refactor(tvix): getEnv(): Return std::optional r/1756

This allows distinguishing between an empty value and no value.

Patch ported from upstream at
https://github.com/NixOS/nix/commit/ba87b08f8529e4d9f8c58d8c625152058ceadb75

Change-Id: I061cc8e16b1a7a0341adfc3b0edca1c0c51d5c97
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1884
Tested-by: BuildkiteCI
Reviewed-by: kanepyork <rikingcoding@gmail.com>

17 files changed, 66 insertions, 66 deletions

diff --git a/third_party/nix/src/libexpr/eval.cc b/third_party/nix/src/libexpr/eval.cc
index b735495f21..28ae46bdfd 100644
--- a/third_party/nix/src/libexpr/eval.cc
+++ b/third_party/nix/src/libexpr/eval.cc
@@ -287,11 +287,11 @@ EvalState::EvalState(const Strings& _searchPath, const ref<Store>& store)
       staticBaseEnv(false, nullptr) {
   expr::InitGC();
 
-  countCalls = getEnv("NIX_COUNT_CALLS", "0") != "0";
+  countCalls = getEnv("NIX_COUNT_CALLS").value_or("0") != "0";
 
   /* 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);
     }
@@ -1644,7 +1644,7 @@ 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);
@@ -1658,7 +1658,7 @@ void EvalState::printStats() {
       nrAttrsets * sizeof(Bindings) + nrAttrsInAttrsets * sizeof(Attr);
 
   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);
diff --git a/third_party/nix/src/libexpr/primops.cc b/third_party/nix/src/libexpr/primops.cc
index 04a44311a2..dd8a509df3 100644
--- a/third_party/nix/src/libexpr/primops.cc
+++ b/third_party/nix/src/libexpr/primops.cc
@@ -413,7 +413,7 @@ static void prim_getEnv(EvalState& state, const Pos& pos, Value** args,
   std::string name = state.forceStringNoCtx(*args[0], pos);
   mkString(v, evalSettings.restrictEval || evalSettings.pureEval
                   ? ""
-                  : getEnv(name));
+                  : getEnv(name).value_or(""));
 }
 
 /* Evaluate the first argument, then return the second argument. */
diff --git a/third_party/nix/src/libstore/build.cc b/third_party/nix/src/libstore/build.cc
index 3dce156653..6e4ca99b15 100644
--- a/third_party/nix/src/libstore/build.cc
+++ b/third_party/nix/src/libstore/build.cc
@@ -2582,7 +2582,7 @@ void DerivationGoal::initEnv() {
   if (fixedOutput) {
     for (auto& i :
          parsedDrv->getStringsAttr("impureEnvVars").value_or(Strings())) {
-      env[i] = getEnv(i);
+      env[i] = getEnv(i).value_or("");
     }
   }
 
diff --git a/third_party/nix/src/libstore/gc.cc b/third_party/nix/src/libstore/gc.cc
index 4f04e09a75..50c52bb009 100644
--- a/third_party/nix/src/libstore/gc.cc
+++ b/third_party/nix/src/libstore/gc.cc
@@ -906,7 +906,8 @@ 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").value_or("");
 
   auto getAvail = [this]() -> uint64_t {
     if (!fakeFreeSpaceFile.empty()) {
diff --git a/third_party/nix/src/libstore/globals.cc b/third_party/nix/src/libstore/globals.cc
index 34f5d4605b..6babb4589f 100644
--- a/third_party/nix/src/libstore/globals.cc
+++ b/third_party/nix/src/libstore/globals.cc
@@ -31,19 +31,22 @@ 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))),
+          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").value_or("1") == "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.empty()) {
     for (auto& fn :
          {"/etc/ssl/certs/ca-certificates.crt",
@@ -58,9 +61,9 @@ Settings::Settings()
   /* Backwards compatibility. */
   // TODO(tazjin): still?
   auto s = getEnv("NIX_REMOTE_SYSTEMS");
-  if (!s.empty()) {
+  if (s) {
     Strings ss;
-    for (auto p : absl::StrSplit(s, absl::ByChar(':'), absl::SkipEmpty())) {
+    for (auto p : absl::StrSplit(*s, absl::ByChar(':'), absl::SkipEmpty())) {
       ss.push_back(absl::StrCat("@", p));
     }
     builders = concatStringsSep(" ", ss);
diff --git a/third_party/nix/src/libstore/globals.hh b/third_party/nix/src/libstore/globals.hh
index 54defff06b..29848fbb4b 100644
--- a/third_party/nix/src/libstore/globals.hh
+++ b/third_party/nix/src/libstore/globals.hh
@@ -61,8 +61,8 @@ class Settings : public Config {
   /* File name of the socket the daemon listens to.  */
   Path nixDaemonSocketFile;
 
-  Setting<std::string> storeUri{this, getEnv("NIX_REMOTE", "auto"), "store",
-                                "The default Nix store to use."};
+  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",
diff --git a/third_party/nix/src/libstore/local-store.hh b/third_party/nix/src/libstore/local-store.hh
index 193050e538..cfcfb35cc3 100644
--- a/third_party/nix/src/libstore/local-store.hh
+++ b/third_party/nix/src/libstore/local-store.hh
@@ -98,8 +98,9 @@ class LocalStore : public LocalFSStore {
  public:
   // Hack for build-remote.cc.
   // TODO(tazjin): remove this when we've got gRPC
-  PathSet locksHeld = absl::StrSplit(
-      getEnv("NIX_HELD_LOCKS"), absl::ByAnyChar(" \t\n\r"), absl::SkipEmpty());
+  PathSet locksHeld =
+      absl::StrSplit(getEnv("NIX_HELD_LOCKS").value_or(""),
+                     absl::ByAnyChar(" \t\n\r"), absl::SkipEmpty());
 
   /* Initialise the local store, upgrading the schema if
      necessary. */
diff --git a/third_party/nix/src/libstore/ssh.cc b/third_party/nix/src/libstore/ssh.cc
index 7d5fe6d109..6043e584dd 100644
--- a/third_party/nix/src/libstore/ssh.cc
+++ b/third_party/nix/src/libstore/ssh.cc
@@ -22,8 +22,8 @@ SSHMaster::SSHMaster(const std::string& host, std::string keyFile,
 
 void SSHMaster::addCommonSSHOpts(Strings& args) {
   for (auto& i :
-       absl::StrSplit(getEnv("NIX_SSHOPTS"), absl::ByAnyChar(" \t\n\r"),
-                      absl::SkipEmpty())) {
+       absl::StrSplit(getEnv("NIX_SSHOPTS").value_or(""),
+                      absl::ByAnyChar(" \t\n\r"), absl::SkipEmpty())) {
     args.push_back(std::string(i));
   }
   if (!keyFile.empty()) {
diff --git a/third_party/nix/src/libutil/util.cc b/third_party/nix/src/libutil/util.cc
index 939b6361d1..aea1e68e3c 100644
--- a/third_party/nix/src/libutil/util.cc
+++ b/third_party/nix/src/libutil/util.cc
@@ -45,9 +45,10 @@ std::string SysError::addErrno(const std::string& s) {
   return s + ": " + strerror(errNo);
 }
 
-std::string getEnv(const std::string& key, const std::string& def) {
+std::optional<std::string> getEnv(const std::string& key) {
   char* value = getenv(key.c_str());
-  return value != nullptr ? std::string(value) : def;
+  if (value == nullptr) return {};
+  return std::string(value);
 }
 
 std::map<std::string, std::string> getEnv() {
@@ -466,8 +467,9 @@ 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();
@@ -507,16 +509,17 @@ Path createTempDir(const Path& tmpRoot, const Path& prefix, bool includePid,
 
 std::string getUserName() {
   auto pw = getpwuid(geteuid());
-  std::string name = pw != nullptr ? pw->pw_name : getEnv("USER", "");
-  if (name.empty()) {
+  std::optional<std::string> name =
+      pw != nullptr ? pw->pw_name : getEnv("USER");
+  if (!name.has_value()) {
     throw Error("cannot figure out user name");
   }
-  return name;
+  return *name;
 }
 
 static Lazy<Path> getHome2([]() {
-  Path homeDir = getEnv("HOME");
-  if (homeDir.empty()) {
+  std::optional<Path> homeDir = getEnv("HOME");
+  if (!homeDir) {
     std::vector<char> buf(16384);
     struct passwd pwbuf;
     struct passwd* pw;
@@ -524,32 +527,24 @@ static Lazy<Path> getHome2([]() {
         (pw == nullptr) || (pw->pw_dir == nullptr) || (pw->pw_dir[0] == 0)) {
       throw Error("cannot determine user's home directory");
     }
-    homeDir = pw->pw_dir;
+    return std::string(pw->pw_dir);
   }
-  return homeDir;
+  return homeDir.value();
 });
 
 Path getHome() { return getHome2(); }
 
 Path getCacheDir() {
-  Path cacheDir = getEnv("XDG_CACHE_HOME");
-  if (cacheDir.empty()) {
-    cacheDir = getHome() + "/.cache";
-  }
-  return cacheDir;
+  return getEnv("XDG_CACHE_HOME").value_or(getHome() + "/.cache");
 }
 
 Path getConfigDir() {
-  Path configDir = getEnv("XDG_CONFIG_HOME");
-  if (configDir.empty()) {
-    configDir = getHome() + "/.config";
-  }
-  return configDir;
+  return getEnv("XDG_CONFIG_HOME").value_or(getHome() + "/.config");
 }
 
 std::vector<Path> getConfigDirs() {
   Path configHome = getConfigDir();
-  std::string configDirs = getEnv("XDG_CONFIG_DIRS");
+  std::string configDirs = getEnv("XDG_CONFIG_DIRS").value_or("");
   std::vector<std::string> result =
       absl::StrSplit(configDirs, absl::ByChar(':'), absl::SkipEmpty());
   result.insert(result.begin(), configHome);
@@ -557,11 +552,7 @@ std::vector<Path> getConfigDirs() {
 }
 
 Path getDataDir() {
-  Path dataDir = getEnv("XDG_DATA_HOME");
-  if (dataDir.empty()) {
-    dataDir = getHome() + "/.local/share";
-  }
-  return dataDir;
+  return getEnv("XDG_DATA_HOME").value_or(getHome() + "/.local/share");
 }
 
 // TODO(grfn): Remove in favor of std::filesystem::create_directories
diff --git a/third_party/nix/src/libutil/util.hh b/third_party/nix/src/libutil/util.hh
index b6d726b46c..b3349c4f39 100644
--- a/third_party/nix/src/libutil/util.hh
+++ b/third_party/nix/src/libutil/util.hh
@@ -26,7 +26,7 @@ struct Source;
 extern const std::string nativeSystem;
 
 /* Return an environment variable. */
-std::string getEnv(const std::string& key, const std::string& def = "");
+std::optional<std::string> getEnv(const std::string& key);
 
 /* Get the entire environment. */
 std::map<std::string, std::string> getEnv();
diff --git a/third_party/nix/src/nix-build/nix-build.cc b/third_party/nix/src/nix-build/nix-build.cc
index 30ab2a8136..26c3089677 100644
--- a/third_party/nix/src/nix-build/nix-build.cc
+++ b/third_party/nix/src/nix-build/nix-build.cc
@@ -377,9 +377,12 @@ 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 opt_shell = getEnv("NIX_BUILD_SHELL");
+    std::string shell;
 
-    if (shell.empty()) {
+    if (opt_shell.has_value()) {
+      shell = opt_shell.value();
+    } else {
       try {
         auto expr = state->parseExprFromString(
             "(import <nixpkgs> {}).bashInteractive", absPath("."));
@@ -427,7 +430,8 @@ 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").value_or(getEnv("XDG_RUNTIME_DIR").value_or("/tmp"));
 
     if (pure) {
       decltype(env) newEnv;
diff --git a/third_party/nix/src/nix-env/nix-env.cc b/third_party/nix/src/nix-env/nix-env.cc
index 830bd514da..ee36510f02 100644
--- a/third_party/nix/src/nix-env/nix-env.cc
+++ b/third_party/nix/src/nix-env/nix-env.cc
@@ -1516,7 +1516,7 @@ static int _main(int argc, char** argv) {
     globals.instSource.autoArgs = myArgs.getAutoArgs(*globals.state);
 
     if (globals.profile.empty()) {
-      globals.profile = getEnv("NIX_PROFILE", "");
+      globals.profile = getEnv("NIX_PROFILE").value_or("");
     }
 
     if (globals.profile.empty()) {
diff --git a/third_party/nix/src/nix-prefetch-url/nix-prefetch-url.cc b/third_party/nix/src/nix-prefetch-url/nix-prefetch-url.cc
index c718ef5f47..b61a38a7f1 100644
--- a/third_party/nix/src/nix-prefetch-url/nix-prefetch-url.cc
+++ b/third_party/nix/src/nix-prefetch-url/nix-prefetch-url.cc
@@ -59,7 +59,7 @@ static int _main(int argc, char** argv) {
   {
     HashType ht = htSHA256;
     std::vector<std::string> args;
-    bool printPath = !getEnv("PRINT_PATH").empty();
+    bool printPath = getEnv("PRINT_PATH").has_value();
     bool fromExpr = false;
     std::string attrPath;
     bool unpack = false;
diff --git a/third_party/nix/src/nix/doctor.cc b/third_party/nix/src/nix/doctor.cc
index 0105b05e97..e65ca69f6b 100644
--- a/third_party/nix/src/nix/doctor.cc
+++ b/third_party/nix/src/nix/doctor.cc
@@ -48,8 +48,8 @@ struct CmdDoctor final : StoreCommand {
   static bool checkNixInPath() {
     PathSet dirs;
 
-    for (auto& dir :
-         absl::StrSplit(getEnv("PATH"), absl::ByChar(':'), absl::SkipEmpty())) {
+    for (auto& dir : absl::StrSplit(getEnv("PATH").value_or(""),
+                                    absl::ByChar(':'), absl::SkipEmpty())) {
       if (pathExists(absl::StrCat(dir, "/nix-env"))) {
         dirs.insert(dirOf(canonPath(absl::StrCat(dir, "/nix-env"), true)));
       }
@@ -72,8 +72,8 @@ struct CmdDoctor final : StoreCommand {
   static bool checkProfileRoots(const ref<Store>& store) {
     PathSet dirs;
 
-    for (auto dir :
-         absl::StrSplit(getEnv("PATH"), absl::ByChar(':'), absl::SkipEmpty())) {
+    for (auto dir : absl::StrSplit(getEnv("PATH").value_or(""),
+                                   absl::ByChar(':'), absl::SkipEmpty())) {
       Path profileDir = dirOf(dir);
       try {
         Path userEnv = canonPath(profileDir, true);
diff --git a/third_party/nix/src/nix/edit.cc b/third_party/nix/src/nix/edit.cc
index 644caa7cbe..b5369e4a8a 100644
--- a/third_party/nix/src/nix/edit.cc
+++ b/third_party/nix/src/nix/edit.cc
@@ -53,7 +53,7 @@ struct CmdEdit final : InstallableCommand {
       throw Error("cannot parse line number '%s'", pos);
     }
 
-    auto editor = getEnv("EDITOR", "cat");
+    auto editor = getEnv("EDITOR").value_or("cat");
 
     Strings args =
         absl::StrSplit(editor, absl::ByAnyChar(" \t\n\r"), absl::SkipEmpty());
diff --git a/third_party/nix/src/nix/run.cc b/third_party/nix/src/nix/run.cc
index e76f8834cf..06c95f856f 100644
--- a/third_party/nix/src/nix/run.cc
+++ b/third_party/nix/src/nix/run.cc
@@ -123,8 +123,8 @@ struct CmdRun final : InstallablesCommand {
       todo.push(path);
     }
 
-    Strings unixPath =
-        absl::StrSplit(getEnv("PATH"), absl::ByChar(':'), absl::SkipEmpty());
+    Strings unixPath = absl::StrSplit(getEnv("PATH").value_or(""),
+                                      absl::ByChar(':'), absl::SkipEmpty());
 
     while (!todo.empty()) {
       Path path = todo.front();
diff --git a/third_party/nix/src/nix/upgrade-nix.cc b/third_party/nix/src/nix/upgrade-nix.cc
index eff51ba158..77e284afba 100644
--- a/third_party/nix/src/nix/upgrade-nix.cc
+++ b/third_party/nix/src/nix/upgrade-nix.cc
@@ -103,8 +103,8 @@ struct CmdUpgradeNix final : MixDryRun, StoreCommand {
   static Path getProfileDir(const ref<Store>& store) {
     Path where;
 
-    for (auto& dir :
-         absl::StrSplit(getEnv("PATH"), absl::ByChar(':'), absl::SkipEmpty())) {
+    for (auto& dir : absl::StrSplit(getEnv("PATH").value_or(""),
+                                    absl::ByChar(':'), absl::SkipEmpty())) {
       if (pathExists(absl::StrCat(dir, "/nix-env"))) {
         where = dir;
         break;