Merge changes I5566a985,I88cf53d3 into main

* changes:
  Support relative and `~/` paths in config settings
  Thread `ApplyConfigOptions` through config parsing

8 files changed, 120 insertions, 77 deletions

diff --git a/src/libfetchers/fetch-settings.cc b/src/libfetchers/fetch-settings.cc
index aeb3c542b..007f2725f 100644
--- a/src/libfetchers/fetch-settings.cc
+++ b/src/libfetchers/fetch-settings.cc
@@ -7,7 +7,7 @@
 
 namespace nix {
 
-template<> AcceptFlakeConfig BaseSetting<AcceptFlakeConfig>::parse(const std::string & str) const
+template<> AcceptFlakeConfig BaseSetting<AcceptFlakeConfig>::parse(const std::string & str, const ApplyConfigOptions & options) const
 {
     if (str == "true") return AcceptFlakeConfig::True;
     else if (str == "ask") return AcceptFlakeConfig::Ask;
diff --git a/src/libstore/globals.cc b/src/libstore/globals.cc
index b534882de..c114e22dc 100644
--- a/src/libstore/globals.cc
+++ b/src/libstore/globals.cc
@@ -126,29 +126,30 @@ Settings::Settings()
 
 void loadConfFile()
 {
-    auto applyConfigFile = [&](const Path & path) {
+    auto applyConfigFile = [&](const ApplyConfigOptions & options) {
         try {
-            std::string contents = readFile(path);
-            globalConfig.applyConfig(contents, path);
-        } catch (SysError &) { }
+            std::string contents = readFile(*options.path);
+            globalConfig.applyConfig(contents, options);
+        } catch (SysError &) {
+        }
     };
 
-    applyConfigFile(settings.nixConfDir + "/nix.conf");
+    applyConfigFile(ApplyConfigOptions{.path = settings.nixConfDir + "/nix.conf"});
 
     /* We only want to send overrides to the daemon, i.e. stuff from
        ~/.nix/nix.conf or the command line. */
     globalConfig.resetOverridden();
 
     auto files = settings.nixUserConfFiles;
+    auto home = getHome();
     for (auto file = files.rbegin(); file != files.rend(); file++) {
-        applyConfigFile(*file);
+        applyConfigFile(ApplyConfigOptions{.path = *file, .home = home});
     }
 
     auto nixConfEnv = getEnv("NIX_CONFIG");
     if (nixConfEnv.has_value()) {
-        globalConfig.applyConfig(nixConfEnv.value(), "NIX_CONFIG");
+        globalConfig.applyConfig(nixConfEnv.value(), ApplyConfigOptions{.fromEnvVar = true});
     }
-
 }
 
 std::vector<Path> getUserConfigFiles()
@@ -274,7 +275,7 @@ NLOHMANN_JSON_SERIALIZE_ENUM(SandboxMode, {
     {SandboxMode::smDisabled, false},
 });
 
-template<> SandboxMode BaseSetting<SandboxMode>::parse(const std::string & str) const
+template<> SandboxMode BaseSetting<SandboxMode>::parse(const std::string & str, const ApplyConfigOptions & options) const
 {
     if (str == "true") return smEnabled;
     else if (str == "relaxed") return smRelaxed;
@@ -317,7 +318,7 @@ template<> void BaseSetting<SandboxMode>::convertToArg(Args & args, const std::s
     });
 }
 
-unsigned int MaxBuildJobsSetting::parse(const std::string & str) const
+unsigned int MaxBuildJobsSetting::parse(const std::string & str, const ApplyConfigOptions & options) const
 {
     if (str == "auto") return std::max(1U, std::thread::hardware_concurrency());
     else {
@@ -325,15 +326,15 @@ unsigned int MaxBuildJobsSetting::parse(const std::string & str) const
             return *n;
         else
             throw UsageError("configuration setting '%s' should be 'auto' or an integer", name);
+        }
     }
-}
 
 
-Paths PluginFilesSetting::parse(const std::string & str) const
+Paths PluginFilesSetting::parse(const std::string & str, const ApplyConfigOptions & options) const
 {
     if (pluginsLoaded)
         throw UsageError("plugin-files set after plugins were loaded, you may need to move the flag before the subcommand");
-    return BaseSetting<Paths>::parse(str);
+    return BaseSetting<Paths>::parse(str, options);
 }
 
 
diff --git a/src/libstore/globals.hh b/src/libstore/globals.hh
index aba99d969..51550b2c3 100644
--- a/src/libstore/globals.hh
+++ b/src/libstore/globals.hh
@@ -26,7 +26,7 @@ struct MaxBuildJobsSetting : public BaseSetting<unsigned int>
         options->addSetting(this);
     }
 
-    unsigned int parse(const std::string & str) const override;
+    unsigned int parse(const std::string & str, const ApplyConfigOptions & options) const override;
 };
 
 struct PluginFilesSetting : public BaseSetting<Paths>
@@ -43,7 +43,7 @@ struct PluginFilesSetting : public BaseSetting<Paths>
         options->addSetting(this);
     }
 
-    Paths parse(const std::string & str) const override;
+    Paths parse(const std::string & str, const ApplyConfigOptions & options) const override;
 };
 
 const uint32_t maxIdsPerBuild =
@@ -1088,6 +1088,7 @@ void loadConfFile();
 
 // Used by the Settings constructor
 std::vector<Path> getUserConfigFiles();
+std::vector<Path> getHomeConfigFile();
 
 extern const std::string nixVersion;
 
diff --git a/src/libutil/config-impl.hh b/src/libutil/config-impl.hh
index 024018e00..748107b6e 100644
--- a/src/libutil/config-impl.hh
+++ b/src/libutil/config-impl.hh
@@ -51,14 +51,14 @@ bool BaseSetting<T>::isAppendable()
     return trait::appendable;
 }
 
-template<> void BaseSetting<Strings>::appendOrSet(Strings newValue, bool append);
-template<> void BaseSetting<StringSet>::appendOrSet(StringSet newValue, bool append);
-template<> void BaseSetting<StringMap>::appendOrSet(StringMap newValue, bool append);
-template<> void BaseSetting<ExperimentalFeatures>::appendOrSet(ExperimentalFeatures newValue, bool append);
-template<> void BaseSetting<DeprecatedFeatures>::appendOrSet(DeprecatedFeatures newValue, bool append);
+template<> void BaseSetting<Strings>::appendOrSet(Strings newValue, bool append, const ApplyConfigOptions & options);
+template<> void BaseSetting<StringSet>::appendOrSet(StringSet newValue, bool append, const ApplyConfigOptions & options);
+template<> void BaseSetting<StringMap>::appendOrSet(StringMap newValue, bool append, const ApplyConfigOptions & options);
+template<> void BaseSetting<ExperimentalFeatures>::appendOrSet(ExperimentalFeatures newValue, bool append, const ApplyConfigOptions & options);
+template<> void BaseSetting<DeprecatedFeatures>::appendOrSet(DeprecatedFeatures newValue, bool append, const ApplyConfigOptions & options);
 
 template<typename T>
-void BaseSetting<T>::appendOrSet(T newValue, bool append)
+void BaseSetting<T>::appendOrSet(T newValue, bool append, const ApplyConfigOptions & options)
 {
     static_assert(
         !trait::appendable,
@@ -69,14 +69,14 @@ void BaseSetting<T>::appendOrSet(T newValue, bool append)
 }
 
 template<typename T>
-void BaseSetting<T>::set(const std::string & str, bool append)
+void BaseSetting<T>::set(const std::string & str, bool append, const ApplyConfigOptions & options)
 {
     if (experimentalFeatureSettings.isEnabled(experimentalFeature)) {
-        auto parsed = parse(str);
+        auto parsed = parse(str, options);
         if (deprecated && (append || parsed != value)) {
             warn("deprecated setting '%s' found (set to '%s')", name, str);
         }
-        appendOrSet(std::move(parsed), append);
+        appendOrSet(std::move(parsed), append, options);
     } else {
         assert(experimentalFeature);
         warn("Ignoring setting '%s' because experimental feature '%s' is not enabled",
@@ -111,7 +111,7 @@ void BaseSetting<T>::convertToArg(Args & args, const std::string & category)
 }
 
 #define DECLARE_CONFIG_SERIALISER(TY) \
-    template<> TY BaseSetting< TY >::parse(const std::string & str) const; \
+    template<> TY BaseSetting< TY >::parse(const std::string & str, const ApplyConfigOptions & options) const; \
     template<> std::string BaseSetting< TY >::to_string() const;
 
 DECLARE_CONFIG_SERIALISER(std::string)
@@ -124,7 +124,7 @@ DECLARE_CONFIG_SERIALISER(ExperimentalFeatures)
 DECLARE_CONFIG_SERIALISER(DeprecatedFeatures)
 
 template<typename T>
-T BaseSetting<T>::parse(const std::string & str) const
+T BaseSetting<T>::parse(const std::string & str, const ApplyConfigOptions & options) const
 {
     static_assert(std::is_integral<T>::value, "Integer required.");
 
diff --git a/src/libutil/config.cc b/src/libutil/config.cc
index 8e20f1321..778da1413 100644
--- a/src/libutil/config.cc
+++ b/src/libutil/config.cc
@@ -1,4 +1,5 @@
 #include "config.hh"
+#include "apply-config-options.hh"
 #include "args.hh"
 #include "abstract-setting-to-json.hh"
 #include "experimental-features.hh"
@@ -17,7 +18,7 @@ Config::Config(StringMap initials)
     : AbstractConfig(std::move(initials))
 { }
 
-bool Config::set(const std::string & name, const std::string & value)
+bool Config::set(const std::string & name, const std::string & value, const ApplyConfigOptions & options)
 {
     bool append = false;
     auto i = _settings.find(name);
@@ -30,7 +31,7 @@ bool Config::set(const std::string & name, const std::string & value)
         } else
             return false;
     }
-    i->second.setting->set(value, append);
+    i->second.setting->set(value, append, options);
     i->second.setting->overridden = true;
     return true;
 }
@@ -91,7 +92,7 @@ void Config::getSettings(std::map<std::string, SettingInfo> & res, bool overridd
 }
 
 
-static void applyConfigInner(const std::string & contents, const std::string & path, std::vector<std::pair<std::string, std::string>> & parsedContents) {
+static void applyConfigInner(const std::string & contents, const ApplyConfigOptions & options, std::vector<std::pair<std::string, std::string>> & parsedContents) {
     unsigned int pos = 0;
 
     while (pos < contents.size()) {
@@ -107,7 +108,7 @@ static void applyConfigInner(const std::string & contents, const std::string & p
         if (tokens.empty()) continue;
 
         if (tokens.size() < 2)
-            throw UsageError("illegal configuration line '%1%' in '%2%'", line, path);
+            throw UsageError("illegal configuration line '%1%' in '%2%'", line, options.relativeDisplay());
 
         auto include = false;
         auto ignoreMissing = false;
@@ -119,24 +120,32 @@ static void applyConfigInner(const std::string & contents, const std::string & p
         }
 
         if (include) {
-            if (tokens.size() != 2)
-                throw UsageError("illegal configuration line '%1%' in '%2%'", line, path);
-            auto p = absPath(tokens[1], dirOf(path));
-            if (pathExists(p)) {
+            if (tokens.size() != 2) {
+                throw UsageError("illegal configuration line '%1%' in '%2%'", line, options.relativeDisplay());
+            }
+            if (!options.path) {
+                throw UsageError("can only include configuration '%1%' from files", tokens[1]);
+            }
+            auto pathToInclude = absPath(tildePath(tokens[1], options.home), dirOf(*options.path));
+            if (pathExists(pathToInclude)) {
+                auto includeOptions = ApplyConfigOptions {
+                    .path = pathToInclude,
+                    .home = options.home,
+                };
                 try {
-                    std::string includedContents = readFile(path);
-                    applyConfigInner(includedContents, p, parsedContents);
+                    std::string includedContents = readFile(pathToInclude);
+                    applyConfigInner(includedContents, includeOptions, parsedContents);
                 } catch (SysError &) {
                     // TODO: Do we actually want to ignore this? Or is it better to fail?
                 }
             } else if (!ignoreMissing) {
-                throw Error("file '%1%' included from '%2%' not found", p, path);
+                throw Error("file '%1%' included from '%2%' not found", pathToInclude, *options.path);
             }
             continue;
         }
 
         if (tokens[1] != "=")
-            throw UsageError("illegal configuration line '%1%' in '%2%'", line, path);
+            throw UsageError("illegal configuration line '%1%' in '%2%'", line, options.relativeDisplay());
 
         std::string name = std::move(tokens[0]);
 
@@ -150,20 +159,20 @@ static void applyConfigInner(const std::string & contents, const std::string & p
     };
 }
 
-void AbstractConfig::applyConfig(const std::string & contents, const std::string & path) {
+void AbstractConfig::applyConfig(const std::string & contents, const ApplyConfigOptions & options) {
     std::vector<std::pair<std::string, std::string>> parsedContents;
 
-    applyConfigInner(contents, path, parsedContents);
+    applyConfigInner(contents, options, parsedContents);
 
     // First apply experimental-feature related settings
     for (const auto & [name, value] : parsedContents)
         if (name == "experimental-features" || name == "extra-experimental-features")
-            set(name, value);
+            set(name, value, options);
 
     // Then apply other settings
     for (const auto & [name, value] : parsedContents)
         if (name != "experimental-features" && name != "extra-experimental-features")
-            set(name, value);
+            set(name, value, options);
 }
 
 void Config::resetOverridden()
@@ -241,7 +250,7 @@ void AbstractSetting::convertToArg(Args & args, const std::string & category)
 
 bool AbstractSetting::isOverridden() const { return overridden; }
 
-template<> std::string BaseSetting<std::string>::parse(const std::string & str) const
+template<> std::string BaseSetting<std::string>::parse(const std::string & str, const ApplyConfigOptions & options) const
 {
     return str;
 }
@@ -251,7 +260,7 @@ template<> std::string BaseSetting<std::string>::to_string() const
     return value;
 }
 
-template<> std::optional<std::string> BaseSetting<std::optional<std::string>>::parse(const std::string & str) const
+template<> std::optional<std::string> BaseSetting<std::optional<std::string>>::parse(const std::string & str, const ApplyConfigOptions & options) const
 {
     if (str == "")
         return std::nullopt;
@@ -264,7 +273,7 @@ template<> std::string BaseSetting<std::optional<std::string>>::to_string() cons
     return value ? *value : "";
 }
 
-template<> bool BaseSetting<bool>::parse(const std::string & str) const
+template<> bool BaseSetting<bool>::parse(const std::string & str, const ApplyConfigOptions & options) const
 {
     if (str == "true" || str == "yes" || str == "1")
         return true;
@@ -297,12 +306,12 @@ template<> void BaseSetting<bool>::convertToArg(Args & args, const std::string &
     });
 }
 
-template<> Strings BaseSetting<Strings>::parse(const std::string & str) const
+template<> Strings BaseSetting<Strings>::parse(const std::string & str, const ApplyConfigOptions & options) const
 {
     return tokenizeString<Strings>(str);
 }
 
-template<> void BaseSetting<Strings>::appendOrSet(Strings newValue, bool append)
+template<> void BaseSetting<Strings>::appendOrSet(Strings newValue, bool append, const ApplyConfigOptions & options)
 {
     if (!append) value.clear();
     value.insert(value.end(), std::make_move_iterator(newValue.begin()),
@@ -314,12 +323,12 @@ template<> std::string BaseSetting<Strings>::to_string() const
     return concatStringsSep(" ", value);
 }
 
-template<> StringSet BaseSetting<StringSet>::parse(const std::string & str) const
+template<> StringSet BaseSetting<StringSet>::parse(const std::string & str, const ApplyConfigOptions & options) const
 {
     return tokenizeString<StringSet>(str);
 }
 
-template<> void BaseSetting<StringSet>::appendOrSet(StringSet newValue, bool append)
+template<> void BaseSetting<StringSet>::appendOrSet(StringSet newValue, bool append, const ApplyConfigOptions & options)
 {
     if (!append) value.clear();
     value.insert(std::make_move_iterator(newValue.begin()), std::make_move_iterator(newValue.end()));
@@ -330,7 +339,7 @@ template<> std::string BaseSetting<StringSet>::to_string() const
     return concatStringsSep(" ", value);
 }
 
-template<> ExperimentalFeatures BaseSetting<ExperimentalFeatures>::parse(const std::string & str) const
+template<> ExperimentalFeatures BaseSetting<ExperimentalFeatures>::parse(const std::string & str, const ApplyConfigOptions & options) const
 {
     ExperimentalFeatures res{};
     for (auto & s : tokenizeString<StringSet>(str)) {
@@ -342,7 +351,7 @@ template<> ExperimentalFeatures BaseSetting<ExperimentalFeatures>::parse(const s
     return res;
 }
 
-template<> void BaseSetting<ExperimentalFeatures>::appendOrSet(ExperimentalFeatures newValue, bool append)
+template<> void BaseSetting<ExperimentalFeatures>::appendOrSet(ExperimentalFeatures newValue, bool append, const ApplyConfigOptions & options)
 {
     if (append)
         value = value | newValue;
@@ -359,7 +368,7 @@ template<> std::string BaseSetting<ExperimentalFeatures>::to_string() const
     return concatStringsSep(" ", stringifiedXpFeatures);
 }
 
-template<> DeprecatedFeatures BaseSetting<DeprecatedFeatures>::parse(const std::string & str) const
+template<> DeprecatedFeatures BaseSetting<DeprecatedFeatures>::parse(const std::string & str, const ApplyConfigOptions & options) const
 {
     DeprecatedFeatures res{};
     for (auto & s : tokenizeString<StringSet>(str)) {
@@ -371,7 +380,7 @@ template<> DeprecatedFeatures BaseSetting<DeprecatedFeatures>::parse(const std::
     return res;
 }
 
-template<> void BaseSetting<DeprecatedFeatures>::appendOrSet(DeprecatedFeatures newValue, bool append)
+template<> void BaseSetting<DeprecatedFeatures>::appendOrSet(DeprecatedFeatures newValue, bool append, const ApplyConfigOptions & options)
 {
     if (append)
         value = value | newValue;
@@ -388,7 +397,7 @@ template<> std::string BaseSetting<DeprecatedFeatures>::to_string() const
     return concatStringsSep(" ", stringifiedDpFeatures);
 }
 
-template<> StringMap BaseSetting<StringMap>::parse(const std::string & str) const
+template<> StringMap BaseSetting<StringMap>::parse(const std::string & str, const ApplyConfigOptions & options) const
 {
     StringMap res;
     for (const auto & s : tokenizeString<Strings>(str)) {
@@ -399,7 +408,7 @@ template<> StringMap BaseSetting<StringMap>::parse(const std::string & str) cons
     return res;
 }
 
-template<> void BaseSetting<StringMap>::appendOrSet(StringMap newValue, bool append)
+template<> void BaseSetting<StringMap>::appendOrSet(StringMap newValue, bool append, const ApplyConfigOptions & options)
 {
     if (!append) value.clear();
     value.insert(std::make_move_iterator(newValue.begin()), std::make_move_iterator(newValue.end()));
@@ -426,34 +435,40 @@ template class BaseSetting<StringMap>;
 template class BaseSetting<ExperimentalFeatures>;
 template class BaseSetting<DeprecatedFeatures>;
 
-static Path parsePath(const AbstractSetting & s, const std::string & str)
+static Path parsePath(const AbstractSetting & s, const std::string & str, const ApplyConfigOptions & options)
 {
-    if (str == "")
+    if (str == "") {
         throw UsageError("setting '%s' is a path and paths cannot be empty", s.name);
-    else
-        return canonPath(str);
+    } else {
+        auto tildeResolvedPath = tildePath(str, options.home);
+        if (options.path) {
+            return absPath(tildeResolvedPath, dirOf(*options.path));
+        } else {
+            return canonPath(tildeResolvedPath);
+        }
+    }
 }
 
-template<> Path PathsSetting<Path>::parse(const std::string & str) const
+template<> Path PathsSetting<Path>::parse(const std::string & str, const ApplyConfigOptions & options) const
 {
-    return parsePath(*this, str);
+    return parsePath(*this, str, options);
 }
 
-template<> std::optional<Path> PathsSetting<std::optional<Path>>::parse(const std::string & str) const
+template<> std::optional<Path> PathsSetting<std::optional<Path>>::parse(const std::string & str, const ApplyConfigOptions & options) const
 {
     if (str == "")
         return std::nullopt;
     else
-        return parsePath(*this, str);
+        return parsePath(*this, str, options);
 }
 
-template<> Paths PathsSetting<Paths>::parse(const std::string & str) const
+template<> Paths PathsSetting<Paths>::parse(const std::string & str, const ApplyConfigOptions & options) const
 {
     auto strings = tokenizeString<Strings>(str);
     Paths parsed;
 
     for (auto str : strings) {
-        parsed.push_back(canonPath(str));
+        parsed.push_back(parsePath(*this, str, options));
     }
 
     return parsed;
@@ -464,10 +479,10 @@ template class PathsSetting<std::optional<Path>>;
 template class PathsSetting<Paths>;
 
 
-bool GlobalConfig::set(const std::string & name, const std::string & value)
+bool GlobalConfig::set(const std::string & name, const std::string & value, const ApplyConfigOptions & options)
 {
     for (auto & config : *configRegistrations)
-        if (config->set(name, value)) return true;
+        if (config->set(name, value, options)) return true;
 
     unknownSettings.emplace(name, value);
 
diff --git a/src/libutil/config.hh b/src/libutil/config.hh
index dbca4b406..59cc281c5 100644
--- a/src/libutil/config.hh
+++ b/src/libutil/config.hh
@@ -10,6 +10,7 @@
 #include "types.hh"
 #include "experimental-features.hh"
 #include "deprecated-features.hh"
+#include "apply-config-options.hh"
 
 namespace nix {
 
@@ -61,7 +62,7 @@ public:
      * Sets the value referenced by `name` to `value`. Returns true if the
      * setting is known, false otherwise.
      */
-    virtual bool set(const std::string & name, const std::string & value) = 0;
+    virtual bool set(const std::string & name, const std::string & value, const ApplyConfigOptions & options = {}) = 0;
 
     struct SettingInfo
     {
@@ -81,7 +82,7 @@ public:
      * - contents: configuration contents to be parsed and applied
      * - path: location of the configuration file
      */
-    void applyConfig(const std::string & contents, const std::string & path = "<unknown>");
+    void applyConfig(const std::string & contents, const ApplyConfigOptions & options = {});
 
     /**
      * Resets the `overridden` flag of all Settings
@@ -155,7 +156,7 @@ public:
 
     Config(StringMap initials = {});
 
-    bool set(const std::string & name, const std::string & value) override;
+    bool set(const std::string & name, const std::string & value, const ApplyConfigOptions & options = {}) override;
 
     void addSetting(AbstractSetting * setting);
 
@@ -200,7 +201,7 @@ protected:
 
     virtual ~AbstractSetting();
 
-    virtual void set(const std::string & value, bool append = false) = 0;
+    virtual void set(const std::string & value, bool append = false, const ApplyConfigOptions & options = {}) = 0;
 
     /**
      * Whether the type is appendable; i.e. whether the `append`
@@ -237,7 +238,7 @@ protected:
      *
      * Used by `set()`.
      */
-    virtual T parse(const std::string & str) const;
+    virtual T parse(const std::string & str, const ApplyConfigOptions & options) const;
 
     /**
      * Append or overwrite `value` with `newValue`.
@@ -247,7 +248,7 @@ protected:
      *
      * @param append Whether to append or overwrite.
      */
-    virtual void appendOrSet(T newValue, bool append);
+    virtual void appendOrSet(T newValue, bool append, const ApplyConfigOptions & options);
 
 public:
 
@@ -284,7 +285,7 @@ public:
      * Uses `parse()` to get the value from `str`, and `appendOrSet()`
      * to set it.
      */
-    void set(const std::string & str, bool append = false) override final;
+    void set(const std::string & str, bool append = false, const ApplyConfigOptions & options = {}) override final;
 
     /**
      * C++ trick; This is template-specialized to compile-time indicate whether
@@ -373,7 +374,7 @@ public:
         options->addSetting(this);
     }
 
-    T parse(const std::string & str) const override;
+    T parse(const std::string & str, const ApplyConfigOptions & options) const override;
 
     void operator =(const T & v) { this->assign(v); }
 };
@@ -384,7 +385,7 @@ struct GlobalConfig : public AbstractConfig
     typedef std::vector<Config*> ConfigRegistrations;
     static ConfigRegistrations * configRegistrations;
 
-    bool set(const std::string & name, const std::string & value) override;
+    bool set(const std::string & name, const std::string & value, const ApplyConfigOptions & options = {}) override;
 
     void getSettings(std::map<std::string, SettingInfo> & res, bool overriddenOnly = false) override;
 
diff --git a/src/libutil/file-system.cc b/src/libutil/file-system.cc
index 1d266067e..8c69c9864 100644
--- a/src/libutil/file-system.cc
+++ b/src/libutil/file-system.cc
@@ -118,6 +118,21 @@ Path realPath(Path const & path)
     return ret;
 }
 
+Path tildePath(Path const & path, const std::optional<Path> & home)
+{
+    if (path.starts_with("~/")) {
+        if (home) {
+            return *home + "/" + path.substr(2);
+        } else {
+            throw UsageError("`~` path not allowed: %1%", path);
+        }
+    } else if (path.starts_with('~')) {
+        throw UsageError("`~` paths must start with `~/`: %1%", path);
+    } else {
+        return path;
+    }
+}
+
 void chmodPath(const Path & path, mode_t mode)
 {
     if (chmod(path.c_str(), mode) == -1)
diff --git a/src/libutil/file-system.hh b/src/libutil/file-system.hh
index e49323e84..0a54d1a3b 100644
--- a/src/libutil/file-system.hh
+++ b/src/libutil/file-system.hh
@@ -63,6 +63,16 @@ Path canonPath(PathView path, bool resolveSymlinks = false);
 Path realPath(Path const & path);
 
 /**
+ * Resolve a tilde path like `~/puppy.nix` into an absolute path.
+ *
+ * If `home` is given, it's substituted for `~/` at the start of the input
+ * `path`. Otherwise, an error is thrown.
+ *
+ * If the path starts with `~` but not `~/`, an error is thrown.
+ */
+Path tildePath(Path const & path, const std::optional<Path> & home = std::nullopt);
+
+/**
  * Change the permissions of a path
  * Not called `chmod` as it shadows and could be confused with
  * `int chmod(char *, mode_t)`, which does not handle errors