worktree: teach worktree to lazy-load "prunable" reason

Add worktree_prune_reason() to allow a caller to discover whether a
worktree is prunable and the reason that it is, much like
worktree_lock_reason() indicates whether a worktree is locked and the
reason for the lock. As with worktree_lock_reason(), retrieve the
prunable reason lazily and cache it in the `worktree` structure.

Helped-by: Eric Sunshine <[email protected]>
Signed-off-by: Rafael Silva <[email protected]>
Reviewed-by: Eric Sunshine <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: raffs

worktree.c

@@ -15,6 +15,7 @@ void free_worktrees(struct worktree **worktrees)
 		free(worktrees[i]->id);
 		free(worktrees[i]->head_ref);
 		free(worktrees[i]->lock_reason);
+		free(worktrees[i]->prune_reason);
 		free(worktrees[i]);
 	}
 	free (worktrees);
@@ -245,6 +246,25 @@ const char *worktree_lock_reason(struct worktree *wt)
 	return wt->lock_reason;
 }
 
+const char *worktree_prune_reason(struct worktree *wt, timestamp_t expire)
+{
+	struct strbuf reason = STRBUF_INIT;
+	char *path = NULL;
+
+	if (is_main_worktree(wt))
+		return NULL;
+	if (wt->prune_reason_valid)
+		return wt->prune_reason;
+
+	if (should_prune_worktree(wt->id, &reason, &path, expire))
+		wt->prune_reason = strbuf_detach(&reason, NULL);
+	wt->prune_reason_valid = 1;
+
+	strbuf_release(&reason);
+	free(path);
+	return wt->prune_reason;
+}
+
 /* convenient wrapper to deal with NULL strbuf */
 static void strbuf_addf_gently(struct strbuf *buf, const char *fmt, ...)
 {

worktree.h

@@ -11,11 +11,13 @@ struct worktree {
 	char *id;
 	char *head_ref;		/* NULL if HEAD is broken or detached */
 	char *lock_reason;	/* private - use worktree_lock_reason */
+	char *prune_reason;     /* private - use worktree_prune_reason */
 	struct object_id head_oid;
 	int is_detached;
 	int is_bare;
 	int is_current;
 	int lock_reason_valid; /* private */
+	int prune_reason_valid; /* private */
 };
 
 /*
@@ -73,6 +75,13 @@ int is_main_worktree(const struct worktree *wt);
  */
 const char *worktree_lock_reason(struct worktree *wt);
 
+/*
+ * Return the reason string if the given worktree should be pruned, otherwise
+ * NULL if it should not be pruned. `expire` defines a grace period to prune
+ * the worktree when its path does not exist.
+ */
+const char *worktree_prune_reason(struct worktree *wt, timestamp_t expire);
+
 /*
  * Return true if worktree entry should be pruned, along with the reason for
  * pruning. Otherwise, return false and the worktree's path in `wtpath`, or