reload: fix infinite reload loop and spurious rebuilds from .devenv/

CHANGELOG.md

@@ -17,6 +17,7 @@
 - Fixed long lines in `devenv shell` getting a hard newline inserted at the wrap point when copying to clipboard. The shell now preserves the soft-wrap when flushing wrapped output into the terminal's scrollback, so clipboard copy keeps the original single line ([#2865](https://github.com/cachix/devenv/issues/2865)).
 - Fixed files declared with the `files` option not being regenerated when an auto-loaded (`devenv allow`) shell reloaded after `devenv update`. enterShell tasks (including `devenv:files`) now re-run on hot-reload, matching a fresh shell entry, instead of only updating environment variables ([#2864](https://github.com/cachix/devenv/issues/2864)).
 - Fixed `devenv test --no-tui` (and any other non-TUI invocation) silently discarding all output from the `enterTest` script, so the test runner's output, traces, and failure messages never reached the terminal or CI logs. Output from commands run in the shell is now printed in non-TUI mode.
+- Fixed `devenv shell` self-triggering hot-reload in an infinite loop after the first reload, and `lib.fileset.fromSource ./.` (and similar `readDir` calls on a parent of `.devenv/`) dragging devenv's own churn (eval cache WAL/SHM, tasks DB, generated shell scripts, `imports.txt`, …) into the Nix tracked-input set and causing spurious rebuilds. The devenv dotfile dir is now excluded from both the reload watch set and the eval cache's tracked inputs, with a carve-out for `.devenv/state/` (`$DEVENV_STATE`) so files users persist there still trigger reload.
 
 ### Improvements
 

devenv-eval-cache/src/db.rs

@@ -15,6 +15,9 @@ pub(crate) fn empty_to_none(s: String) -> Option<String> {
 // Create a constant for embedded migrations
 pub const MIGRATIONS: sqlx::migrate::Migrator = sqlx::migrate!();
 
+/// Filename of the SQLite eval cache database under the devenv dotfile dir.
+pub const DB_FILENAME: &str = "nix-eval-cache.db";
+
 /// The row type for the `file_input` table.
 #[derive(Clone, Debug, PartialEq)]
 pub struct FileInputRow {

devenv-eval-cache/src/ffi_cache.rs

@@ -67,7 +67,19 @@ pub struct CachingConfig {
     /// Additional paths to watch for changes beyond those detected during eval.
     pub extra_watch_paths: Vec<PathBuf>,
     /// Paths to exclude from cache invalidation (e.g., generated files).
+    /// Prefix-matched: any source whose path starts with one of these is
+    /// dropped, unless `excluded_path_exceptions` carves it back in by a
+    /// longer (more specific) prefix.
     pub excluded_paths: Vec<PathBuf>,
+    /// Carve-outs that override `excluded_paths`. Combined with
+    /// `excluded_paths` under longest-prefix-match: for each source, the
+    /// most specific matching entry wins. Ties favor the exception so a
+    /// broad exclude with an equal-depth exception is still tracked. This
+    /// lets callers exclude a parent broadly, carve out a subdirectory,
+    /// and then re-exclude a leaf inside it by adding a longer entry to
+    /// `excluded_paths` (e.g. exclude `.devenv/`, keep `.devenv/state/`,
+    /// re-exclude `.devenv/state/tasks.db`).
+    pub excluded_path_exceptions: Vec<PathBuf>,
     /// Environment variable names to exclude from cache invalidation
     /// (e.g., vars already tracked via NixArgs).
     pub excluded_envs: Vec<String>,
@@ -156,12 +168,30 @@ pub fn ops_to_inputs(ops: impl IntoIterator<Item = EvalOp>, config: &CachingConf
                     continue;
                 }
 
-                // Skip excluded paths
-                if config
+                // Longest-prefix-match between `excluded_paths` and
+                // `excluded_path_exceptions`. The most specific matching
+                // rule wins; ties favor the exception. This lets callers
+                // re-exclude a leaf inside an otherwise-allowed carve-out
+                // (e.g. exclude `.devenv/`, allow `.devenv/state/`,
+                // re-exclude `.devenv/state/tasks.db`).
+                let best_excluded = config
                     .excluded_paths
                     .iter()
-                    .any(|excluded| source.starts_with(excluded))
-                {
+                    .filter(|p| source.starts_with(p))
+                    .map(|p| p.components().count())
+                    .max();
+                let best_allowed = config
+                    .excluded_path_exceptions
+                    .iter()
+                    .filter(|p| source.starts_with(p))
+                    .map(|p| p.components().count())
+                    .max();
+                let drop = match (best_excluded, best_allowed) {
+                    (None, _) => false,
+                    (Some(_), None) => true,
+                    (Some(e), Some(a)) => e > a,
+                };
+                if drop {
                     continue;
                 }
 
@@ -282,6 +312,90 @@ mod tests {
         assert!(inputs.is_empty());
     }
 
+    #[test]
+    fn test_ops_to_inputs_excluded_path_exceptions_kept() {
+        // Broad exclude with a narrow carve-out: anything under /excluded is
+        // dropped, except /excluded/keep — used to ignore devenv's own state
+        // dir while still tracking user files under $DEVENV_STATE.
+        let config = CachingConfig {
+            excluded_paths: vec![PathBuf::from("/excluded")],
+            excluded_path_exceptions: vec![PathBuf::from("/excluded/keep")],
+            ..Default::default()
+        };
+        let ops = vec![
+            EvalOp::ReadFile {
+                source: PathBuf::from("/excluded/internal.db"),
+            },
+            EvalOp::ReadFile {
+                source: PathBuf::from("/excluded/keep/user-file.txt"),
+            },
+        ];
+        let inputs = ops_to_inputs(ops, &config);
+        assert_eq!(inputs.len(), 1);
+        match &inputs[0] {
+            Input::File(desc) => {
+                assert_eq!(desc.path, PathBuf::from("/excluded/keep/user-file.txt"))
+            }
+            _ => panic!("expected file input"),
+        }
+    }
+
+    #[test]
+    fn test_ops_to_inputs_longest_prefix_re_excludes_leaf() {
+        // Re-exclude a leaf inside an exception: `excluded_paths` covers a
+        // broad parent, `excluded_path_exceptions` carves out a subdir,
+        // and a longer entry in `excluded_paths` re-excludes a leaf inside
+        // that subdir. Models the devenv layout: exclude `.devenv/`, keep
+        // `.devenv/state/`, but drop devenv-managed `state/tasks.db*`.
+        // `Path::starts_with` matches at component boundaries, so each
+        // sqlite sibling (`-wal`, `-shm`) is its own component and must be
+        // listed explicitly — they are *not* covered by a `tasks.db`
+        // prefix.
+        let config = CachingConfig {
+            excluded_paths: vec![
+                PathBuf::from("/d"),
+                PathBuf::from("/d/state/tasks.db"),
+                PathBuf::from("/d/state/tasks.db-wal"),
+                PathBuf::from("/d/state/tasks.db-shm"),
+                PathBuf::from("/d/state/git-hooks"),
+            ],
+            excluded_path_exceptions: vec![PathBuf::from("/d/state")],
+            ..Default::default()
+        };
+        let ops = vec![
+            // Dropped by `/d`.
+            EvalOp::ReadFile {
+                source: PathBuf::from("/d/shell-env.sh"),
+            },
+            // Kept by `/d/state` carve-out.
+            EvalOp::ReadFile {
+                source: PathBuf::from("/d/state/postgres/data"),
+            },
+            // Dropped: leaf exclusions are deeper than the carve-out.
+            EvalOp::ReadFile {
+                source: PathBuf::from("/d/state/tasks.db"),
+            },
+            EvalOp::ReadFile {
+                source: PathBuf::from("/d/state/tasks.db-wal"),
+            },
+            EvalOp::ReadFile {
+                source: PathBuf::from("/d/state/tasks.db-shm"),
+            },
+            EvalOp::ReadFile {
+                source: PathBuf::from("/d/state/git-hooks/config.json"),
+            },
+        ];
+        let inputs = ops_to_inputs(ops, &config);
+        let kept: Vec<_> = inputs
+            .iter()
+            .map(|i| match i {
+                Input::File(d) => d.path.clone(),
+                _ => panic!(),
+            })
+            .collect();
+        assert_eq!(kept, vec![PathBuf::from("/d/state/postgres/data")]);
+    }
+
     #[test]
     fn test_ops_to_inputs_filters_excluded_envs() {
         let config = CachingConfig {

devenv-eval-cache/src/lib.rs

@@ -20,4 +20,4 @@ pub use ffi_cache::{CachingConfig, EvalCacheKey, InputTracker, ops_to_inputs};
 pub use resource_manager::{ResourceManager, ResourceSpec};
 
 // Re-export database query functions for file tracking
-pub use db::{get_all_tracked_file_paths, get_file_inputs_by_key_hash};
+pub use db::{DB_FILENAME, get_all_tracked_file_paths, get_file_inputs_by_key_hash};

devenv-nix-backend/src/backend.rs

@@ -325,7 +325,39 @@ impl NixCBackend {
                     force_refresh: self.cache_settings.refresh_eval_cache,
                     extra_watch_paths: core_config_watch_paths(&self.paths.root),
                     excluded_envs: vec!["NIXPKGS_CONFIG".to_string()],
-                    excluded_paths: vec![self.nixpkgs_config_path.clone()],
+                    // Exclude devenv's own dotfile dir from the tracked input
+                    // set. Files inside (eval cache + WAL/SHM, tasks DB,
+                    // generated shell scripts, imports.txt, etc.) are
+                    // rewritten on every evaluation or build; tracking them
+                    // would let `lib.fileset.fromSource ./.` (or any other
+                    // `readDir` on a parent) drag devenv's own churn into
+                    // the cache key and trigger spurious rebuilds.
+                    excluded_paths: {
+                        let state = self.paths.dotfile.join("state");
+                        vec![
+                            self.nixpkgs_config_path.clone(),
+                            self.paths.dotfile.clone(),
+                            // Re-exclude devenv-managed leaves that the
+                            // broader `state/` carve-out below would
+                            // otherwise re-admit. The tasks DB is rewritten
+                            // on every task run and the git-hooks state on
+                            // every reload; tracking either would invalidate
+                            // the eval cache and self-trigger reload loops.
+                            // `tasks.db-wal`/`tasks.db-shm` are listed
+                            // separately because path prefix matching is
+                            // component-wise, not byte-wise.
+                            state.join("tasks.db"),
+                            state.join("tasks.db-wal"),
+                            state.join("tasks.db-shm"),
+                            state.join("git-hooks"),
+                        ]
+                    },
+                    // Carve-out: keep `.devenv/state/` tracked. That's the
+                    // documented `$DEVENV_STATE` area where users persist
+                    // files they want reload/eval to react to. The
+                    // devenv-internal leaves above re-exclude themselves by
+                    // longest-prefix-match.
+                    excluded_path_exceptions: vec![self.paths.dotfile.join("state")],
                 };
                 let service = CachingEvalService::with_config(pool.clone(), config.clone());
                 let invalidation_flag = self.devenv_value_invalidated.clone();

devenv/src/devenv/mod.rs

@@ -385,7 +385,7 @@ impl Devenv {
         if cache_settings.eval_cache {
             eval_cache_pool
                 .get_or_try_init(|| async {
-                    let db_path = devenv_dotfile.join("nix-eval-cache.db");
+                    let db_path = devenv_dotfile.join(devenv_eval_cache::DB_FILENAME);
                     let db = devenv_cache_core::db::Database::new(
                         db_path,
                         &devenv_eval_cache::db::MIGRATIONS,