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RE: [PATCH]Fix that GDB will get hang on Windows when using pipe to get stdout and stderr from stub


Hi Eli,

Can you help to review this proposal? The GDB trunk also has this issue.
Thanks.

BR,
Terry

> -----Original Message-----
> From: gdb-patches-owner@sourceware.org [mailto:gdb-patches-
> owner@sourceware.org] On Behalf Of Terry Guo
> Sent: Tuesday, June 26, 2012 9:13 AM
> To: gdb-patches@sourceware.org
> Cc: eliz@gnu.org; Joey Ye; Matthew Gretton-Dann; 'Pedro Alves';
> daniel.jacobowitz@gmail.com
> Subject: [PATCH]Fix that GDB will get hang on Windows when using pipe
> to get stdout and stderr from stub
> 
> Hi,
> 
> I noticed a cross-built MINGW arm-none-eabi GDB will get hang on
> Windows
> when use pipe to get stderr and stdout from stub. The command used to
> start
> stub in GDB is "target extended-remote |
> stub-that-write-stderr-before-stdout". For my case, after send
> "$vFlashDone#ea" to stub, GDB get hang. The GDB source show that GDB
> will
> keep waiting for ACK message from stdout of stub, after send the packet.
> Unfortunately my stub will write some kind of log information into
> stderr
> and this action takes place before stub write ACK message to its stdout.
> So
> the only pipe is occupied by stderr which is waiting for GDB to consume,
> while GDB keep waiting for message from the stdout which hasn't pipe to
> use.
> We finally end up with a deadlock on pipe between GDB/stderr/stdout.
> 
> The following patch can avoid such deadlock by letting GDB also probe
> and
> consume stderr when waiting for stdout. Please review and comment.
> 
> The Linux version GDB hasn't such issue. I think it's because we use
> different way to handle PIPE as stated in functions pipe_open and
> pipe_windows_open. For Linux we have two socketpair kind pipes, one for
> stdout and one for stderr. While for windows, we only have one pipe
> which is
> created by _pipe function.
> 
> BR,
> Terry
> 
> 2012-06-25  Terry Guo  <terry.guo@arm.com>
> 
> 	* ser_base (ser_base_read_error_fd): New function.
> 	(do_ser_base_readchar): Poll error file descriptor as well as
> 	standard output.
> 	(generic_readchar): Refactor error handling.
> 
> diff --git a/gdb/ser-base.c b/gdb/ser-base.c
> index 368afa6..ee6db54 100644
> --- a/gdb/ser-base.c
> +++ b/gdb/ser-base.c
> @@ -223,6 +223,63 @@ ser_base_wait_for (struct serial *scb, int timeout)
>      }
>  }
> 
> +/* Read any error output we might have.  */
> +
> +void
> +ser_base_read_error_fd (struct serial *scb, int close_fd)
> +{
> +  if (scb->error_fd != -1)
> +    {
> +      ssize_t s;
> +      char buf[81];
> +
> +      for (;;)
> +        {
> + 	  char *current;
> + 	  char *newline;
> +	  int to_read = 80;
> +	  int num_bytes = -1;
> +
> +	  if (scb->ops->avail)
> +	    num_bytes = (scb->ops->avail)(scb, scb->error_fd);
> +
> +	  if (num_bytes != -1)
> +	    to_read = (num_bytes < to_read) ? num_bytes : to_read;
> +
> +	  if (to_read == 0)
> +	    break;
> +
> +	  s = read (scb->error_fd, &buf, to_read);
> +	  if ((s == -1) || (s == 0 && !close_fd))
> +	    break;
> +
> +	  if (s == 0 && close_fd)
> +	    {
> +	      /* End of file.  */
> +	      close (scb->error_fd);
> +	      scb->error_fd = -1;
> +	      break;
> +	    }
> +
> +	  /* In theory, embedded newlines are not a problem.
> +	     But for MI, we want each output line to have just
> +	     one newline for legibility.  So output things
> +	     in newline chunks.  */
> +	  buf[s] = '\0';
> +	  current = buf;
> +	  while ((newline = strstr (current, "\n")) != NULL)
> +	    {
> +	      *newline = '\0';
> +	      fputs_unfiltered (current, gdb_stderr);
> +	      fputs_unfiltered ("\n", gdb_stderr);
> +	      current = newline + 1;
> +	    }
> +
> +	  fputs_unfiltered (current, gdb_stderr);
> +	}
> +    }
> +}
> +
>  /* Read a character with user-specified timeout.  TIMEOUT is number of
> seconds
>     to wait, or -1 to wait forever.  Use timeout of 0 to effect a poll.
> Returns
>     char if successful.  Returns -2 if timeout expired, EOF if line
> dropped
> @@ -273,6 +330,11 @@ do_ser_base_readchar (struct serial *scb, int
> timeout)
>  	  status = SERIAL_TIMEOUT;
>  	  break;
>  	}
> +
> +      /* We also need to check and consume the stderr because it could
> +         come before the stdout for some stubs.  If we just sit and
> wait
> +         for stdout, we would hit a deadlock for that case.  */
> +      ser_base_read_error_fd (scb, 0);
>      }
> 
>    if (status < 0)
> @@ -344,53 +406,7 @@ generic_readchar (struct serial *scb, int timeout,
>  	}
>      }
>    /* Read any error output we might have.  */
> -  if (scb->error_fd != -1)
> -    {
> -      ssize_t s;
> -      char buf[81];
> -
> -      for (;;)
> -        {
> - 	  char *current;
> - 	  char *newline;
> -	  int to_read = 80;
> -
> -	  int num_bytes = -1;
> -	  if (scb->ops->avail)
> -	    num_bytes = (scb->ops->avail)(scb, scb->error_fd);
> -	  if (num_bytes != -1)
> -	    to_read = (num_bytes < to_read) ? num_bytes : to_read;
> -
> -	  if (to_read == 0)
> -	    break;
> -
> -	  s = read (scb->error_fd, &buf, to_read);
> -	  if (s == -1)
> -	    break;
> -	  if (s == 0)
> -	    {
> -	      /* EOF */
> -	      close (scb->error_fd);
> -	      scb->error_fd = -1;
> -	      break;
> -	    }
> -
> -	  /* In theory, embedded newlines are not a problem.
> -	     But for MI, we want each output line to have just
> -	     one newline for legibility.  So output things
> -	     in newline chunks.  */
> -	  buf[s] = '\0';
> -	  current = buf;
> -	  while ((newline = strstr (current, "\n")) != NULL)
> -	    {
> -	      *newline = '\0';
> -	      fputs_unfiltered (current, gdb_stderr);
> -	      fputs_unfiltered ("\n", gdb_stderr);
> -	      current = newline + 1;
> -	    }
> -	  fputs_unfiltered (current, gdb_stderr);
> -	}
> -    }
> +  ser_base_read_error_fd (scb, 1);
> 
>    reschedule (scb);
>    return ch;
> 
> 




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