Avoid a crash in source_cache::extract_lines

If the first requested line is larger than the number of lines in the
source buffer, source_cache::extract_lines could crash, because it
would try to pass string::npos" to string::substr.

This patch avoids the crash by checking for this case.

This version of the patch changes get_source_lines to return
std::string.

gdb/ChangeLog
2019-03-14  Tom Tromey  <tromey@adacore.com>

	* source-cache.h (class source_cache) <get_source_lines>: Return
	std::string.
	* source-cache.c (source_cache::extract_lines): Handle case where
	first_pos==npos.  Return std::string.
	(source_cache::get_source_lines): Update.

3 files changed, 25 insertions, 11 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 787949c..50cbd2e 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,5 +1,13 @@
 2019-03-14  Tom Tromey  <tromey@adacore.com>
 
+	* source-cache.h (class source_cache) <get_source_lines>: Return
+	std::string.
+	* source-cache.c (source_cache::extract_lines): Handle case where
+	first_pos==npos.  Return std::string.
+	(source_cache::get_source_lines): Update.
+
+2019-03-14  Tom Tromey  <tromey@adacore.com>
+
 	* NEWS: Add item for "style sources" commands.
 	* source-cache.c (source_cache::get_source_lines): Check
 	source_styling.
diff --git a/gdb/source-cache.c b/gdb/source-cache.c
index 3b15b3f..1ed2601 100644
--- a/gdb/source-cache.c
+++ b/gdb/source-cache.c
@@ -84,9 +84,9 @@ source_cache::get_plain_source_lines (struct symtab *s, int first_line,
 
 /* See source-cache.h.  */
 
-bool
+std::string
 source_cache::extract_lines (const struct source_text &text, int first_line,
-			     int last_line, std::string *lines)
+			     int last_line)
 {
   int lineno = 1;
   std::string::size_type pos = 0;
@@ -102,16 +102,17 @@ source_cache::extract_lines (const struct source_text &text, int first_line,
       pos = new_pos;
       if (lineno == last_line || pos == std::string::npos)
 	{
+	  if (first_pos == std::string::npos)
+	    return {};
 	  if (pos == std::string::npos)
 	    pos = text.contents.size ();
-	  *lines = text.contents.substr (first_pos, pos - first_pos);
-	  return true;
+	  return text.contents.substr (first_pos, pos - first_pos);
 	}
       ++lineno;
       ++pos;
     }
 
-  return false;
+  return {};
 }
 
 #ifdef HAVE_SOURCE_HIGHLIGHT
@@ -187,7 +188,10 @@ source_cache::get_source_lines (struct symtab *s, int first_line,
       for (const auto &item : m_source_map)
 	{
 	  if (item.fullname == fullname)
-	    return extract_lines (item, first_line, last_line, lines);
+	    {
+	      *lines = extract_lines (item, first_line, last_line);
+	      return true;
+	    }
 	}
 
       const char *lang_name = get_language_name (SYMTAB_LANGUAGE (s));
@@ -208,8 +212,9 @@ source_cache::get_source_lines (struct symtab *s, int first_line,
 	      if (m_source_map.size () > MAX_ENTRIES)
 		m_source_map.erase (m_source_map.begin ());
 
-	      return extract_lines (m_source_map.back (), first_line,
-				    last_line, lines);
+	      *lines = extract_lines (m_source_map.back (), first_line,
+				      last_line);
+	      return true;
 	    }
 	}
     }
diff --git a/gdb/source-cache.h b/gdb/source-cache.h
index dd23266..e2e25a1 100644
--- a/gdb/source-cache.h
+++ b/gdb/source-cache.h
@@ -65,9 +65,10 @@ private:
 			       int last_line, std::string *lines_out);
   /* A helper function for get_plain_source_lines that extracts the
      desired source lines from TEXT, putting them into LINES_OUT.  The
-     arguments and return value are as for get_source_lines.  */
-  bool extract_lines (const struct source_text &text, int first_line,
-		      int last_line, std::string *lines_out);
+     arguments are as for get_source_lines.  The return value is the
+     desired lines.  */
+  std::string extract_lines (const struct source_text &text, int first_line,
+			     int last_line);
 
   /* The contents of the cache.  */
   std::vector<source_text> m_source_map;