Memoize per-function nil results in the root cache (#1836)

Previously a root function returning nil stored nil in the cache,
which is indistinguishable from a missing entry, so on the next call
the function was re-run.  When a project's root is found by the third
function in projectile-project-root-functions, the first two
re-walked the tree on every call - and projectile-project-root is hot
in mode-line / company / spaceline updates.

Store the symbol 'none for unsuccessful entries and treat it as a
cache hit on lookup.  The overall-failure marker already used 'none
in a separate slot, so this just extends the same convention to the
per-function entries.

1 files changed, 25 insertions, 1 deletions

diff --git a/test/projectile-test.el b/test/projectile-test.el
index 52f24b1..88d1da7 100644
--- a/test/projectile-test.el
+++ b/test/projectile-test.el
@@ -1283,7 +1283,31 @@ Just delegates OPERATION and ARGS for all operations except for`shell-command`'.
         ;; Second "buffer": same default-directory, different override.
         ;; The cache must not return the previous buffer's answer.
         (let ((projectile-project-root beta-root))
-          (expect (projectile-project-root) :to-equal beta-root)))))))
+          (expect (projectile-project-root) :to-equal beta-root))))))
+
+  (it "caches per-function nil results so earlier misses aren't re-walked (#1836)"
+    (projectile-test-with-sandbox
+     (projectile-test-with-files
+      ("projectA/.git/"
+       "projectA/src/")
+      (let* ((calls 0)
+             (always-nil (lambda (_dir) (cl-incf calls) nil))
+             (projectile-project-root-functions
+              (list always-nil 'projectile-root-bottom-up))
+             (projectile-project-root-files-bottom-up '(".git"))
+             (dir (expand-file-name "projectA/src/"))
+             (projectile-project-root-cache (make-hash-table :test 'equal)))
+        ;; first call: always-nil is invoked, then bottom-up wins
+        (expect (file-truename (projectile-project-root dir))
+                :to-equal (file-truename (expand-file-name "projectA/")))
+        (expect calls :to-equal 1)
+        ;; second call: the cached 'none for always-nil should be honoured
+        (expect (file-truename (projectile-project-root dir))
+                :to-equal (file-truename (expand-file-name "projectA/")))
+        (expect calls :to-equal 1)
+        ;; and the cache holds the 'none sentinel for the failing function
+        (expect (gethash (cons always-nil dir) projectile-project-root-cache)
+                :to-be 'none))))))
 
 (describe "projectile-file-exists-p"
   (it "returns t if file exists"