gdbsupport: allow passing format string to scoped_debug_start_end

A little thing that bothers me with scoped_debug_start_end is that it's
not possible to pass a format string to add context to the messages: the
start and end messages are fixed.

It was done like this at the time because there's the risk that debug
output is not enabled on entry (when the constructor runs) but is
enabled on exit (when the destructor runs).  For example, a user
debugging from a top-gdb may manually enable a debug_foo variable.  If
debug output is disabled while the constructor runs, we won't render the
format string (to minimize overhead) so it won't be available in the
destructor.

I think it would be nice to be able to use a format string along with
scoped_debug_start_end, and I think it's unfortunate that such a narrow
use case prevents it.  So with this patch, I propose that we allow
passing a format string to scoped_debug_start_end, and if the rare
situation described above happens, then we just show a "sorry, message
not available" kind of message.

The following patch makes use of this.

gdbsupport/ChangeLog:

	* common-debug.h (struct scoped_debug_start_end)
	<scoped_debug_start_end>: Change start_msg/end_msg for
	start_prefix/end_prefix.  Add format string parameter and make
	variadic.
	<~scoped_debug_start_end>: Adjust.
	<m_end_msg>: Rename to...
	<m_end_prefix>: ... this.
	<m_with_format>: New.
	<m_msg>: New.
	(scoped_debug_start_end): Make variadic.
	(scoped_debug_enter_exit): Adjust.

Change-Id: I9427ce8877a246a46694b3a1fec3837dc6954d6e

Author: simark

gdbsupport/ChangeLog

@@ -1,3 +1,17 @@
+2021-04-24  Simon Marchi  <[email protected]>
+
+	* common-debug.h (struct scoped_debug_start_end)
+	<scoped_debug_start_end>: Change start_msg/end_msg for
+	start_prefix/end_prefix.  Add format string parameter and make
+	variadic.
+	<~scoped_debug_start_end>: Adjust.
+	<m_end_msg>: Rename to...
+	<m_end_prefix>: ... this.
+	<m_with_format>: New.
+	<m_msg>: New.
+	(scoped_debug_start_end): Make variadic.
+	(scoped_debug_enter_exit): Adjust.
+
 2021-04-24  Simon Marchi  <[email protected]>
 
 	* observable.h (class observable) <struct observer> <observer>:

gdbsupport/common-debug.h

@@ -20,8 +20,11 @@
 #ifndef COMMON_COMMON_DEBUG_H
 #define COMMON_COMMON_DEBUG_H
 
+#include "gdbsupport/gdb_optional.h"
 #include "gdbsupport/preprocessor.h"
 
+#include <stdarg.h>
+
 /* Set to true to enable debugging of hardware breakpoint/
    watchpoint support code.  */
 
@@ -94,20 +97,39 @@ struct scoped_debug_start_end
 
      MODULE and FUNC are forwarded to debug_prefixed_printf.
 
-     START_MSG and END_MSG are the statements to print on construction and
-     destruction, respectively.  */
+     START_PREFIX and END_PREFIX are the statements to print on construction and
+     destruction, respectively.
+
+     If the FMT format string is non-nullptr, then a `: ` is appended to the
+     messages, followed by the rendering of that format string.  The format
+     string is rendered during construction and is re-used as is for the
+     message on exit.  */
 
   scoped_debug_start_end (bool &debug_enabled, const char *module,
-			  const char *func, const char *start_msg,
-			  const char *end_msg)
+			  const char *func, const char *start_prefix,
+			  const char *end_prefix, const char *fmt, ...)
+    ATTRIBUTE_NULL_PRINTF (7, 8)
     : m_debug_enabled (debug_enabled),
       m_module (module),
       m_func (func),
-      m_end_msg (end_msg)
+      m_end_prefix (end_prefix),
+      m_with_format (fmt != nullptr)
   {
     if (m_debug_enabled)
       {
-	debug_prefixed_printf (m_module, m_func, "%s", start_msg);
+	if (fmt != nullptr)
+	  {
+	    va_list args;
+	    va_start (args, fmt);
+	    m_msg = string_vprintf (fmt, args);
+	    va_end (args);
+
+	    debug_prefixed_printf (m_module, m_func, "%s: %s",
+				   start_prefix, m_msg->c_str ());
+	  }
+	else
+	  debug_prefixed_printf (m_module, m_func, "%s", start_prefix);
+
 	++debug_print_depth;
 	m_must_decrement_print_depth = true;
       }
@@ -125,35 +147,55 @@ struct scoped_debug_start_end
 
     if (m_debug_enabled)
       {
-	debug_prefixed_printf (m_module, m_func, "%s", m_end_msg);
+	if (m_with_format)
+	  {
+	    if (m_msg.has_value ())
+	      debug_prefixed_printf (m_module, m_func, "%s: %s",
+				     m_end_prefix, m_msg->c_str ());
+	    else
+	      {
+		/* A format string was passed to the constructor, but debug
+		   control variable wasn't set at the time, so we don't have the
+		   rendering of the format string.  */
+		debug_prefixed_printf (m_module, m_func, "%s: <%s debugging was not enabled on entry>",
+				       m_end_prefix, m_module);
+	      }
+	  }
+	else
+	  debug_prefixed_printf (m_module, m_func, "%s", m_end_prefix);
       }
   }
 
 private:
   bool &m_debug_enabled;
   const char *m_module;
   const char *m_func;
-  const char *m_end_msg;
+  const char *m_end_prefix;
+
+  /* The result of formatting the format string in the constructor.  */
+  gdb::optional<std::string> m_msg;
+
+  /* True is a non-nullptr format was passed to the constructor.  */
+  bool m_with_format;
 
   /* This is used to handle the case where debugging is enabled during
      construction but not during destruction, or vice-versa.  We want to make
      sure there are as many increments are there are decrements.  */
-
   bool m_must_decrement_print_depth = false;
 };
 
 /* Helper to define a module-specific start/end debug macro.  */
 
-#define scoped_debug_start_end(debug_enabled, module, msg) \
+#define scoped_debug_start_end(debug_enabled, module, fmt, ...) \
   scoped_debug_start_end CONCAT(scoped_debug_start_end, __LINE__) \
-    (debug_enabled, module, __func__, "start: " msg, "end: " msg)
+    (debug_enabled, module, __func__, "start", "end", fmt, ##__VA_ARGS__)
 
 /* Helper to define a module-specific enter/exit debug macro.  This is a special
    case of `scoped_debug_start_end` where the start and end messages are "enter"
    and "exit", to denote entry and exit of a function.  */
 
 #define scoped_debug_enter_exit(debug_enabled, module) \
   scoped_debug_start_end CONCAT(scoped_debug_start_end, __LINE__) \
-    (debug_enabled, module, __func__, "enter", "exit")
+    (debug_enabled, module, __func__, "enter", "exit", nullptr)
 
 #endif /* COMMON_COMMON_DEBUG_H */