Merge pull request #487 from nix-community/GH-486

Issue a warning if we are falling back to the old shell environment

README.md

@@ -275,51 +275,52 @@ nix invocation.
 
 #### Tracked files
 
-`nix-direnv` makes a performance trade-off and only considers changes in a
-limited number of files when deciding to update its cache.
+As a convenience, `nix-direnv` adds common files to direnv's watched file list
+automatically.
 
-- for `use nix` this is:
+The list of additionally tracked files is as follows:
 
+- for `use nix`:
   - `~/.direnvrc`
   - `~/.config/direnv/direnvrc`
   - `.envrc`,
   - A single nix file. In order of preference:
     - The file argument to `use nix`
-    - `shell.nix` if it exists
     - `default.nix` if it exists
+    - `shell.nix` if it exists
 
-- for `use flake` this is:
+- for `use flake`:
   - `~/.direnvrc`
   - `~/.config/direnv/direnvrc`
   - `.envrc`
   - `flake.nix`
   - `flake.lock`
   - `devshell.toml` if it exists
 
-To add more files to be checked use `watch_file` like this
-
-```shell
-watch_file your-file.nix
-use nix # or use flake
-```
-
-Or - if you don't mind the overhead (runtime and conceptual) of watching all
-nix-files:
+Users are free to use direnv's builtin `watch_file` function to track additional
+files. `watch_file` must be invoked before either `use flake` or `use nix` to
+take effect.
 
-```shell
-watch_file $(find . -name "*.nix" -printf '"%p" ')
-```
+#### Environment Variables
 
-Note that this will re-execute direnv for any nix change, regardless of whether
-that change is meaningful for the devShell in use.
+nix-direnv sets the following environment variables for user consumption. All
+other environment variables are either a product of the underlying nix
+invocation or are purely incidental and should not be relied upon.
 
-`watch_file` must be invoked before either `use flake` or `use nix` to take
-effect.
+- `NIX_DIRENV_DID_FALLBACK`: Set when the current revision of your nix shell or
+  flake's devShell are invalid and nix-direnv has loaded the last known working
+  shell.
 
 ## General direnv tips
 
-- [Changing where direnv stores its cache](https://github.com/direnv/direnv/wiki/Customizing-cache-location)
-- [Quickly setting up direnv in a new nix project](https://github.com/nix-community/nix-direnv/wiki/Shell-integration)
+- [Changing where direnv stores its cache][cache_location]
+- [Quickly setting up direnv in a new nix project][new_project]
+- [Disable the diff notice (requires direnv 2.34+)][hide_diff_notice]: Note that
+  this goes into direnv's TOML configuration!
+
+[cache_location]: https://github.com/direnv/direnv/wiki/Customizing-cache-location
+[new_project]: https://github.com/nix-community/nix-direnv/wiki/Shell-integration
+[hide_diff_notice]: https://direnv.net/man/direnv.toml.1.html#codehideenvdiffcode
 
 ## Other projects in the field
 

direnvrc

@@ -298,6 +298,7 @@ use_flake() {
       local tmp_profile_rc
       local tmp_profile="${layout_dir}/flake-tmp-profile.$$"
       if tmp_profile_rc=$(_nix print-dev-env --profile "$tmp_profile" "$@"); then
+        # If we've gotten here, the user's current devShell is valid and we should cache it
         _nix_clean_old_gcroots "$layout_dir"
 
         # We need to update our cache
@@ -318,13 +319,20 @@ use_flake() {
           flake_input_paths="${flake_input_paths/${store_path}/}"
         done
 
-        _nix_direnv_info "renewed cache"
+        _nix_direnv_info "Renewed cache"
+      else
+        # The user's current flake failed to evaluate,
+        # but there is already a prior profile_rc,
+        # which is probably more useful than nothing.
+        # Fallback to use that (which means just leaving profile_rc alone!)
+        _nix_direnv_warning "Evaluating current devShell failed. Falling back to previous environment!"
+        export NIX_DIRENV_DID_FALLBACK=1
       fi
     fi
   else
     if [[ -e ${profile_rc} ]]; then
       # Our cache is valid, use that
-      _nix_direnv_info "using cached dev shell"
+      _nix_direnv_info "Using cached dev shell"
     else
       # We don't have a profile_rc to use!
       _nix_direnv_error "use_flake failed - Is your flake's devShell working?"
@@ -469,12 +477,15 @@ use_nix() {
         echo "$tmp_profile_rc" >"$profile_rc"
         _nix_add_gcroot "$tmp_profile" "$profile"
         rm -f "$tmp_profile" "$tmp_profile"*
-        _nix_direnv_info "renewed cache"
+        _nix_direnv_info "Renewed cache"
+      else
+        _nix_direnv_warning "Evaluating current nix shell failed. Falling back to previous environment!"
+        export NIX_DIRENV_DID_FALLBACK=1
       fi
     fi
   else
     if [[ -e ${profile_rc} ]]; then
-      _nix_direnv_info "using cached dev shell"
+      _nix_direnv_info "Using cached dev shell"
     else
       _nix_direnv_error "use_nix failed - Is your nix shell working?"
       return 1

flake.nix

@@ -43,7 +43,7 @@
           };
 
           devShells.default = pkgs.callPackage ./shell.nix {
-            packages = [ config.treefmt.build.wrapper ];
+            packages = [ config.treefmt.build.wrapper pkgs.shellcheck ];
           };
 
           checks =

tests/test_gc.py

@@ -24,7 +24,7 @@ def common_test(direnv_project: DirenvProject) -> None:
     )
     sys.stderr.write(out1.stderr)
     assert out1.returncode == 0
-    assert "renewed cache" in out1.stderr
+    assert "Renewed cache" in out1.stderr
     assert "Executing shellHook." in out1.stderr
 
     run(["nix-collect-garbage"])
@@ -37,7 +37,7 @@ def common_test(direnv_project: DirenvProject) -> None:
     )
     sys.stderr.write(out2.stderr)
     assert out2.returncode == 0
-    assert "using cached dev shell" in out2.stderr
+    assert "Using cached dev shell" in out2.stderr
     assert "Executing shellHook." in out2.stderr
 
 

tests/test_use_nix.py

@@ -30,7 +30,7 @@ def direnv_exec(
     sys.stderr.write(out.stderr)
     assert out.returncode == 0
     assert out.stdout == "OK\n"
-    assert "renewed cache" in out.stderr
+    assert "Renewed cache" in out.stderr
 
 
 @pytest.mark.parametrize("strict_env", [False, True])