[gdb] Handle EINTR in run_under_shell

When building gdb with -O2 -fsanitize=thread and running test-case
gdb.base/bg-exec-sigint-bp-cond.exp, I run into:
...
(gdb) c&^M
Continuing.^M
(gdb) Quit^M
(gdb) quit_count=1
^M
Breakpoint 2, foo () at bg-exec-sigint-bp-cond.c:23^M
23        return 0;^M
FAIL: $exp: no force memory write: \
  SIGINT does not interrupt background execution
...

What happens is that:
- the breakpoint hits
- while evaluating the condition of the breakpoint,
  $_shell("kill -INT <pid-of-gdb>") is called, handled by run_under_shell
- in run_under_shell, a vfork is issued
- in the vfork child, execl executes the kill command
- in the vfork parent, waitpid is called to wait for the result of the kill
  command
- waitpid returns -1 with errno set to EINTR
- run_under_shell doesn't check the result of waitpid, and returns the
  value of local variable status.  Since waitpid returned -1, status was
  not assigned a value, so it's uninitialized, and happens to be
  non-zero
- the breakpoint condition evaluates to true, because
  $_shell("kill -INT <pid-of-gdb>") != 0
- the breakpoint triggers a stop, which the test-case doesn't expect.

Fix this by using gdb::handle_eintr to call waitpid in run_under_shell.

Also handle the case that waitpid returns an error other than EINTR, using
perror_with_name.

Tested on x86_64-linux.

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

PR gdb/30695
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30695

1 files changed, 6 insertions, 1 deletions

diff --git a/gdb/cli/cli-cmds.c b/gdb/cli/cli-cmds.c
index ea2e156..65ac7d6 100644
--- a/gdb/cli/cli-cmds.c
+++ b/gdb/cli/cli-cmds.c
@@ -55,6 +55,7 @@
 #include "extension.h"
 #include "gdbsupport/pathstuff.h"
 #include "gdbsupport/gdb_tilde_expand.h"
+#include "gdbsupport/eintr.h"
 
 #ifdef TUI
 #include "tui/tui.h"
@@ -921,7 +922,11 @@ run_under_shell (const char *arg, int from_tty)
     }
 
   if (pid != -1)
-    waitpid (pid, &status, 0);
+    {
+      int ret = gdb::handle_eintr (-1, ::waitpid, pid, &status, 0);
+      if (ret == -1)
+	perror_with_name ("Cannot get status of shell command");
+    }
   else
     error (_("Fork failed"));
   return status;