flakes: search up to git or filesystem boundary

While parsing a flakeref, upon not finding a flake.nix, search upwards
until git or filesystem boundary.

doc/manual/src/release-notes/rl-next.md

@@ -8,3 +8,5 @@
 
 * New built-in function: `builtins.groupBy`, with the same functionality as
   Nixpkgs' `lib.groupBy`, but faster.
+
+* Nix now searches for a flake.nix up until git or filesystem boundary.

src/libexpr/flake/flakeref.cc

@@ -117,8 +117,27 @@ std::pair<FlakeRef, std::string> parseFlakeRefWithFragment(
             if (!S_ISDIR(lstat(path).st_mode))
                 throw BadURL("path '%s' is not a flake (because it's not a directory)", path);
 
-            if (!allowMissing && !pathExists(path + "/flake.nix"))
-                throw BadURL("path '%s' is not a flake (because it doesn't contain a 'flake.nix' file)", path);
+            if (!allowMissing && !pathExists(path + "/flake.nix")){
+                notice("path '%s' does not contain a 'flake.nix', searching up",path);
+
+                // Save device to detect filesystem boundary
+                dev_t device = lstat(path).st_dev;
+                bool found = false;
+                while (path != "/") {
+                    if (pathExists(path + "/flake.nix")) {
+                        found = true;
+                        break;
+                    } else if (pathExists(path + "/.git"))
+                        throw Error("unable to find a flake before encountering git boundary at '%s'", path);
+                    else {
+                        if (lstat(path).st_dev != device)
+                            throw Error("unable to find a flake before encountering filesystem boundary at '%s'", path);
+                    }
+                    path = dirOf(path);
+                }
+                if (!found)
+                    throw BadURL("could not find a flake.nix file");
+            }
 
             auto flakeRoot = path;
             std::string subdir;

tests/flake-searching.sh

@@ -0,0 +1,24 @@
+source common.sh
+
+clearStore
+
+cp ./simple.nix ./simple.builder.sh ./config.nix $TEST_HOME
+cd $TEST_HOME
+cat <<EOF > flake.nix
+{
+    outputs = a: {
+       defaultPackage.$system = import ./simple.nix;
+       packages.$system.test = import ./simple.nix;
+    };
+}
+EOF
+mkdir subdir
+cd subdir
+
+for i in "" . "$PWD" .# .#test; do
+    nix build $i || fail "flake should be found by searching up directories"
+done
+
+for i in "path:$PWD"; do
+    ! nix build $i || fail "flake should not search up directories when using 'path:'"
+done

tests/local.mk

@@ -47,6 +47,7 @@ nix_tests = \
   describe-stores.sh \
   flakes.sh \
   flake-local-settings.sh \
+  flake-searching.sh \
   build.sh \
   repl.sh ca/repl.sh \
   ca/build.sh \