[Bug cli/30346] New: [gdb/cli] CLI in ansi terminal has off-by-one width problem

["vries at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=30346

            Bug ID: 30346
           Summary: [gdb/cli] CLI in ansi terminal has off-by-one width
                    problem
           Product: gdb
           Version: HEAD
            Status: NEW
          Severity: normal
          Priority: P2
         Component: cli
          Assignee: unassigned at sourceware dot org
          Reporter: vries at gcc dot gnu.org
  Target Milestone: ---

[ Previously filed at PR30337 comment 3. ]

Say we have a terminal with:
...
$ echo $COLUMNS
40
...

For contrast, lets start with the terminal in xterm mode, and type 'a' for a
while:
...
$ TERM=xterm gdb -q
         1         2         3         4
1234567890123456789012345678901234567890
(gdb) aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aa
...
All 40 positions in the line are used, before we wrap to the next line.

Now we start gdb with the terminal in ansi mode:
...
$ TERM=ansi gdb -q
         1         2         3         4
1234567890123456789012345678901234567890
(gdb) aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aa
...

The line wraps after 38 chars.

Based on the readline behaviour of hiding one char to deal with the lack of
autowrap, we'd expect wrapping after 39 chars.

The problem is that we start out with COLUMNS=40, then ask readline how wide
the screen is, which replies 39.  Then we use the 39 to set the screensize
using readline, which results in an effective width of 38.

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