Bug#42790 Test 'main.innodb' fails from "InnoDB: Error: in ALTER TABLE"

      
The problem here seem to be that when mysql
is redirecting stderr to a file, stderr becomes 
buffered, whereas it is unbuffered by definition.

The solution is to unbuffer it by setting buffer
to null.

sql/log.cc:
  use setbuf(stderr, NULL) to set the buffer to null.
sql/mysqld.cc:
  use setbuf(stderr, NULL) to set the buffer to null.

sql/log.cc

@@ -4650,10 +4650,14 @@ bool flush_error_log()
       uchar buf[IO_SIZE];
 
       freopen(err_temp,"a+",stderr);
+      setbuf(stderr, NULL);
       (void) my_delete(err_renamed, MYF(0));
       my_rename(log_error_file,err_renamed,MYF(0));
       if (freopen(log_error_file,"a+",stdout))
+      {
         freopen(log_error_file,"a+",stderr);
+        setbuf(stderr, NULL);
+      }
 
       if ((fd = my_open(err_temp, O_RDONLY, MYF(0))) >= 0)
       {
@@ -4669,7 +4673,10 @@ bool flush_error_log()
 #else
    my_rename(log_error_file,err_renamed,MYF(0));
    if (freopen(log_error_file,"a+",stdout))
+   {
      freopen(log_error_file,"a+",stderr);
+     setbuf(stderr, NULL);
+   }
    else
      result= 1;
 #endif

sql/mysqld.cc

@@ -3703,7 +3703,10 @@ static int init_server_components()
 #ifndef EMBEDDED_LIBRARY
       if (freopen(log_error_file, "a+", stdout))
 #endif
+      {
         freopen(log_error_file, "a+", stderr);
+        setbuf(stderr, NULL);
+      }
     }
   }
 
@@ -4331,6 +4334,7 @@ we force server id to 2, but this MySQL server will not act as a slave.");
   {
     freopen(log_error_file,"a+",stdout);
     freopen(log_error_file,"a+",stderr);
+    setbuf(stderr, NULL);
     FreeConsole();				// Remove window
   }
 #endif