ox-latex: Fix exporting longtable with multiline header

* lisp/ox-latex.el (org-latex-table-row): Use all the rows when
constructing header definition.
* testing/lisp/test-ox-latex.el (test-ox-latex/longtable): Add new
test.

Reported-by: Brett Presnell <presnell@member.fsf.org>
Link: https://orgmode.org/list/87mt9zywco.fsf@localhost

lisp/ox-latex.el

@@ -4046,6 +4046,18 @@ a communication channel."
 		(org-export-get-previous-element table-row info) info))
 	  "")
 	 (t "\\midrule"))
+      ;; Memorize table header in case it is multiline. We need this
+      ;; information to define contents before "\\endhead" in longtable environments.
+      (when (org-export-table-row-in-header-p table-row info)
+        (let ((table-head-cache (plist-get info :org-latex-table-head-cache)))
+          (unless (hash-table-p table-head-cache)
+            (setq table-head-cache (make-hash-table :test #'eq))
+            (plist-put info :org-latex-table-head-cache table-head-cache))
+          (if-let ((head-contents (gethash (org-element-parent table-row) table-head-cache)))
+              (puthash (org-element-parent table-row) (concat head-contents org-latex-line-break-safe "\n" contents)
+                       table-head-cache)
+            (puthash (org-element-parent table-row) contents table-head-cache))))
+      ;; Return LaTeX string as the transcoder.
       (concat
        ;; When BOOKTABS are activated enforce top-rule even when no
        ;; hline was specifically marked.
@@ -4075,7 +4087,7 @@ a communication channel."
 		     "")
 		    (booktabsp "\\toprule\n")
 		    (t "\\hline\n"))
-		   contents
+		   (gethash (org-element-parent table-row) (plist-get info :org-latex-table-head-cache))
 		   (if booktabsp "\\midrule" "\\hline")
 		   (if booktabsp "\\midrule" "\\hline")
 		   columns

testing/lisp/test-ox-latex.el

@@ -58,5 +58,40 @@ lorem ipsum dolor\\\\[0pt]
 lorem ipsum dolor\\\\[0pt]
 \\end{verse}"))))
 
+(ert-deftest test-ox-latex/longtable ()
+  "Test table export with longtable environment."
+  (org-test-with-exported-text
+      'latex
+      "#+attr_latex: :environment longtable
+| First        | Second |
+| Column       | Column |
+|--------------+--------|
+| a            |      1 |
+| b            |      2 |
+| \\pagebreak c |      3 |
+| d            |      4 |
+"
+    (goto-char (point-min))
+    (should
+     (search-forward
+      "\\begin{longtable}{lr}
+First & Second\\\\[0pt]
+Column & Column\\\\[0pt]
+\\hline
+\\endfirsthead"))
+    (goto-char (point-min))
+    (should
+     (search-forward
+      "First & Second\\\\[0pt]
+Column & Column \\\\[0pt]
+
+\\hline
+\\endhead"))
+    (goto-char (point-min))
+    (should
+     (search-forward
+      "\\hline\\multicolumn{2}{r}{Continued on next page} \\\\
+\\endfoot"))))
+
 (provide 'test-ox-latex)
 ;;; test-ox-latex.el ends here