[PATCH] Fix missing null check for tangle links during export

[Tom Gillespie <tgbugs@gmail.com>]

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Hi,
   Here is a patch that fixes broken tangling during export. I don't have a
simple reproduction, but was able to run git bisect to find the originating
commit. The commit text explains the issue in detail. Best,
Tom

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From 0b4e35453874db0acf046e46c571cdef5f39f59a Mon Sep 17 00:00:00 2001
From: Tom Gillespie <tgbugs@gmail.com>
Date: Mon, 19 Dec 2022 21:25:02 -0500
Subject: [PATCH] Fix missing null check for tangle links during export

* lisp/ob-tangle.el (org-babel-tangle--unbracketed-link): Add the let
bound variable l to the and statement to prevent l from being passed
to `string-match' in the event that it is nil.

When tangling nested noweb blocks during export it is possible for the
results of `org-store-link' to return nil.  This commit ensures that
the value returned for l is only passed to `string-match' when it is
non-nil, avoiding a `wrong-type-argument' (stringp nil) error.

Handling of comments and nesting of babel blocks is known to have
issues.  The bug is from 8a781d35dc68f20fa2a5546c98ba3d9b77ee3cda
where new code was introduced to obtain the value for bare which was
not present in early code.  I'm guessing that the bug appears now
because `string-match' is called on l (aka link) at a point in time
when it was never previously called and it was thus masked because in
the old version it was impossible to call `string-match' when `link'
was nil because another variable was always nil, masking the issue.
---
 lisp/ob-tangle.el | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/lisp/ob-tangle.el b/lisp/ob-tangle.el
index bd17bda32..fd6b6f3b9 100644
--- a/lisp/ob-tangle.el
+++ b/lisp/ob-tangle.el
@@ -500,7 +500,8 @@ The PARAMS are the 3rd element of the info for the same src block."
                  (cl-letf (((symbol-function 'org-store-link-functions)
                             (lambda () nil)))
                    (org-store-link nil))))
-             (bare (and (string-match org-link-bracket-re l)
+             (bare (and l
+                        (string-match org-link-bracket-re l)
                         (match-string 1 l))))
         (when bare
           (if (and org-babel-tangle-use-relative-file-links
-- 
2.38.2