[gdb/cli] Allow source-highlight to autodetect language

Currently when gdb asks the source-highlight library to highlight a file, it
tells it what language file to use.

For instance, if gdb learns from the debug info that the file is language_c,
the language file "c.lang" is used.  This mapping is hardcoded in
get_language_name.

However, if gdb doesn't know what language file to use, it falls back to using
python pygments, and in absence of that, unhighlighted source text.

In the case of python pygments, it autodetects which language to use based on
the file name.

Add the same capability when using the source-highlight library.

Tested on x86_64-linux.

Verified that it works by:
- making get_language_name return nullptr for language_c, and
- checking that source-highlight still manages to highlight a hello world.

Reviewed-By: Guinevere Larsen <[email protected]>
Approved-By: Tom Tromey <[email protected]>

PR cli/30966
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30966

Author: vries

gdb/source-cache.c

@@ -37,6 +37,7 @@
 #include <sstream>
 #include <srchilite/sourcehighlight.h>
 #include <srchilite/langmap.h>
+#include <srchilite/settings.h>
 #endif
 
 /* The number of source files we'll cache.  */
@@ -205,22 +206,35 @@ try_source_highlight (std::string &contents ATTRIBUTE_UNUSED,
     return false;
 
   const char *lang_name = get_language_name (lang);
-  if (lang_name == nullptr)
-    return false;
 
   /* The global source highlight object, or null if one was
      never constructed.  This is stored here rather than in
      the class so that we don't need to include anything or do
      conditional compilation in source-cache.h.  */
   static srchilite::SourceHighlight *highlighter;
 
+  /* The global source highlight language map object.  */
+  static srchilite::LangMap *langmap;
+
   bool styled = false;
   try
     {
       if (highlighter == nullptr)
 	{
 	  highlighter = new srchilite::SourceHighlight ("esc.outlang");
 	  highlighter->setStyleFile ("esc.style");
+
+	  const std::string &datadir = srchilite::Settings::retrieveDataDir ();
+	  langmap = new srchilite::LangMap (datadir, "lang.map");
+	}
+
+      std::string detected_lang;
+      if (lang_name == nullptr)
+	{
+	  detected_lang = langmap->getMappedFileNameFromFileName (fullname);
+	  if (detected_lang.empty ())
+	    return false;
+	  lang_name = detected_lang.c_str ();
 	}
 
       std::istringstream input (contents);