[PATCH 5/6] vsh: Restore original rl_line_buffer after completion

[Michal Privoznik <mprivozn@xxxxxxxxxx>]

Problem with readline is its API. It's basically a bunch of
global variables with no clear dependencies between them. In this
specific case that I'm seeing: in interactive mode the
cmdComplete() causes instant crash of virsh/virt-admin:

==27999== Invalid write of size 1
==27999==    at 0x516EF71: _rl_init_line_state (readline.c:742)
==27999==    by 0x5170054: rl_initialize (readline.c:1192)
==27999==    by 0x516E5E4: readline (readline.c:379)
==27999==    by 0x1B7024: vshReadline (vsh.c:3048)
==27999==    by 0x140DCF: main (virsh.c:905)
==27999==  Address 0x0 is not stack'd, malloc'd or (recently) free'd

This is because readline keeps a copy of pointer to
rl_line_buffer and the moment cmdComplete() returns and readline
takes over, it accesses the copy which is now a dangling pointer.

To fix this, just keep the original state of rl_line_buffer and
restore it.

Fixes: 41400ac1dda55b817388a4050aa823051bda2e05
Fixes: a0e1ada63c0afdc2af3b9405cbf637d8bd28700c
Signed-off-by: Michal Privoznik <mprivozn@xxxxxxxxxx>
---
 tools/vsh.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/tools/vsh.c b/tools/vsh.c
index de869248b4..c91d756885 100644
--- a/tools/vsh.c
+++ b/tools/vsh.c
@@ -3511,6 +3511,7 @@ cmdComplete(vshControl *ctl, const vshCmd *cmd)
     const vshClientHooks *hooks = ctl->hooks;
     const char *lastArg = NULL;
     const char **args = NULL;
+    char *old_rl_line_buffer = NULL;
     g_auto(GStrv) matches = NULL;
     char **iter;
 
@@ -3531,6 +3532,7 @@ cmdComplete(vshControl *ctl, const vshCmd *cmd)
 
     vshReadlineInit(ctl);
 
+    old_rl_line_buffer = g_steal_pointer(&rl_line_buffer);
     if (!(rl_line_buffer = g_strdup(vshCommandOptArgvString(cmd, "string"))))
         rl_line_buffer = g_strdup("");
 
@@ -3540,6 +3542,7 @@ cmdComplete(vshControl *ctl, const vshCmd *cmd)
 
     matches = vshReadlineCompletion(lastArg, 0, 0);
     g_clear_pointer(&rl_line_buffer, g_free);
+    rl_line_buffer = g_steal_pointer(&old_rl_line_buffer);
 
     if (!matches)
         return false;
-- 
2.44.1