[PATCH] gdbsupport/even-loop.cc: simplify !HAVE_POLL paths

[Pedro Alves <pedro@palves.net>]

On 2022-05-14 02:14, Youling Tang wrote:
> Hi, Pedro
> On 05/13/2022 06:21 PM, Pedro Alves wrote:
>> On 2022-05-13 11:09, Youling Tang wrote:
>>> Hi, Pedro
>>>
>>> On 05/13/2022 05:28 PM, Pedro Alves wrote:
>>>> On 2022-05-13 08:49, Youling Tang wrote:
>>>>> By searching event-loop.cc we found that there are only the following 2
>>>>> places to assign values to use_poll,
>>>>> $ grep -nr use_poll gdbsupport/event-loop.cc
>>>>>     88:static unsigned char use_poll = USE_POLL;
>>>>>     263:    use_poll = 0;
>>>>>
>>>>> This USE_POLL is defined as follows,
>>>>>    #ifdef HAVE_POLL
>>>>>    #define USE_POLL 1
>>>>>    #else
>>>>>    #define USE_POLL 0
>>>>>    #endif
>>>>>
>>>>> So use_poll may be 1 only if HAVE_POLL is defined. Removed "ifdef
>>>>> HAVE_POLL" judgment in use_poll control block.
>>>>>
>>>>> Signed-off-by: Youling Tang <tangyouling@loongson.cn>
>>>> Maybe I'm missing something, but ISTM this can't possible work on hosts that don'
>>>>    have poll at all.  Like e.g., mingw:
>>>>
>>>>    $ grep POLL *
>>>>    config.h:/* #undef HAVE_POLL */
>>>>    config.h:/* #undef HAVE_POLL_H */
>>>>    config.h:/* #undef HAVE_SYS_POLL_H */
>>>>
>>>> Surely they'll fail to compile after the patch?
>>> You are right my oversight.
>>>
>>> But we can remove these internal_error handling, similar to the following modification:
>> We can remove _every_ internal_error call in GDB, since by design, they are not supposed
>> to ever execute, unless we have something wrong.  This is the scenario here, guarding against
>> someone mistakenly doing that change that would result in use_poll as 1 on a host that doesn't
>> support poll.  Why change it?
> Thank you for letting me know the design background, this change is not needed.

Hmm, so the issue was the possibility of ending up with 'use_poll' true, and HAVE_POLL
not defined.  How about if we ALSO guard the definition of 'use_poll' with HAVE_POLL?
Then it's way way more unlikely that such a bad scenario happens by accident, making it
OK to remove the internal_error calls throughout.

Like in this following patch.  WDYT?

From 2931c1c66fa0ebcb3fce7dbc7032b9c4243fa4f2 Mon Sep 17 00:00:00 2001
From: Pedro Alves <pedro@palves.net>
Date: Mon, 16 May 2022 10:11:15 +0100
Subject: [PATCH] gdbsupport/even-loop.cc: simplify !HAVE_POLL paths

gdbsupport/even-loop.cc throughout handles the case of use_poll being
true on a system where HAVE_POLL is not defined, by calling
internal_error if that situation ever happens.

Simplify this by moving the "use_poll" global itself under HAVE_POLL,
so that it's way more unlikely to ever end up in such a situation.
Then, move the code that checks the value of use_poll under HAVE_POLL
too, and remove the internal_error calls.  Like, from:

    if (use_poll)
      {
  #ifdef HAVE_POLL
        // poll code
  #else
      internal_error (....);
  #endif /* HAVE_POLL */
      }
    else
      {
	// select code
      }

to

  #ifdef HAVE_POLL
    if (use_poll)
      {
        // poll code
      }
    else
  #endif /* HAVE_POLL */
      {
	// select code
      }

While at it, make use_poll be a bool.  The current code is using
unsigned char most probably to save space, but I don't think it really
matters here.

Co-Authored-By: Youling Tang <tangyouling@loongson.cn>
Change-Id: I0dd74fdd4d393ccd057906df4cd75e8e83c1cdb4
---
 gdbsupport/event-loop.cc | 88 ++++++++++++----------------------------
 1 file changed, 27 insertions(+), 61 deletions(-)

diff --git a/gdbsupport/event-loop.cc b/gdbsupport/event-loop.cc
index 385b45b2de1..18f57ef8d61 100644
--- a/gdbsupport/event-loop.cc
+++ b/gdbsupport/event-loop.cc
@@ -78,14 +78,12 @@ struct file_handler
   struct file_handler *next_file;
 };
 
-/* Do we use poll or select ? */
 #ifdef HAVE_POLL
-#define USE_POLL 1
-#else
-#define USE_POLL 0
-#endif /* HAVE_POLL */
-
-static unsigned char use_poll = USE_POLL;
+/* Do we use poll or select?  Some systems have poll, but then it's
+   not useable with all kinds of files.  We probe that whenever a new
+   file handler is added.  */
+static bool use_poll = true;
+#endif
 
 #ifdef USE_WIN32API
 #include <windows.h>
@@ -249,12 +247,10 @@ add_file_handler (int fd, handler_func *proc, gdb_client_data client_data,
 		  std::string &&name, bool is_ui)
 {
 #ifdef HAVE_POLL
-  struct pollfd fds;
-#endif
-
   if (use_poll)
     {
-#ifdef HAVE_POLL
+      struct pollfd fds;
+
       /* Check to see if poll () is usable.  If not, we'll switch to
 	 use select.  This can happen on systems like
 	 m68k-motorola-sys, `poll' cannot be used to wait for `stdin'.
@@ -263,23 +259,15 @@ add_file_handler (int fd, handler_func *proc, gdb_client_data client_data,
       fds.fd = fd;
       fds.events = POLLIN;
       if (poll (&fds, 1, 0) == 1 && (fds.revents & POLLNVAL))
-	use_poll = 0;
-#else
-      internal_error (__FILE__, __LINE__,
-		      _("use_poll without HAVE_POLL"));
-#endif /* HAVE_POLL */
+	use_poll = false;
     }
   if (use_poll)
     {
-#ifdef HAVE_POLL
       create_file_handler (fd, POLLIN, proc, client_data, std::move (name),
 			   is_ui);
-#else
-      internal_error (__FILE__, __LINE__,
-		      _("use_poll without HAVE_POLL"));
-#endif
     }
   else
+#endif /* HAVE_POLL */
     create_file_handler (fd, GDB_READABLE | GDB_EXCEPTION,
 			 proc, client_data, std::move (name), is_ui);
 }
@@ -321,9 +309,9 @@ create_file_handler (int fd, int mask, handler_func * proc,
       file_ptr->next_file = gdb_notifier.first_file_handler;
       gdb_notifier.first_file_handler = file_ptr;
 
+#ifdef HAVE_POLL
       if (use_poll)
 	{
-#ifdef HAVE_POLL
 	  gdb_notifier.num_fds++;
 	  if (gdb_notifier.poll_fds)
 	    gdb_notifier.poll_fds =
@@ -336,12 +324,9 @@ create_file_handler (int fd, int mask, handler_func * proc,
 	  (gdb_notifier.poll_fds + gdb_notifier.num_fds - 1)->fd = fd;
 	  (gdb_notifier.poll_fds + gdb_notifier.num_fds - 1)->events = mask;
 	  (gdb_notifier.poll_fds + gdb_notifier.num_fds - 1)->revents = 0;
-#else
-	  internal_error (__FILE__, __LINE__,
-			  _("use_poll without HAVE_POLL"));
-#endif /* HAVE_POLL */
 	}
       else
+#endif /* HAVE_POLL */
 	{
 	  if (mask & GDB_READABLE)
 	    FD_SET (fd, &gdb_notifier.check_masks[0]);
@@ -402,10 +387,6 @@ delete_file_handler (int fd)
 {
   file_handler *file_ptr, *prev_ptr = NULL;
   int i;
-#ifdef HAVE_POLL
-  int j;
-  struct pollfd *new_poll_fds;
-#endif
 
   /* Find the entry for the given file.  */
 
@@ -419,9 +400,12 @@ delete_file_handler (int fd)
   if (file_ptr == NULL)
     return;
 
+#ifdef HAVE_POLL
   if (use_poll)
     {
-#ifdef HAVE_POLL
+      int j;
+      struct pollfd *new_poll_fds;
+
       /* Create a new poll_fds array by copying every fd's information
 	 but the one we want to get rid of.  */
 
@@ -442,12 +426,9 @@ delete_file_handler (int fd)
       xfree (gdb_notifier.poll_fds);
       gdb_notifier.poll_fds = new_poll_fds;
       gdb_notifier.num_fds--;
-#else
-      internal_error (__FILE__, __LINE__,
-		      _("use_poll without HAVE_POLL"));
-#endif /* HAVE_POLL */
     }
   else
+#endif /* HAVE_POLL */
     {
       if (file_ptr->mask & GDB_READABLE)
 	FD_CLR (fd, &gdb_notifier.check_masks[0]);
@@ -510,9 +491,6 @@ static void
 handle_file_event (file_handler *file_ptr, int ready_mask)
 {
   int mask;
-#ifdef HAVE_POLL
-  int error_mask;
-#endif
 
     {
 	{
@@ -526,9 +504,11 @@ handle_file_event (file_handler *file_ptr, int ready_mask)
 	  /* See if the desired events (mask) match the received
 	     events (ready_mask).  */
 
+#ifdef HAVE_POLL
 	  if (use_poll)
 	    {
-#ifdef HAVE_POLL
+	      int error_mask;
+
 	      /* POLLHUP means EOF, but can be combined with POLLIN to
 		 signal more data to read.  */
 	      error_mask = POLLHUP | POLLERR | POLLNVAL;
@@ -547,12 +527,9 @@ handle_file_event (file_handler *file_ptr, int ready_mask)
 		}
 	      else
 		file_ptr->error = 0;
-#else
-	      internal_error (__FILE__, __LINE__,
-			      _("use_poll without HAVE_POLL"));
-#endif /* HAVE_POLL */
 	    }
 	  else
+#endif /* HAVE_POLL */
 	    {
 	      if (ready_mask & GDB_EXCEPTION)
 		{
@@ -599,9 +576,9 @@ gdb_wait_for_event (int block)
   if (block)
     update_wait_timeout ();
 
+#ifdef HAVE_POLL
   if (use_poll)
     {
-#ifdef HAVE_POLL
       int timeout;
 
       if (block)
@@ -616,12 +593,9 @@ gdb_wait_for_event (int block)
 	 signal.  */
       if (num_found == -1 && errno != EINTR)
 	perror_with_name (("poll"));
-#else
-      internal_error (__FILE__, __LINE__,
-		      _("use_poll without HAVE_POLL"));
-#endif /* HAVE_POLL */
     }
   else
+#endif /* HAVE_POLL */
     {
       struct timeval select_timeout;
       struct timeval *timeout_p;
@@ -670,9 +644,9 @@ gdb_wait_for_event (int block)
   /* To level the fairness across event descriptors, we handle them in
      a round-robin-like fashion.  The number and order of descriptors
      may change between invocations, but this is good enough.  */
+#ifdef HAVE_POLL
   if (use_poll)
     {
-#ifdef HAVE_POLL
       int i;
       int mask;
 
@@ -699,12 +673,9 @@ gdb_wait_for_event (int block)
       mask = (gdb_notifier.poll_fds + i)->revents;
       handle_file_event (file_ptr, mask);
       return 1;
-#else
-      internal_error (__FILE__, __LINE__,
-		      _("use_poll without HAVE_POLL"));
-#endif /* HAVE_POLL */
     }
   else
+#endif /* HAVE_POLL */
     {
       /* See comment about even source fairness above.  */
       int mask = 0;
@@ -856,16 +827,11 @@ update_wait_timeout (void)
 	}
 
       /* Update the timeout for select/ poll.  */
-      if (use_poll)
-	{
 #ifdef HAVE_POLL
-	  gdb_notifier.poll_timeout = timeout.tv_sec * 1000;
-#else
-	  internal_error (__FILE__, __LINE__,
-			  _("use_poll without HAVE_POLL"));
-#endif /* HAVE_POLL */
-	}
+      if (use_poll)
+	gdb_notifier.poll_timeout = timeout.tv_sec * 1000;
       else
+#endif /* HAVE_POLL */
 	{
 	  gdb_notifier.select_timeout.tv_sec = timeout.tv_sec;
 	  gdb_notifier.select_timeout.tv_usec = timeout.tv_usec;

base-commit: b7ff32f191ed7e708412e9faa31cf691f08ca695
-- 
2.36.0