[gdb/testsuite] Set TSAN_OPTIONS by default to history_size=7

I build gdb with -fsanitize=thread and ran the testsuite, and ran into the
case that a race is detected, but we see the full stack trace only for one of
the two accesses, and the other one is showing "failed to restore the stack".

Try to prevent this by setting ThreadSanitizer flag history_size [1] to the
maximum (7) by default, as suggested here [2].

Tested on x86_64-linux.

Approved-By: Tom Tromey <[email protected]>

[1] https://github.com/google/sanitizers/wiki/ThreadSanitizerFlags
[2] https://groups.google.com/g/thread-sanitizer/c/VzSWE7UxhIE

Author: vries

gdb/testsuite/lib/gdb.exp

@@ -71,6 +71,14 @@ proc set_sanitizer_default { env_var var_id val } {
 set_sanitizer_default TSAN_OPTIONS suppressions \
     $srcdir/../tsan-suppressions.txt
 
+# When using ThreadSanitizer we may run into the case that a race is detected,
+# but we see the full stack trace only for one of the two accesses, and the
+# other one is showing "failed to restore the stack".
+# Try to prevent this by setting history_size to the maximum (7) by default.
+# See also the ThreadSanitizer docs (
+# https://github.com/google/sanitizers/wiki/ThreadSanitizerFlags ).
+set_sanitizer_default TSAN_OPTIONS history_size 7
+
 # If GDB is built with ASAN (and because there are leaks), it will output a
 # leak report when exiting as well as exit with a non-zero (failure) status.
 # This can affect tests that are sensitive to what GDB prints on stderr or its