[2/4] gdbserver: Add debug-file option

diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index f410d026b8..1716466609 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -21332,8 +21332,11 @@  The @option{--debug} option tells @code{gdbserver} to display extra
 status information about the debugging process.
 @cindex @option{--remote-debug}, @code{gdbserver} option
 The @option{--remote-debug} option tells @code{gdbserver} to display
-remote protocol debug output.  These options are intended for
-@code{gdbserver} development and for bug reports to the developers.
+remote protocol debug output.
+@cindex @option{--debug-file}, @code{gdbserver} option
+The @option{--debug-file=filename} option tells @code{gdbserver} to
+write any debug output to the given file.  These options are intended
+for @code{gdbserver} development and for bug reports to the developers.
 
 @cindex @option{--debug-format}, @code{gdbserver} option
 The @option{--debug-format=option1[,option2,...]} option tells
@@ -21433,6 +21436,10 @@  Disable or enable general debugging messages.
 Disable or enable specific debugging messages associated with the remote
 protocol (@pxref{Remote Protocol}).
 
+@item monitor set debug-file filename
+@itemx monitor set debug-file
+Send any debug output to the given file, or to stderr.
+
 @item monitor set debug-format option1@r{[},option2,...@r{]}
 Specify additional text to add to debugging messages.
 Possible options are:
@@ -44563,6 +44570,11 @@  Instruct @code{gdbserver} to display remote protocol debug output.
 This option is intended for @code{gdbserver} development and for bug reports to
 the developers.
 
+@item --debug-file=filename
+Instruct @code{gdbserver} to send any debug output to the given file.
+This option is intended for @code{gdbserver} development and for bug reports to
+the developers.
+
 @item --debug-format=option1@r{[},option2,...@r{]}
 Instruct @code{gdbserver} to include extra information in each line
 of debugging output.
diff --git a/gdb/gdbserver/debug.c b/gdb/gdbserver/debug.c
index 7c4c77afe2..53647c1ea6 100644
--- a/gdb/gdbserver/debug.c
+++ b/gdb/gdbserver/debug.c
@@ -23,6 +23,9 @@ 
 int remote_debug = 0;
 #endif
 
+/* Output file for debugging.  Default to standard error.  */
+FILE *debug_file = stderr;
+
 /* Enable miscellaneous debugging output.  The name is historical - it
    was originally used to debug LinuxThreads support.  */
 int debug_threads;
@@ -30,6 +33,39 @@  int debug_threads;
 /* Include timestamps in debugging output.  */
 int debug_timestamp;
 
+#if !defined (IN_PROCESS_AGENT)
+
+/* See debug.h.  */
+
+void
+debug_set_output (const char *new_debug_file)
+{
+  /* Close any existing file and reset to standard error.  */
+  if (debug_file != stderr)
+    {
+      fclose (debug_file);
+    }
+  debug_file = stderr;
+
+  /* Catch empty filenames.  */
+  if (new_debug_file == nullptr || strlen (new_debug_file) == 0)
+    return;
+
+  FILE *fptr = fopen (new_debug_file, "w");
+
+  if (fptr == nullptr)
+    {
+      debug_printf ("Cannot open %s for writing. %s. Switching to stderr.\n",
+		    new_debug_file, strerror (errno));
+      return;
+    }
+
+  debug_file = fptr;
+  return;
+}
+
+#endif
+
 /* Print a debugging message.
    If the text begins a new line it is preceded by a timestamp.
    We don't get fancy with newline checking, we just check whether the
@@ -50,11 +86,11 @@  debug_vprintf (const char *format, va_list ap)
       seconds s = duration_cast<seconds> (now.time_since_epoch ());
       microseconds us = duration_cast<microseconds> (now.time_since_epoch ()) - s;
 
-      fprintf (stderr, "%ld.%06ld ", (long) s.count (), (long) us.count ());
+      fprintf (debug_file, "%ld.%06ld ", (long) s.count (), (long) us.count ());
     }
 #endif
 
-  vfprintf (stderr, format, ap);
+  vfprintf (debug_file, format, ap);
 
 #if !defined (IN_PROCESS_AGENT)
   if (*format)
@@ -69,7 +105,7 @@  debug_vprintf (const char *format, va_list ap)
 void
 debug_flush (void)
 {
-  fflush (stderr);
+  fflush (debug_file);
 }
 
 /* Notify the user that the code is entering FUNCTION_NAME.
diff --git a/gdb/gdbserver/debug.h b/gdb/gdbserver/debug.h
index c8d5e3365e..f65c91c9eb 100644
--- a/gdb/gdbserver/debug.h
+++ b/gdb/gdbserver/debug.h
@@ -21,6 +21,11 @@ 
 
 #if !defined (IN_PROCESS_AGENT)
 extern int remote_debug;
+
+/* Switch all debug output to DEBUG_FILE.  If DEBUG_FILE is nullptr or an
+   empty string, or if the file cannot be opened, then debug output is sent to
+   stderr.  */
+void debug_set_output (const char *debug_file);
 #endif
 
 extern int debug_threads;
diff --git a/gdb/gdbserver/server.c b/gdb/gdbserver/server.c
index 3f6c849dbc..36510ad1b2 100644
--- a/gdb/gdbserver/server.c
+++ b/gdb/gdbserver/server.c
@@ -1403,6 +1403,10 @@  handle_monitor_command (char *mon, char *own_buf)
 	  write_enn (own_buf);
 	}
     }
+  else if (strcmp (mon, "set debug-file") == 0)
+    debug_set_output (nullptr);
+  else if (startswith (mon, "set debug-file "))
+    debug_set_output (mon + sizeof ("set debug-file ") - 1);
   else if (strcmp (mon, "help") == 0)
     monitor_show_help ();
   else if (strcmp (mon, "exit") == 0)
@@ -3649,6 +3653,8 @@  captured_main (int argc, char *argv[])
 	}
       else if (strcmp (*next_arg, "--remote-debug") == 0)
 	remote_debug = 1;
+      else if (startswith (*next_arg, "--debug-file="))
+	debug_set_output ((*next_arg) + sizeof ("--debug-file=") -1);
       else if (strcmp (*next_arg, "--disable-packet") == 0)
 	{
 	  gdbserver_show_disableable (stdout);