Use mmdc's ability to read from STDIN

[ayorgo]

As mermaid-cli allows piping from STDIN it would be nice to bypass the creation of a temporary file in this mode.

This would make ob-mermaid work with containerized installations without affecting current usage scenarios.

For example, with the following script at /some_path/mmdc:

#!/usr/bin/env sh
docker run --rm -i -u `id -u`:`id -g` -v $(pwd):/data minlag/mermaid-cli:10.6.1 mmdc "$@"

I am able to generate diagrams as follows:

$ cat test.mmd | ./mmdc -o test.png

So if this behaviour was reflected in ob-mermaid.el the following configuration would be enough to make it work with containers:

(setq ob-mermaid-cli-path "/some_path/mmdc")

[bradprob]

A workaround of the suggested request might be:

(use-package ob-mermaid
  :ensure t
  :config
  (setq ob-mermaid-use-wrapper t)
  (setq ob-mermaid-cli-path "/PATH/TO/WRAPPER/SCRIPT.sh")
  (defun org-babel-execute:mermaid (body params)
    (let* ((out-file (or (cdr (assoc :file params))
                         (error "mermaid requires a \":file\" header argument")))
	   (theme (cdr (assoc :theme params)))
	   (width (cdr (assoc :width params)))
	   (height (cdr (assoc :height params)))
	   (background-color (cdr (assoc :background-color params)))
	   (mermaid-config-file (cdr (assoc :mermaid-config-file params)))
	   (css-file (cdr (assoc :css-file params)))
	   (pupeteer-config-file (cdr (assoc :pupeteer-config-file params)))
           (temp-file (org-babel-temp-file "mermaid-"))
           (mmdc (or ob-mermaid-cli-path
                     (executable-find "mmdc")
                     (error "`ob-mermaid-cli-path' is not set and mmdc is not in `exec-path'")))
           (cmd (concat (unless ob-mermaid-use-wrapper
                          (concat (shell-quote-argument (expand-file-name mmdc))
                                  " -i "
                                  (org-babel-process-file-name temp-file)
                                  " -o "
                                  (org-babel-process-file-name out-file)))
		        (when theme
			  (concat " -t " theme))
		        (when background-color
			  (concat " -b " background-color))
		        (when width
			  (concat " -w " width))
		        (when height
			  (concat " -H " height))
		        (when mermaid-config-file
			  (concat " -c " (org-babel-process-file-name mermaid-config-file)))
		        (when css-file
			  (concat " -C " (org-babel-process-file-name css-file)))
                        (when pupeteer-config-file
                          (concat " -p " (org-babel-process-file-name pupeteer-config-file))))))
      (unless (file-executable-p mmdc)
        ;; cannot happen with `executable-find', so we complain about
        ;; `ob-mermaid-cli-path'
        (error "Cannot find or execute %s, please check `ob-mermaid-cli-path'" mmdc))
      (with-temp-file temp-file (insert body))
      (message "%s" cmd)
      (if ob-mermaid-use-wrapper
          (shell-command (format "cat %s | %s %s -o %s" temp-file mmdc cmd out-file))
        (org-babel-eval cmd ""))
      nil)))