getEnv(): Return std::optional

This allows distinguishing between an empty value and no value.

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())
-            return std::stoll(readFile(fakeFreeSpaceFile));
+        if (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))))
-    , nixDataDir(canonPath(getEnv("NIX_DATA_DIR", NIX_DATA_DIR)))
-    , nixLogDir(canonPath(getEnv("NIX_LOG_DIR", NIX_LOG_DIR)))
-    , nixStateDir(canonPath(getEnv("NIX_STATE_DIR", NIX_STATE_DIR)))
-    , nixConfDir(canonPath(getEnv("NIX_CONF_DIR", NIX_CONF_DIR)))
-    , nixLibexecDir(canonPath(getEnv("NIX_LIBEXEC_DIR", NIX_LIBEXEC_DIR)))
-    , nixBinDir(canonPath(getEnv("NIX_BIN_DIR", NIX_BIN_DIR)))
+    , nixStore(canonPath(getEnv("NIX_STORE_DIR").value_or(getEnv("NIX_STORE").value_or(NIX_STORE_DIR))))
+    , nixDataDir(canonPath(getEnv("NIX_DATA_DIR").value_or(NIX_DATA_DIR)))
+    , nixLogDir(canonPath(getEnv("NIX_LOG_DIR").value_or(NIX_LOG_DIR)))
+    , nixStateDir(canonPath(getEnv("NIX_STATE_DIR").value_or(NIX_STATE_DIR)))
+    , nixConfDir(canonPath(getEnv("NIX_CONF_DIR").value_or(NIX_CONF_DIR)))
+    , nixLibexecDir(canonPath(getEnv("NIX_LIBEXEC_DIR").value_or(NIX_LIBEXEC_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});