[gdb/tui] Fix fingerprint for cmd-only layout

I added a cmd-only layout:
...
(gdb) tui new-layout cmd cmd 1
...
and set it:
...
(gdb) layout cmd
...
which gave me the expect result: only the cmd window in the screen.

However, after going back to layout src:
...
(gdb) layout src
...
I got a source window with only one line in it, and the cmd window taking most
of the screen.

I traced this back to tui_set_layout, where for both the old and the new
layout the fingerprint of the cmd window in the layout is taken.  If the
fingerprint is the same, an effort will be done to preserve the command
window size.

The fingerprint is "VC" for both the old (cmd) and new (src) layouts, which
explains the behaviour.

I think this is essentially a bug in the finger print calculation, and it
should be "C" for the cmd layout.

Fix this by not adding a V or H in the fingerprint if the list size is one.

Tested on x86_64-linux.

Reviewed-By: Tom Tromey <tom@tromey.com>

3 files changed, 13 insertions, 3 deletions

diff --git a/gdb/testsuite/gdb.tui/new-layout.exp b/gdb/testsuite/gdb.tui/new-layout.exp
index eaebac8..3085cc3 100644
--- a/gdb/testsuite/gdb.tui/new-layout.exp
+++ b/gdb/testsuite/gdb.tui/new-layout.exp
@@ -142,3 +142,12 @@ foreach_with_prefix layout $layouts {
 			   "$gdb_prompt\\s+"]
     }
 }
+
+Term::command "layout src"
+Term::command "winheight cmd 8"
+Term::check_box "before cmd_only: src box in src layout" 0 0 80 15
+
+Term::command "layout cmd_only"
+
+Term::command "layout src"
+Term::check_box "after cmd_only: src box in src layout" 0 0 80 15
diff --git a/gdb/tui/tui-layout.c b/gdb/tui/tui-layout.c
index 50c568f..159445d 100644
--- a/gdb/tui/tui-layout.c
+++ b/gdb/tui/tui-layout.c
@@ -1101,7 +1101,7 @@ tui_layout_split::layout_fingerprint () const
   for (auto &item : m_splits)
     {
       std::string fp = item.layout->layout_fingerprint ();
-      if (!fp.empty ())
+      if (!fp.empty () && m_splits.size () != 1)
 	return std::string (m_vertical ? "V" : "H") + fp;
     }
 
diff --git a/gdb/tui/tui-layout.h b/gdb/tui/tui-layout.h
index ff354fb..a6d3485 100644
--- a/gdb/tui/tui-layout.h
+++ b/gdb/tui/tui-layout.h
@@ -111,7 +111,8 @@ public:
      non-empty string made of 'V' and 'H' characters, followed by a single
      'C' character.  Each 'V' and 'H' represents a vertical or horizontal
      layout that must be passed through in order to find the cmd
-     window.
+     window.  A vertical or horizontal layout of just one window does not add
+     a 'V' or 'H' character.
 
      Of course, layouts are built recursively, so, when called on a partial
      layout, if this object represents a single window, then either the
@@ -119,7 +120,7 @@ public:
      containing a single 'C' is returned.
 
      For object representing layouts, if the layout contains the cmd
-     window then we will get back a valid fingerprint string (contains 'V'
+     window then we will get back a valid fingerprint string (may contain 'V'
      and 'H', ends with 'C'), or, if this layout doesn't contain the cmd
      window, an empty string is returned.  */
   virtual std::string layout_fingerprint () const = 0;