NIX_PROFILE").value_or("");
 
         if (globals.profile == "") {
             Path profileLink = getHome() + "/.nix-profile";

src/nix/command.cc

@@ -156,7 +156,7 @@ void StorePathCommand::run(ref<Store> store)
 
 Strings editorFor(const Pos & pos)
 {
-    auto editor = getEnv("EDITOR", "cat");
+    auto editor = getEnv("EDITOR").value_or("cat");
     auto args = tokenizeString<Strings>(editor);
     if (pos.line > 0 && (
         editor.find("emacs") != std::string::npos ||

src/nix/doctor.cc

@@ -68,7 +68,7 @@ struct CmdDoctor : StoreCommand
     {
         PathSet dirs;
 
-        for (auto & dir : tokenizeString<Strings>(getEnv("PATH"), ":"))
+        for (auto & dir : tokenizeString<Strings>(getEnv("PATH").value_or(""), ":"))
             if (pathExists(dir + "/nix-env"))
                 dirs.insert(dirOf(canonPath(dir + "/nix-env", true)));
 
@@ -87,7 +87,7 @@ struct CmdDoctor : StoreCommand
     {
         PathSet dirs;
 
-        for (auto & dir : tokenizeString<Strings>(getEnv("PATH"), ":")) {
+        for (auto & dir : tokenizeString<Strings>(getEnv("PATH").value_or(""), ":")) {
             Path profileDir = dirOf(dir);
             try {
                 Path userEnv = canonPath(profileDir, true);

src/nix/run.cc

@@ -128,7 +128,7 @@ struct CmdRun : InstallablesCommand
         std::queue<Path> todo;
         for (auto & path : outPaths) todo.push(path);
 
-        auto unixPath = tokenizeString<Strings>(getEnv("PATH"), ":");
+        auto unixPath = tokenizeString<Strings>(getEnv("PATH").value_or(""), ":");
 
         while (!todo.empty()) {
             Path path = todo.front();

src/nix/upgrade-nix.cc

@@ -106,7 +106,7 @@ struct CmdUpgradeNix : MixDryRun, StoreCommand
     {
         Path where;
 
-        for (auto & dir : tokenizeString<Strings>(getEnv("PATH"), ":"))
+        for (auto & dir : tokenizeString<Strings>(getEnv("PATH").value_or(""), ":"))
             if (pathExists(dir + "/nix-env")) {
                 where = dir;
                 break;