runtime(java): Manage circularity for every :syn-included syntax file

With "g:markdown_fenced_languages" defined and "java" added
to its list, a circular dependency between the Markdown and
Java syntax files will be made.  To break it, no Markdown
documentation comments will be recognised in fenced blocks
in Markdown files; in order to view Java source files,
"java" must be removed from "g:markdown_fenced_languages",
and this task can be automated as follows.

1) Add to "~/.after/ftplugin/java.vim":
------------------------------------------------------------
if exists("g:markdown_fenced_languages") &&
	\ !(exists("g:java_ignore_javadoc") ||
	\ exists("g:java_ignore_markdown"))
    let s:idx = index(g:markdown_fenced_languages, 'java')
    if s:idx > -1
	call remove(g:markdown_fenced_languages, s:idx)
    endif
    unlet s:idx
endif
------------------------------------------------------------

2) Optionally add to "~/.after/ftplugin/markdown.vim":
------------------------------------------------------------
if exists("g:markdown_fenced_languages") &&
	\ index(g:markdown_fenced_languages, 'java') < 0
    call add(g:markdown_fenced_languages, 'java')
endif
------------------------------------------------------------

(Make sure that the above snippets appear in the files under
the "ftplugin" NOT "syntax" directory.)

Finally, unless the new version of the syntax file is made
available from "$VIMRUNTIME" (and from "~/.vim/syntax" if
necessary), OTHER discoverable file versions will be used
whose behaviour may interfere with this fix.

related: #15740
closes: #15796

Signed-off-by: Aliaksei Budavei <0x000c70@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/syntax/java.vim

@@ -3,16 +3,32 @@
 " Maintainer:		Aliaksei Budavei <0x000c70 AT gmail DOT com>
 " Former Maintainer:	Claudio Fleiner <claudio@fleiner.com>
 " Repository:		https://github.com/zzzyxwvut/java-vim.git
-" Last Change:		2024 Sep 28
+" Last Change:		2024 Oct 03
 
-" Please check :help java.vim for comments on some of the options available.
+" Please check ":help java.vim" for comments on some of the options
+" available.
 
-" quit when a syntax file was already loaded
+" Do not aggregate syntax items from circular inclusion.
-if !exists("g:main_syntax")
+if exists("b:current_syntax")
-  if exists("b:current_syntax")
+  finish
-    finish
+endif
+
+if exists("g:main_syntax")
+  " Reject attendant circularity for every :syn-included syntax file,
+  " but ACCEPT FAILURE when "g:main_syntax" is set to "java".
+  if g:main_syntax == 'html'
+    if !exists("g:java_ignore_html")
+      let g:java_ignore_html = 1
+      let s:clear_java_ignore_html = 1
+    endif
+  elseif g:main_syntax == 'markdown'
+    if !exists("g:java_ignore_markdown")
+      let g:java_ignore_markdown = 1
+      let s:clear_java_ignore_markdown = 1
+    endif
   endif
-  " we define it here so that included files can test for it
+else
+  " Allow syntax files that include this file test for its inclusion.
   let g:main_syntax = 'java'
 endif
 
@@ -364,15 +380,17 @@ if !exists("g:java_ignore_javadoc") && (s:with_html || s:with_markdown) && g:mai
 
   " Include HTML syntax coloring for Javadoc comments.
   if s:with_html
-    syntax include @javaHtml syntax/html.vim
+    try
-    unlet b:current_syntax
+      syntax include @javaHtml syntax/html.vim
+    finally
+      unlet! b:current_syntax
+    endtry
   endif
 
   " Include Markdown syntax coloring (v7.2.437) for Javadoc comments.
   if s:with_markdown
     try
       syntax include @javaMarkdown syntax/markdown.vim
-      unlet b:current_syntax
       let s:ff.WithMarkdown = s:ff.LeftConstant
     catch /\<E48[45]:/
       call s:ReportOnce(v:exception)
@@ -383,6 +401,8 @@ if !exists("g:java_ignore_javadoc") && (s:with_html || s:with_markdown) && g:mai
       hi clear markdownCodeBlock
       hi clear markdownCodeDelimiter
       hi clear markdownLinkDelimiter
+    finally
+      unlet! b:current_syntax
     endtry
   endif
 
@@ -839,6 +859,14 @@ if g:main_syntax == 'java'
   unlet g:main_syntax
 endif
 
+if exists("s:clear_java_ignore_html")
+  unlet! s:clear_java_ignore_html g:java_ignore_html
+endif
+
+if exists("s:clear_java_ignore_markdown")
+  unlet! s:clear_java_ignore_markdown g:java_ignore_markdown
+endif
+
 let b:spell_options = "contained"
 let &cpo = s:cpo_save
 unlet s:cpo_save s:ff s:with_html s:with_markdown

runtime/syntax/testdir/dumps/markdown_circularity_00.dump

@@ -0,0 +1,20 @@
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runtime/syntax/testdir/dumps/markdown_circularity_01.dump

@@ -0,0 +1,20 @@
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+@16|{| @57
+@20>S|y|s|t|e|m|.|o|u|t|.|p|r|i|n|t|l|n|(|"|.|"|)|;| @30
+@16|}| @57
+@12|}| @61
+@12|`@2| @59
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+@4|}| @69
+|}| @73
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+|~+0#e000e06&@2| +0#0000000&@71
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+@75
+|`+0#e000e06&@2|j|a|v|a| +0#0000000&@67
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runtime/syntax/testdir/dumps/markdown_circularity_02.dump

@@ -0,0 +1,20 @@
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+|/+0#0000e05&|*@1| +0#e000e06&|J|a|v|a| |s|y|n|t|a|x| |c|i|r|c|u|l|a|r|i|t|y| |t|e|s|t|s|.| +0#0000e05&|*|/| +0#0000000&@37
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+|{| @73
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+@4>/+0#0000e05&@2| +0#0000000&@67
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+@4|p+0#00e0003&|u|b|l|i|c| +0#0000000&|s+0#00e0003&|t|a|t|i|c| +0#0000000&|v+0#00e0003&|o|i|d| +0#0000000&|m|a|i|n|(|S+0#e000002&|t|r|i|n|g|[+0#0000000&|]| |a|r|g|s|)| @32
+@4|{| @69
+@8|S+0#e000e06&|y|s|t|e|m|.|o|u|t|.|p|r|i|n|t|l|n|(|"@2| +0#0000000&@44
+| +0#e000002&@11|<|p|r|e|>|<|c|o|d|e|>| +0#0000000&@51
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+| +0#e000002&@11|<|/|c|o|d|e|>|<|/|p|r|e|>| +0#0000000&@49
+@57|3|7|,|5| @9|8@1|%| 

runtime/syntax/testdir/dumps/markdown_circularity_03.dump

@@ -0,0 +1,20 @@
+| +0#e000002#ffffff0@11|<|/|c|o|d|e|>|<|/|p|r|e|>| +0#0000000&@49
+| +0#e000002&@7|"+0#e000e06&@2|)|;+0#0000000&| @61
+@4|}| @69
+|}| @73
+>`+0#e000e06&@2| +0#0000000&@71
+|~+0#4040ff13&| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+| +0#0000000&@56|5|4|,|1| @9|B|o|t| 

runtime/syntax/testdir/input/markdown_circularity.markdown

@@ -0,0 +1,54 @@
+No code is recognised in HTML snippets.
+
+~~~html
+<pre><code>
+/** HTML syntax circularity tests. */
+class HTMLSyntaxCircularityTests
+{
+    /// @hidden
+    ///
+    /// @param args optional command-line arguments
+    public static void main(String[] args)
+    {
+        System.out.println("""
+            ```java
+            class SyntaxCircularityTests
+            {
+                public static void main(String[] args)
+                {
+                    System.out.println(".");
+                }
+            }
+            ```
+        """);
+    }
+}
+</code></pre>
+~~~
+
+
+Markdown documentation comments are not recognised in Java snippets.
+
+```java
+/** Java syntax circularity tests. */
+class JavaSyntaxCircularityTests
+{
+    /// @hidden
+    ///
+    /// @param args optional command-line arguments
+    public static void main(String[] args)
+    {
+        System.out.println("""
+            <pre><code>
+            class SyntaxCircularityTests
+            {
+                public static void main(String[] args)
+                {
+                    System.out.println(".");
+                }
+            }
+            </code></pre>
+        """);
+    }
+}
+```

runtime/syntax/testdir/input/setup/markdown_circularity.vim

@@ -0,0 +1,4 @@
+unlet! g:java_ignore_javadoc g:java_ignore_markdown
+let g:java_highlight_debug = 1
+let g:java_highlight_java_lang = 1
+let g:markdown_fenced_languages = ['html', 'java']