Fix PR gdb/17820

This patch is a comprehensive fix for PR 17820 which reports that
using "set history size unlimited" inside one's gdbinit file doesn't
really work.

There are three small changes in this patch.  The most important change
this patch makes is to decode the argument of the "size" subcommand
using add_setshow_zuinteger_unlimited_cmd() instead of using
add_setshow_uinteger_cmd().  The new decoder takes an int * and maps
unlimited to -1 whereas the old decoder takes an unsigned int * and maps
unlimited to UINT_MAX.  Using the new decoder simplifies our handling of
unlimited and makes it easier to interface with readline which itself
expects a signed-int history size.

The second change is the factoring of the [stifle|unstifle]_history logic
into a common function which is now used by both init_history() and
set_history_size_command().  This is technically the change that fixes
the PR itself.

Thirdly, this patch initializes history_size_setshow_var to -2 to mean
that the variable has not been set yet.  Now init_history() tests for -2
instead of 0 to determine whether to give the variable a default value.
This means that having "set history size 0" in one's gdbinit file will
actually keep the history size at 0 and not reset it to 256.

gdb/ChangeLog:

	PR gdb/17820
	* top.c (history_size_setshow_var): Change type to signed.
	Initialize to -2.  Update documentation.
	(set_readline_history_size): Define.
	(set_history_size_command): Use it.  Remove logic for handling
	out-of-range sizes.
	(init_history): Use set_readline_history_size().  Test for a
	value of -2 instead of 0 when determining whether to set a
	default history size.
	(init_main): Decode the argument of the "size" command as a
	zuinteger_unlimited.

gdb/testsuite/ChangeLog:

	PR gdb/17820
	* gdb.base/gdbinit-history.exp: New test.
	* gdb.base/gdbinit-history/unlimited/.gdbinit: New file.
	* gdb.base/gdbinit-history/zero/.gdbinit: New file.

1 files changed, 21 insertions, 30 deletions

diff --git a/gdb/top.c b/gdb/top.c
index deb4108..74e1e07 100644
--- a/gdb/top.c
+++ b/gdb/top.c
@@ -687,8 +687,8 @@ show_write_history_p (struct ui_file *file, int from_tty,
 }
 
 /* The variable associated with the "set/show history size"
-   command.  */
-static unsigned int history_size_setshow_var;
+   command.  The value -1 means unlimited, and -2 means undefined.  */
+static int history_size_setshow_var = -2;
 
 static void
 show_history_size (struct ui_file *file, int from_tty,
@@ -1610,34 +1610,26 @@ show_commands (char *args, int from_tty)
     }
 }
 
-/* Called by do_setshow_command.  */
-static void
-set_history_size_command (char *args, int from_tty, struct cmd_list_element *c)
-{
-  /* Readline's history interface works with 'int', so it can only
-     handle history sizes up to INT_MAX.  The command itself is
-     uinteger, so UINT_MAX means "unlimited", but we only get that if
-     the user does "set history size 0" -- "set history size <UINT_MAX>"
-     throws out-of-range.  */
-  if (history_size_setshow_var > INT_MAX
-      && history_size_setshow_var != UINT_MAX)
-    {
-      unsigned int new_value = history_size_setshow_var;
+/* Update the size of our command history file to HISTORY_SIZE.
 
-      /* Restore previous value before throwing.  */
-      if (history_is_stifled ())
-	history_size_setshow_var = history_max_entries;
-      else
-	history_size_setshow_var = UINT_MAX;
+   A HISTORY_SIZE of -1 stands for unlimited.  */
 
-      error (_("integer %u out of range"), new_value);
-    }
+static void
+set_readline_history_size (int history_size)
+{
+  gdb_assert (history_size >= -1);
 
-  /* Commit the new value to readline's history.  */
-  if (history_size_setshow_var == UINT_MAX)
+  if (history_size == -1)
     unstifle_history ();
   else
-    stifle_history (history_size_setshow_var);
+    stifle_history (history_size);
+}
+
+/* Called by do_setshow_command.  */
+static void
+set_history_size_command (char *args, int from_tty, struct cmd_list_element *c)
+{
+  set_readline_history_size (history_size_setshow_var);
 }
 
 void
@@ -1705,12 +1697,10 @@ init_history (void)
       history_size_setshow_var = var;
     }
   /* If the init file hasn't set a size yet, pick the default.  */
-  else if (history_size_setshow_var == 0)
+  else if (history_size_setshow_var == -2)
     history_size_setshow_var = 256;
 
-  /* Note that unlike "set history size 0", "HISTSIZE=0" really sets
-     the history size to 0...  */
-  stifle_history (history_size_setshow_var);
+  set_readline_history_size (history_size_setshow_var);
 
   tmpenv = getenv ("GDBHISTFILE");
   if (tmpenv)
@@ -1859,7 +1849,8 @@ Without an argument, saving is enabled."),
 			   show_write_history_p,
 			   &sethistlist, &showhistlist);
 
-  add_setshow_uinteger_cmd ("size", no_class, &history_size_setshow_var, _("\
+  add_setshow_zuinteger_unlimited_cmd ("size", no_class,
+				       &history_size_setshow_var, _("\
 Set the size of the command history,"), _("\
 Show the size of the command history,"), _("\
 ie. the number of previous commands to keep a record of.\n\