gdb/breakpoint: do not update the condition string if parsing the condition fails

The condition of a breakpoint can be set with the 'cond' command.  If
the condition has errors that make it problematic to evaluate, it
appears like GDB rejects the condition, but updates the breakpoint's
condition string, which causes incorrect/unintuitive behavior.

For instance:

  $ gdb ./test
  Reading symbols from ./test...
  (gdb) break 5
  Breakpoint 1 at 0x1155: file test.c, line 5.
  (gdb) cond 1 gibberish
  No symbol "gibberish" in current context.

At this point, it looks like the condition was rejected.
But "info breakpoints" shows the following:

  (gdb) info breakpoints
  Num     Type           Disp Enb Address            What
  1       breakpoint     keep y   0x0000000000001155 in main at test.c:5
          stop only if gibberish

Running the code gives the following behavior, where re-insertion of
the breakpoint causes failures.

  (gdb) run
  Starting program: test
  warning: failed to reevaluate condition for breakpoint 1: No symbol "gibberish" in current context.
  warning: failed to reevaluate condition for breakpoint 1: No symbol "gibberish" in current context.
  warning: failed to reevaluate condition for breakpoint 1: No symbol "gibberish" in current context.
  warning: failed to reevaluate condition for breakpoint 1: No symbol "gibberish" in current context.
  warning: failed to reevaluate condition for breakpoint 1: No symbol "gibberish" in current context.
  [Inferior 1 (process 19084) exited normally]
  (gdb)

This broken behavior occurs because GDB updates the condition string
of the breakpoint *before* checking that it parses successfully.
When parsing fails, the update has already taken place.

Fix the problem by updating the condition string *after* parsing the
condition.  We get the following behavior when this patch is applied:

  $ gdb ./test
  Reading symbols from ./test...
  (gdb) break 5
  Breakpoint 1 at 0x1155: file test.c, line 5.
  (gdb) cond 1 gibberish
  No symbol "gibberish" in current context.
  (gdb) info breakpoints
  Num     Type           Disp Enb Address            What
  1       breakpoint     keep y   0x0000000000001155 in main at test.c:5
  (gdb) run
  Starting program: test

  Breakpoint 1, main () at test.c:5
  5         a = a + 1; /* break-here */
  (gdb) c
  Continuing.
  [Inferior 1 (process 15574) exited normally]
  (gdb)

A side note: The problem does not occur if the condition is given
at the time of breakpoint definition, as in "break 5 if gibberish",
because the parsing of the condition fails during symtab-and-line
creation, before the breakpoint is created.

Finally, the code included the following comment:

  /* I don't know if it matters whether this is the string the user
     typed in or the decompiled expression.  */

This comment did not make sense to me because the condition string is
the user-typed input.  The patch updates this comment, too.

gdb/ChangeLog:
2020-07-30  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* breakpoint.c (set_breakpoint_condition): Update the
	condition string after parsing the new condition successfully.

gdb/testsuite/ChangeLog:
2020-07-30  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* gdb.base/condbreak-bad.c: New test.
	* gdb.base/condbreak-bad.exp: New file.

1 files changed, 9 insertions, 8 deletions

diff --git a/gdb/breakpoint.c b/gdb/breakpoint.c
index 4142084..a3a7c17 100644
--- a/gdb/breakpoint.c
+++ b/gdb/breakpoint.c
@@ -834,9 +834,6 @@ void
 set_breakpoint_condition (struct breakpoint *b, const char *exp,
 			  int from_tty)
 {
-  xfree (b->cond_string);
-  b->cond_string = NULL;
-
   if (is_watchpoint (b))
     {
       struct watchpoint *w = (struct watchpoint *) b;
@@ -859,6 +856,9 @@ set_breakpoint_condition (struct breakpoint *b, const char *exp,
 
   if (*exp == 0)
     {
+      xfree (b->cond_string);
+      b->cond_string = nullptr;
+
       if (from_tty)
 	printf_filtered (_("Breakpoint %d now unconditional.\n"), b->number);
     }
@@ -866,11 +866,6 @@ set_breakpoint_condition (struct breakpoint *b, const char *exp,
     {
       const char *arg = exp;
 
-      /* I don't know if it matters whether this is the string the user
-	 typed in or the decompiled expression.  */
-      b->cond_string = xstrdup (arg);
-      b->condition_not_parsed = 0;
-
       if (is_watchpoint (b))
 	{
 	  struct watchpoint *w = (struct watchpoint *) b;
@@ -896,6 +891,12 @@ set_breakpoint_condition (struct breakpoint *b, const char *exp,
 		error (_("Junk at end of expression"));
 	    }
 	}
+
+      /* We know that the new condition parsed successfully.  The
+	 condition string of the breakpoint can be safely updated.  */
+      xfree (b->cond_string);
+      b->cond_string = xstrdup (exp);
+      b->condition_not_parsed = 0;
     }
   mark_breakpoint_modified (b);