add NIX_USER_CONF_FILES

Motivation: maintain project-level configuration files.

Document the whole situation a bit better so that it corresponds to the
implementation, and add NIX_USER_CONF_FILES that allows overriding
which user files Nix will load during startup.

doc/manual/command-ref/conf-file.xml

@@ -19,26 +19,30 @@
 
 <refsection><title>Description</title>
 
-<para>Nix reads settings from two configuration files:</para>
+<para>By default Nix reads settings from the following places:</para>
 
-<itemizedlist>
+<para>The system-wide configuration file
+<filename><replaceable>sysconfdir</replaceable>/nix/nix.conf</filename>
+(i.e. <filename>/etc/nix/nix.conf</filename> on most systems), or
+<filename>$NIX_CONF_DIR/nix.conf</filename> if
+<envar>NIX_CONF_DIR</envar> is set. Values loaded in this file are not forwarded to the Nix daemon. The
+client assumes that the daemon has already loaded them.
+</para>
 
-  <listitem>
+<para>User-specific configuration files:</para>
-    <para>The system-wide configuration file
-    <filename><replaceable>sysconfdir</replaceable>/nix/nix.conf</filename>
-    (i.e. <filename>/etc/nix/nix.conf</filename> on most systems), or
-    <filename>$NIX_CONF_DIR/nix.conf</filename> if
-    <envar>NIX_CONF_DIR</envar> is set.</para>
-  </listitem>
 
-  <listitem>
+<para>
-    <para>The user configuration file
+  If <envar>NIX_USER_CONF_FILES</envar> is set, then each path separated by
-    <filename>$XDG_CONFIG_HOME/nix/nix.conf</filename>, or
+  <literal>:</literal> will be loaded in reverse order.
-    <filename>~/.config/nix/nix.conf</filename> if
+</para>
-    <envar>XDG_CONFIG_HOME</envar> is not set.</para>
-  </listitem>
 
-</itemizedlist>
+<para>
+  Otherwise it will look for <filename>nix/nix.conf</filename> files in
+  <envar>XDG_CONFIG_DIRS</envar> and <envar>XDG_CONFIG_HOME</envar>.
+
+  The default location is <filename>$HOME/.config/nix.conf</filename> if
+  those environment variables are unset.
+</para>
 
 <para>The configuration files consist of
 <literal><replaceable>name</replaceable> =

doc/manual/command-ref/env-common.xml

@@ -33,7 +33,7 @@
 
     will cause Nix to look for paths relative to
     <filename>/home/eelco/Dev</filename> and
-    <filename>/etc/nixos</filename>, in that order.  It is also
+    <filename>/etc/nixos</filename>, in this order.  It is also
     possible to match paths against a prefix.  For example, the value
 
     <screen>
@@ -137,12 +137,19 @@ $ mount -o bind /mnt/otherdisk/nix /nix</screen>
 
 <varlistentry><term><envar>NIX_CONF_DIR</envar></term>
 
-  <listitem><para>Overrides the location of the Nix configuration
+  <listitem><para>Overrides the location of the system Nix configuration
   directory (default
   <filename><replaceable>prefix</replaceable>/etc/nix</filename>).</para></listitem>
 
 </varlistentry>
 
+<varlistentry><term><envar>NIX_USER_CONF_FILES</envar></term>
+
+  <listitem><para>Overrides the location of the user Nix configuration files
+  to load from (defaults to the XDG spec locations). The variable is treated
+  as a list separated by the <literal>:</literal> token.</para></listitem>
+
+</varlistentry>
 
 <varlistentry><term><envar>TMPDIR</envar></term>
 

src/libmain/shared.cc

@@ -260,7 +260,10 @@ void printVersion(const string & programName)
         cfg.push_back("signed-caches");
 #endif
         std::cout << "Features: " << concatStringsSep(", ", cfg) << "\n";
-        std::cout << "Configuration file: " << settings.nixConfDir + "/nix.conf" << "\n";
+        std::cout << "System configuration file: " << settings.nixConfDir + "/nix.conf" << "\n";
+        std::cout << "User configuration files: " <<
+            concatStringsSep(":", settings.nixUserConfFiles)
+            << "\n";
         std::cout << "Store directory: " << settings.nixStore << "\n";
         std::cout << "State directory: " << settings.nixStateDir << "\n";
     }

src/libstore/globals.cc

@@ -31,6 +31,7 @@ Settings::Settings()
     , 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)))
+    , nixUserConfFiles(getUserConfigFiles())
     , 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))
@@ -77,13 +78,29 @@ void loadConfFile()
        ~/.nix/nix.conf or the command line. */
     globalConfig.resetOverriden();
 
-    auto dirs = getConfigDirs();
+    auto files = settings.nixUserConfFiles;
-    // Iterate over them in reverse so that the ones appearing first in the path take priority
+    for (auto file = files.rbegin(); file != files.rend(); file++) {
-    for (auto dir = dirs.rbegin(); dir != dirs.rend(); dir++) {
+        globalConfig.applyConfigFile(*file);
-        globalConfig.applyConfigFile(*dir + "/nix/nix.conf");
     }
 }
 
+std::vector<Path> getUserConfigFiles()
+{
+    // Use the paths specified in NIX_USER_CONF_FILES if it has been defined
+    auto nixConfFiles = getEnv("NIX_USER_CONF_FILES");
+    if (nixConfFiles.has_value()) {
+        return tokenizeString<std::vector<string>>(nixConfFiles.value(), ":");
+    }
+
+    // Use the paths specified by the XDG spec
+    std::vector<Path> files;
+    auto dirs = getConfigDirs();
+    for (auto & dir : dirs) {
+        files.insert(files.end(), dir + "/nix/nix.conf");
+    }
+    return files;
+}
+
 unsigned int Settings::getDefaultCores()
 {
     return std::max(1U, std::thread::hardware_concurrency());

src/libstore/globals.hh

@@ -53,9 +53,12 @@ public:
     /* The directory where state is stored. */
     Path nixStateDir;
 
-    /* The directory where configuration files are stored. */
+    /* The directory where system configuration files are stored. */
     Path nixConfDir;
 
+    /* A list of user configuration files to load. */
+    std::vector<Path> nixUserConfFiles;
+
     /* The directory where internal helper programs are stored. */
     Path nixLibexecDir;
 
@@ -378,6 +381,9 @@ void initPlugins();
 
 void loadConfFile();
 
+// Used by the Settings constructor
+std::vector<Path> getUserConfigFiles();
+
 extern const string nixVersion;
 
 }

src/libutil/util.hh

@@ -16,6 +16,7 @@
 #include <sstream>
 #include <optional>
 #include <future>
+#include <iterator>
 
 #ifndef HAVE_STRUCT_DIRENT_D_TYPE
 #define DT_UNKNOWN 0

tests/common.sh.in

@@ -11,6 +11,7 @@ export NIX_LOCALSTATE_DIR=$TEST_ROOT/var
 export NIX_LOG_DIR=$TEST_ROOT/var/log/nix
 export NIX_STATE_DIR=$TEST_ROOT/var/nix
 export NIX_CONF_DIR=$TEST_ROOT/etc
+unset NIX_USER_CONF_FILES
 export _NIX_TEST_SHARED=$TEST_ROOT/shared
 if [[ -n $NIX_STORE ]]; then
     export _NIX_TEST_NO_SANDBOX=1
@@ -21,6 +22,8 @@ export NIX_REMOTE=$NIX_REMOTE_
 unset NIX_PATH
 export TEST_HOME=$TEST_ROOT/test-home
 export HOME=$TEST_HOME
+unset XDG_CONFIG_HOME
+unset XDG_CONFIG_DIRS
 unset XDG_CACHE_HOME
 mkdir -p $TEST_HOME
 

tests/config.sh

@@ -0,0 +1,18 @@
+source common.sh
+
+# Test that files are loaded from XDG by default
+export XDG_CONFIG_HOME=/tmp/home
+export XDG_CONFIG_DIRS=/tmp/dir1:/tmp/dir2
+files=$(nix-build --verbose --version | grep "User config" | cut -d ':' -f2- | xargs)
+[[ $files == "/tmp/home/nix/nix.conf:/tmp/dir1/nix/nix.conf:/tmp/dir2/nix/nix.conf" ]]
+
+# Test that setting NIX_USER_CONF_FILES overrides all the default user config files
+export NIX_USER_CONF_FILES=/tmp/file1.conf:/tmp/file2.conf
+files=$(nix-build --verbose --version | grep "User config" | cut -d ':' -f2- | xargs)
+[[ $files == "/tmp/file1.conf:/tmp/file2.conf" ]]
+
+# Test that it's possible to load the config from a custom location
+here=$(readlink -f "$(dirname "${BASH_SOURCE[0]}")")
+export NIX_USER_CONF_FILES=$here/config/nix-with-substituters.conf
+var=$(nix show-config | grep '^substituters =' | cut -d '=' -f 2 | xargs)
+[[ $var == https://example.com ]]

tests/config/nix-with-substituters.conf

@@ -0,0 +1,2 @@
+experimental-features = nix-command
+substituters = https://example.com

tests/local.mk

@@ -1,5 +1,6 @@
 nix_tests = \
   init.sh hash.sh lang.sh add.sh simple.sh dependencies.sh \
+  config.sh \
   gc.sh \
   gc-concurrent.sh \
   gc-auto.sh \