feat(coverage): add backend registry

The coverage-core module now has a registry protocol so per-language backends can plug in without touching the core.

A backend is a plist with :name, :detect, :run, and :lcov-path. cj/coverage-register-backend appends to cj/coverage-backends, or replaces an existing entry with the same :name at its original position (so first-registered wins on ties).

cj/--coverage-backend-for-project resolves which backend applies to a project root. Resolution order:

1. An OVERRIDE argument (typically buffer-local cj/coverage-backend from .dir-locals.el) wins if supplied, and errors if it names an unregistered backend.
2. Otherwise, walk the registry in order and return the first backend whose :detect returns non-nil for the given root.

Tests cover Normal (register and retrieve, re-register replaces in place, first detect wins), Boundary (empty registry, no match, override bypasses detect, detect receives the root), and Error (override names an unknown backend).

With the registry in place, the elisp backend (and later python / typescript / go) can self-register on load without any changes to coverage-core.

2 files changed, 162 insertions, 0 deletions

diff --git a/modules/coverage-core.el b/modules/coverage-core.el
index e7540434..f1e8ae88 100644
--- a/modules/coverage-core.el
+++ b/modules/coverage-core.el
@@ -12,6 +12,59 @@
 
 ;;; Code:
 
+(require 'seq)
+
+(defvar cj/coverage-backends nil
+  "Registry of coverage backends in priority order.
+Each entry is a plist with at least :name, :detect, :run, and :lcov-path.
+Use `cj/coverage-register-backend' to add or replace an entry.")
+
+(defvar-local cj/coverage-backend nil
+  "Override: name of the coverage backend to use for the current project.
+When nil (the default), resolution runs each registered backend's :detect
+function in registration order.  Typically set buffer-locally via
+`.dir-locals.el' to pin a specific backend.")
+
+(defun cj/coverage-register-backend (backend)
+  "Register BACKEND, a plist with :name, :detect, :run, :lcov-path.
+Appends to `cj/coverage-backends' at the end, or replaces the existing
+entry with the same :name in its current position."
+  (let ((name (plist-get backend :name)))
+	(if (cj/--coverage-backend-by-name name)
+		(setq cj/coverage-backends
+			  (mapcar (lambda (b)
+						(if (eq (plist-get b :name) name) backend b))
+					  cj/coverage-backends))
+	  (setq cj/coverage-backends
+			(append cj/coverage-backends (list backend))))))
+
+(defun cj/--coverage-backend-by-name (name)
+  "Return the registered backend whose :name equals NAME, or nil."
+  (seq-find (lambda (b) (eq name (plist-get b :name)))
+			cj/coverage-backends))
+
+(defun cj/--coverage-backend-for-project (root &optional override)
+  "Resolve the coverage backend to use for ROOT.
+OVERRIDE, if non-nil, is a backend name symbol (typically the value of
+`cj/coverage-backend' from .dir-locals.el).  When given, the named
+backend is returned regardless of any :detect functions.  Signals
+`user-error' when OVERRIDE names a backend that isn't registered.
+
+When OVERRIDE is nil, each backend's :detect is called in turn with
+ROOT as its sole argument; the first that returns non-nil wins.
+Returns the backend plist, or nil when no backend matches."
+  (cond
+   (override
+	(or (cj/--coverage-backend-by-name override)
+		(user-error
+		 "Unknown coverage backend: %s (registered: %s)"
+		 override
+		 (mapcar (lambda (b) (plist-get b :name)) cj/coverage-backends))))
+   (t
+	(seq-find (lambda (backend)
+				(funcall (plist-get backend :detect) root))
+			  cj/coverage-backends))))
+
 (defun cj/--coverage-parse-lcov (file)
   "Parse FILE as LCOV and return a hash table of covered lines.
 Keys are source-file paths (strings).  Values are hash tables whose
diff --git a/tests/test-coverage-core--backend-registry.el b/tests/test-coverage-core--backend-registry.el
new file mode 100644
index 00000000..ac1f6f7d
--- /dev/null
+++ b/tests/test-coverage-core--backend-registry.el
@@ -0,0 +1,109 @@
+;;; test-coverage-core--backend-registry.el --- Tests for coverage backend registry -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Unit tests for the backend registry.
+;;
+;; A backend is a plist with at least :name, :detect, :run, and :lcov-path
+;; keys.  `cj/coverage-register-backend' adds or replaces an entry.
+;; `cj/--coverage-backend-for-project' resolves which backend applies to
+;; a project root, honoring an optional override (buffer-local
+;; `cj/coverage-backend' from .dir-locals.el in real use).
+
+;;; Code:
+
+(require 'ert)
+
+(add-to-list 'load-path (expand-file-name "modules" user-emacs-directory))
+(require 'coverage-core)
+
+(defmacro test-coverage-registry-with-empty (&rest body)
+  "Run BODY with `cj/coverage-backends' rebound to an empty list."
+  (declare (indent 0))
+  `(let ((cj/coverage-backends nil))
+	 ,@body))
+
+;;; Normal cases
+
+(ert-deftest test-coverage-backend-register-adds-entry ()
+  "Normal: registering a backend makes it retrievable by name."
+  (test-coverage-registry-with-empty
+	(cj/coverage-register-backend
+	 '(:name elisp :detect (lambda (_) t) :run ignore :lcov-path ignore))
+	(should (= 1 (length cj/coverage-backends)))
+	(should (eq 'elisp (plist-get (car cj/coverage-backends) :name)))))
+
+(ert-deftest test-coverage-backend-register-replaces-in-place ()
+  "Normal: re-registering by name replaces the existing entry at the same position."
+  (test-coverage-registry-with-empty
+	(cj/coverage-register-backend
+	 '(:name elisp :detect (lambda (_) nil) :run ignore :lcov-path ignore))
+	(cj/coverage-register-backend
+	 '(:name python :detect (lambda (_) nil) :run ignore :lcov-path ignore))
+	(cj/coverage-register-backend
+	 '(:name elisp :detect (lambda (_) t) :run ignore :lcov-path ignore))
+	(should (= 2 (length cj/coverage-backends)))
+	(should (eq 'elisp (plist-get (nth 0 cj/coverage-backends) :name)))
+	(should (eq 'python (plist-get (nth 1 cj/coverage-backends) :name)))
+	(should (funcall (plist-get (nth 0 cj/coverage-backends) :detect) "/tmp"))))
+
+(ert-deftest test-coverage-backend-for-project-first-detect-wins ()
+  "Normal: resolution returns the first backend whose :detect matches."
+  (test-coverage-registry-with-empty
+	(cj/coverage-register-backend
+	 '(:name a :detect (lambda (_) nil) :run ignore :lcov-path ignore))
+	(cj/coverage-register-backend
+	 '(:name b :detect (lambda (_) t)   :run ignore :lcov-path ignore))
+	(cj/coverage-register-backend
+	 '(:name c :detect (lambda (_) t)   :run ignore :lcov-path ignore))
+	(let ((backend (cj/--coverage-backend-for-project "/tmp")))
+	  (should (eq 'b (plist-get backend :name))))))
+
+;;; Boundary cases
+
+(ert-deftest test-coverage-backend-for-project-empty-registry ()
+  "Boundary: empty registry returns nil, not an error."
+  (test-coverage-registry-with-empty
+	(should (null (cj/--coverage-backend-for-project "/tmp")))))
+
+(ert-deftest test-coverage-backend-for-project-no-match ()
+  "Boundary: no backend's :detect matches returns nil."
+  (test-coverage-registry-with-empty
+	(cj/coverage-register-backend
+	 '(:name a :detect (lambda (_) nil) :run ignore :lcov-path ignore))
+	(cj/coverage-register-backend
+	 '(:name b :detect (lambda (_) nil) :run ignore :lcov-path ignore))
+	(should (null (cj/--coverage-backend-for-project "/tmp")))))
+
+(ert-deftest test-coverage-backend-for-project-override-bypasses-detect ()
+  "Boundary: OVERRIDE returns the named backend without calling :detect."
+  (test-coverage-registry-with-empty
+	(cj/coverage-register-backend
+	 '(:name a :detect (lambda (_) nil) :run ignore :lcov-path ignore))
+	(cj/coverage-register-backend
+	 '(:name b :detect (lambda (_) nil) :run ignore :lcov-path ignore))
+	(let ((backend (cj/--coverage-backend-for-project "/tmp" 'b)))
+	  (should (eq 'b (plist-get backend :name))))))
+
+(ert-deftest test-coverage-backend-for-project-detect-receives-root ()
+  "Boundary: the :detect function is called with the project root."
+  (test-coverage-registry-with-empty
+	(let ((captured nil))
+	  (cj/coverage-register-backend
+	   `(:name a
+			   :detect ,(lambda (root) (setq captured root) t)
+			   :run ignore :lcov-path ignore))
+	  (cj/--coverage-backend-for-project "/my/root")
+	  (should (equal "/my/root" captured)))))
+
+;;; Error cases
+
+(ert-deftest test-coverage-backend-for-project-override-unknown-errors ()
+  "Error: OVERRIDE that names an unregistered backend signals user-error."
+  (test-coverage-registry-with-empty
+	(cj/coverage-register-backend
+	 '(:name a :detect (lambda (_) t) :run ignore :lcov-path ignore))
+	(should-error (cj/--coverage-backend-for-project "/tmp" 'bogus)
+				  :type 'user-error)))
+
+(provide 'test-coverage-core--backend-registry)
+;;; test-coverage-core--backend-registry.el ends here