Add "undefined-command" error code at end of ^error result...

... when trying to execute an undefined GDB/MI command. When trying
to execute a GDB/MI command which does not exist, the current error
result record looks like this:

    -unsupported
    ^error,msg="Undefined MI command: unsupported"

The only indication that the command does not exist is the error
message. It would be a little fragile for a consumer to rely solely
on the contents of the error message in order to determine whether
a command exists or not.

This patch improves the situation by adding concept of error
code, starting with one well-defined error code ("undefined-command")
identifying errors due to a non-existant command. Here is the new
output:

    -unsupported
    ^error,msg="Undefined MI command: unsupported",code="undefined-command"

This error code is only displayed when the corresponding error
condition is met. Otherwise, the error record remains unchanged.
For instance:

    -symbol-list-lines foo.adb
    ^error,msg="-symbol-list-lines: Unknown source file name."

For frontends to be able to know whether they can rely on this
variable, a new entry "undefined-command-error-code" has been
added to the "-list-features" command.  Another option would be
to always generate an error="..." variable (for the default case,
we could decide for instance that the error code is the empty string).
But it seems more efficient to provide that info in "-list-features"
and then only add the error code when meaningful.

gdb/ChangeLog:

        (from Pedro Alves  <palves@redhat.com>)
        (from Joel Brobecker  <brobecker@adacore.com>)
        * exceptions.h (enum_errors) <UNDEFINED_COMMAND_ERROR>: New enum.
        * mi/mi-parse.c (mi_parse): Throw UNDEFINED_COMMAND_ERROR instead
        of a regular error when the GDB/MI command does not exist.
        * mi/mi-main.c (mi_cmd_list_features): Add
        "undefined-command-error-code".
        (mi_print_exception): Print an "undefined-command"
        error code if EXCEPTION.ERROR is UNDEFINED_COMMAND_ERROR.
        * NEWS: Add entry documenting the new "code" variable in
        "^error" result records.

gdb/doc/ChangeLog:

        * gdb.texinfo (GDB/MI Result Records): Fix the syntax of the
        "^error" result record concerning the error message.  Document
        the error code that may also be part of that result record.
        (GDB/MI Miscellaneous Commands): Document the
        "undefined-command-error-code" element in the output of
        the "-list-features" GDB/MI command.

gdb/testsuite/ChangeLog:

        * gdb.mi/mi-undefined-cmd.exp: New testcase.

2 files changed, 25 insertions, 3 deletions

diff --git a/gdb/doc/ChangeLog b/gdb/doc/ChangeLog
index 329c293..fdb05c5 100644
--- a/gdb/doc/ChangeLog
+++ b/gdb/doc/ChangeLog
@@ -1,5 +1,14 @@
 2013-12-03  Joel Brobecker  <brobecker@adacore.com>
 
+	* gdb.texinfo (GDB/MI Result Records): Fix the syntax of the
+	"^error" result record concerning the error message.  Document
+	the error code that may also be part of that result record.
+	(GDB/MI Miscellaneous Commands): Document the
+	"undefined-command-error-code" element in the output of
+	the "-list-features" GDB/MI command.
+
+2013-12-03  Joel Brobecker  <brobecker@adacore.com>
+
 	* gdb.texinfo (GDB/MI Miscellaneous Commands): Document
 	the new -info-gdb-mi-command GDB/MI command.  Document
 	the meaning of "-info-gdb-mi-command" in the output of
diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index 5b1021b..75633c9 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -29333,10 +29333,19 @@ which threads are resumed.
 @findex ^connected
 @value{GDBN} has connected to a remote target.
 
-@item "^error" "," @var{c-string}
+@item "^error" "," "msg=" @var{c-string} [ "," "code=" @var{c-string} ]
 @findex ^error
-The operation failed.  The @code{@var{c-string}} contains the corresponding
-error message.
+The operation failed.  The @code{msg=@var{c-string}} variable contains
+the corresponding error message.
+
+If present, the @code{code=@var{c-string}} variable provides an error
+code on which consumers can rely on to detect the corresponding
+error condition.  At present, only one error code is defined:
+
+@table @samp
+@item "undefined-command"
+Indicates that the command causing the error does not exist.
+@end table
 
 @item "^exit"
 @findex ^exit
@@ -35204,6 +35213,10 @@ Indicates that all @sc{gdb/mi} commands accept the @option{--language}
 option (@pxref{Context management}).
 @item info-gdb-mi-command
 Indicates support for the @code{-info-gdb-mi-command} command.
+@item undefined-command-error-code
+Indicates support for the "undefined-command" error code in error result
+records, produced when trying to execute an undefined @sc{gdb/mi} command
+(@pxref{GDB/MI Result Records}).
 @end table
 
 @subheading The @code{-list-target-features} Command