MDEV-6870 Not possible to use FIFO file as a general_log file

Remove the too restrictive bugfix for bug#67088.
FIFO can be used for general/slow logs, but lseek() and fsync() on
FIFO fail. And open() needs to be non-blocking, in case the other
end isn't reading.

mysql-test/r/log_errchk.result

-call mtr.add_suppression("Could not use");
-# Case 1: Setting fife file to general_log_file and slow_query_log_file
+# Case 1: Setting fifo file to general_log_file and slow_query_log_file
 #         system variable.
 SET GLOBAL general_log_file="MYSQLTEST_VARDIR/tmp/general_log.fifo";;
 ERROR 42000: Variable 'general_log_file' can't be set to the value of 'MYSQLTEST_VARDIR/tmp/general_log.fifo'
@@ -7,4 +6,6 @@ SET GLOBAL slow_query_log_file="MYSQLTEST_VARDIR/tmp/slow_log.fifo";;
 ERROR 42000: Variable 'slow_query_log_file' can't be set to the value of 'MYSQLTEST_VARDIR/tmp/slow_log.fifo'
 # Case 2: Starting server with fifo file as general log file
 #         and slow query log file.
+call mtr.add_suppression("Could not use .* for logging \\(error 6\\)");
+call mtr.add_suppression("File '.*' not found \\(Errcode: 6\\)");
 Setting fifo file as general log file and slow query log failed.

mysql-test/t/log_errchk.test

@@ -6,7 +6,8 @@
 # Bug#14757009 : WHEN THE GENERAL_LOG IS A SOCKET AND THE READER GOES AWAY,
 #                MYSQL QUITS WORKING.
 #
-call mtr.add_suppression("Could not use");
+# MDEV-6870 Not possible to use FIFO file as a general_log file
+#
 
 --let $gen_log_file= $MYSQLTEST_VARDIR/tmp/general_log.fifo
 --let $slow_query_log_file= $MYSQLTEST_VARDIR/tmp/slow_log.fifo
@@ -15,10 +16,9 @@ call mtr.add_suppression("Could not use");
 --exec mkfifo $gen_log_file
 --exec mkfifo $slow_query_log_file
 
---echo # Case 1: Setting fife file to general_log_file and slow_query_log_file
+--echo # Case 1: Setting fifo file to general_log_file and slow_query_log_file
 --echo #         system variable.
-# Only regular files can be set to general log. Setting fifo file to general log
-# reports an error.
+# Setting fifo file to general log reports an error because the other end is closed
 --replace_result $MYSQLTEST_VARDIR MYSQLTEST_VARDIR
 --error ER_WRONG_VALUE_FOR_VAR
 --eval SET GLOBAL general_log_file="$gen_log_file";
@@ -38,11 +38,12 @@ call mtr.add_suppression("Could not use");
 --exec echo "restart: --general-log-file=$gen_log_file --slow-query-log-file=$slow_query_log_file" > $MYSQLTEST_VARDIR/tmp/mysqld.1.expect
 --source include/wait_until_connected_again.inc
 
-# With fix error should be reported in the error log file if file is not a
-# regular file.
+# Error 6 is reported, because the other end is closed
+call mtr.add_suppression("Could not use .* for logging \\(error 6\\)");
+call mtr.add_suppression("File '.*' not found \\(Errcode: 6\\)");
 --perl
   my $file= $ENV{'GREP_FILE'};
-  my $pattern= "Turning logging off for the whole duration";
+  my $pattern= "for logging \\(error 6\\)\\. Turning logging off for the whole duration";
   open(FILE, "$file") or die("Unable to open $file: $!\n");
   my $count = 0;
   while (<FILE>) {

sql/log.cc

@@ -2360,6 +2360,7 @@ bool MYSQL_LOG::open(
   MY_STAT f_stat;
   File file= -1;
   my_off_t seek_offset;
+  bool is_fifo = false;
   int open_flags= O_CREAT | O_BINARY;
   DBUG_ENTER("MYSQL_LOG::open");
   DBUG_PRINT("enter", ("log_type: %d", (int) log_type_arg));
@@ -2376,15 +2377,17 @@ bool MYSQL_LOG::open(
                                  log_type_arg, io_cache_type_arg))
     goto err;
 
-  /* File is regular writable file */
-  if (my_stat(log_file_name, &f_stat, MYF(0)) && !MY_S_ISREG(f_stat.st_mode))
-    goto err;
+  is_fifo = my_stat(log_file_name, &f_stat, MYF(0)) &&
+            MY_S_ISFIFO(f_stat.st_mode);
 
   if (io_cache_type == SEQ_READ_APPEND)
     open_flags |= O_RDWR | O_APPEND;
   else
     open_flags |= O_WRONLY | (log_type == LOG_BIN ? 0 : O_APPEND);
 
+  if (is_fifo)
+    open_flags |= O_NONBLOCK;
+
   db[0]= 0;
 
 #ifdef HAVE_PSI_INTERFACE
@@ -2396,7 +2399,9 @@ bool MYSQL_LOG::open(
                              MYF(MY_WME | ME_WAITTANG))) < 0)
     goto err;
 
-  if ((seek_offset= mysql_file_tell(file, MYF(MY_WME))))
+  if (is_fifo)
+    seek_offset= 0;
+  else if ((seek_offset= mysql_file_tell(file, MYF(MY_WME))))
     goto err;
 
   if (init_io_cache(&log_file, file, IO_SIZE, io_cache_type, seek_offset, 0,
@@ -2488,7 +2493,7 @@ void MYSQL_LOG::close(uint exiting)
   {
     end_io_cache(&log_file);
 
-    if (mysql_file_sync(log_file.file, MYF(MY_WME)) && ! write_error)
+    if (log_type == LOG_BIN && mysql_file_sync(log_file.file, MYF(MY_WME)) && ! write_error)
     {
       write_error= 1;
       sql_print_error(ER_THD_OR_DEFAULT(current_thd, ER_ERROR_ON_WRITE), name, errno);