Support relative and ~/ paths in config settings

Change-Id: I5566a9858ba255f4ac5051d1368c7dfb24460f0a

doc/manual/rl-next/relative-and-tilde-paths-in-config.md

@@ -0,0 +1,30 @@
+---
+synopsis: Relative and tilde paths in configuration
+issues: [fj#482]
+cls: [1851, 1863, 1864]
+category: Features
+credits: [9999years]
+---
+
+[Configuration settings](@docroot@/command-ref/conf-file.md) can now refer to
+files with paths relative to the file they're written in or relative to your
+home directory (with `~/`).
+
+This makes settings like
+[`repl-overlays`](@docroot@/command-ref/conf-file.md#conf-repl-overlays) and
+[`secret-key-files`](@docroot@/command-ref/conf-file.md#conf-repl-overlays)
+much easier to set, especially if you'd like to refer to files in an existing
+dotfiles repo cloned into your home directory.
+
+If you put `repl-overlays = repl.nix` in your `~/.config/nix/nix.conf`, it'll
+load `~/.config/nix/repl.nix`. Similarly, you can set `repl-overlays =
+~/.dotfiles/repl.nix` to load a file relative to your home directory.
+
+Configuration files can also
+[`include`](@docroot@/command-ref/conf-file.md#file-format) paths relative to
+your home directory.
+
+Only user configuration files (like `$XDG_CONFIG_HOME/nix/nix.conf` or the
+files listed in `$NIX_USER_CONF_FILES`) can use tilde paths relative to your
+home directory. Configuration listed in the `$NIX_CONFIG` environment variable
+may not use relative paths.

src/libstore/globals.cc

@@ -131,8 +131,9 @@ void loadConfFile()
     globalConfig.resetOverridden();
 
     auto files = settings.nixUserConfFiles;
+    auto home = getHome();
     for (auto file = files.rbegin(); file != files.rend(); file++) {
-        applyConfigFile(ApplyConfigOptions{.path = *file});
+        applyConfigFile(ApplyConfigOptions{.path = *file, .home = home});
     }
 
     auto nixConfEnv = getEnv("NIX_CONFIG");

src/libutil/config.cc

@@ -126,7 +126,7 @@ static void applyConfigInner(const std::string & contents, const ApplyConfigOpti
             if (!options.path) {
                 throw UsageError("can only include configuration '%1%' from files", tokens[1]);
             }
-            auto pathToInclude = absPath(tokens[1], dirOf(*options.path));
+            auto pathToInclude = absPath(tildePath(tokens[1], options.home), dirOf(*options.path));
             if (pathExists(pathToInclude)) {
                 auto includeOptions = ApplyConfigOptions {
                     .path = pathToInclude,
@@ -437,10 +437,16 @@ template class BaseSetting<DeprecatedFeatures>;
 
 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
+    } else {
-        return canonPath(str);
+        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 ApplyConfigOptions & options) const

src/libutil/file-system.cc

@@ -117,6 +117,21 @@ 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)

src/libutil/file-system.hh

@@ -62,6 +62,16 @@ 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

tests/unit/libutil/paths-setting.cc

@@ -48,7 +48,60 @@ TEST_F(PathsSettingTest, parse)
     ASSERT_THAT(config.paths.parse("/puppy/../doggy.nix", {}), Eq<Paths>({"/doggy.nix"}));
 }
 
-TEST_F(PathsSettingTest, append) {
+TEST_F(PathsSettingTest, parseRelative)
+{
+    auto options = ApplyConfigOptions{.path = "/doggy/kinds/config.nix"};
+    auto config = mkConfig();
+    ASSERT_THAT(
+        config.paths.parse("puppy.nix", options),
+        Eq<Paths>({"/doggy/kinds/puppy.nix"})
+    );
+
+    // Splits on whitespace:
+    ASSERT_THAT(
+        config.paths.parse("puppy.nix /doggy.nix", options), Eq<Paths>({"/doggy/kinds/puppy.nix", "/doggy.nix"})
+    );
+
+    // Canonicizes paths:
+    ASSERT_THAT(config.paths.parse("../soft.nix", options), Eq<Paths>({"/doggy/soft.nix"}));
+
+    // Canonicizes paths:
+    ASSERT_THAT(config.paths.parse("./soft.nix", options), Eq<Paths>({"/doggy/kinds/soft.nix"}));
+}
+
+TEST_F(PathsSettingTest, parseHome)
+{
+    auto options = ApplyConfigOptions{
+        .path = "/doggy/kinds/config.nix",
+        .home = "/home/puppy"
+    };
+    auto config = mkConfig();
+
+    ASSERT_THAT(
+        config.paths.parse("puppy.nix", options),
+        Eq<Paths>({"/doggy/kinds/puppy.nix"})
+    );
+
+    ASSERT_THAT(
+        config.paths.parse("~/.config/nix/puppy.nix", options),
+        Eq<Paths>({"/home/puppy/.config/nix/puppy.nix"})
+    );
+
+    // Splits on whitespace:
+    ASSERT_THAT(
+        config.paths.parse("~/puppy.nix ~/doggy.nix", options),
+        Eq<Paths>({"/home/puppy/puppy.nix", "/home/puppy/doggy.nix"})
+    );
+
+    // Canonicizes paths:
+    ASSERT_THAT(config.paths.parse("~/../why.nix", options), Eq<Paths>({"/home/why.nix"}));
+
+    // Home paths for other users not allowed. Needs to start with `~/`.
+    ASSERT_THROW(config.paths.parse("~root/config.nix", options), Error);
+}
+
+TEST_F(PathsSettingTest, append)
+{
     auto config = mkConfig();
 
     ASSERT_TRUE(config.paths.isAppendable());