Salvage comments added by Ingo while working on

Bug#52114 and Bug#50788.
The bugs themselves are regressions that are introduced
by an incomplete fix for Bug#36171 and will not be pushed.

mysys/mf_iocache.c

@@ -455,7 +455,9 @@ my_bool reinit_io_cache(IO_CACHE *info, enum cache_type type,
 
   RETURN
     0      we succeeded in reading all data
-    1      Error: can't read requested characters
+    1      Error: couldn't read requested characters. In this case:
+             If info->error == -1, we got a read error.
+             Otherwise info->error contains the number of bytes in Buffer.
 */
 
 int _my_b_read(register IO_CACHE *info, uchar *Buffer, size_t Count)
@@ -464,6 +466,7 @@ int _my_b_read(register IO_CACHE *info, uchar *Buffer, size_t Count)
   my_off_t pos_in_file;
   DBUG_ENTER("_my_b_read");
 
+  /* If the buffer is not empty yet, copy what is available. */
   if ((left_length= (size_t) (info->read_end-info->read_pos)))
   {
     DBUG_ASSERT(Count >= left_length);	/* User is not using my_b_read() */
@@ -475,7 +478,7 @@ int _my_b_read(register IO_CACHE *info, uchar *Buffer, size_t Count)
   /* pos_in_file always point on where info->buffer was read */
   pos_in_file=info->pos_in_file+ (size_t) (info->read_end - info->buffer);
 
-  /* 
+  /*
     Whenever a function which operates on IO_CACHE flushes/writes
     some part of the IO_CACHE to disk it will set the property
     "seek_not_done" to indicate this to other functions operating
@@ -502,19 +505,38 @@ int _my_b_read(register IO_CACHE *info, uchar *Buffer, size_t Count)
     }
   }
 
+  /*
+    Calculate, how much we are within a IO_SIZE block. Ideally this
+    should be zero.
+  */
   diff_length= (size_t) (pos_in_file & (IO_SIZE-1));
+
+  /*
+    If more than a block plus the rest of the current block is wanted,
+    we do read directly, without filling the buffer.
+  */
   if (Count >= (size_t) (IO_SIZE+(IO_SIZE-diff_length)))
   {					/* Fill first intern buffer */
     size_t read_length;
     if (info->end_of_file <= pos_in_file)
-    {					/* End of file */
+    {
+      /* End of file. Return, what we did copy from the buffer. */
       info->error= (int) left_length;
       DBUG_RETURN(1);
     }
+    /*
+      Crop the wanted count to a multiple of IO_SIZE and subtract,
+      what we did already read from a block. That way, the read will
+      end aligned with a block.
+    */
     length=(Count & (size_t) ~(IO_SIZE-1))-diff_length;
     if ((read_length= my_read(info->file,Buffer, length, info->myflags))
 	!= length)
     {
+      /*
+        If we didn't get, what we wanted, we either return -1 for a read
+        error, or (it's end of file), how much we got in total.
+      */
       info->error= (read_length == (size_t) -1 ? -1 :
 		    (int) (read_length+left_length));
       DBUG_RETURN(1);
@@ -526,15 +548,27 @@ int _my_b_read(register IO_CACHE *info, uchar *Buffer, size_t Count)
     diff_length=0;
   }
 
+  /*
+    At this point, we want less than one and a partial block.
+    We will read a full cache, minus the number of bytes, we are
+    within a block already. So we will reach new alignment.
+  */
   max_length= info->read_length-diff_length;
+  /* We will not read past end of file. */
   if (info->type != READ_FIFO &&
       max_length > (info->end_of_file - pos_in_file))
     max_length= (size_t) (info->end_of_file - pos_in_file);
+  /*
+    If there is nothing left to read,
+      we either are done, or we failed to fulfill the request.
+    Otherwise, we read max_length into the cache.
+  */
   if (!max_length)
   {
     if (Count)
     {
-      info->error= left_length;		/* We only got this many char */
+      /* We couldn't fulfil the request. Return, how much we got. */
+      info->error= left_length;
       DBUG_RETURN(1);
     }
     length=0;				/* Didn't read any chars */
@@ -543,13 +577,23 @@ int _my_b_read(register IO_CACHE *info, uchar *Buffer, size_t Count)
                             info->myflags)) < Count ||
 	   length == (size_t) -1)
   {
+    /*
+      We got an read error, or less than requested (end of file).
+      If not a read error, copy, what we got.
+    */
     if (length != (size_t) -1)
       memcpy(Buffer, info->buffer, length);
     info->pos_in_file= pos_in_file;
+    /* For a read error, return -1, otherwise, what we got in total. */
     info->error= length == (size_t) -1 ? -1 : (int) (length+left_length);
     info->read_pos=info->read_end=info->buffer;
     DBUG_RETURN(1);
   }
+  /*
+    Count is the remaining number of bytes requested.
+    length is the amount of data in the cache.
+    Read Count bytes from the cache.
+  */
   info->read_pos=info->buffer+Count;
   info->read_end=info->buffer+length;
   info->pos_in_file=pos_in_file;

sql/sql_base.cc

@@ -1931,7 +1931,6 @@ TABLE_LIST *find_table_in_list(TABLE_LIST *table,
 }
 
 
-
 /**
   Test that table is unique (It's only exists once in the table list)
 
@@ -2054,6 +2053,11 @@ next:
   or (in case of MyISAMMRG) one of its children are not used later
   in the query.
 
+  For MyISAMMRG tables, it is assumed that all the underlying
+  tables of @c table (if any) are listed right after it and that
+  their @c parent_l field points at the main table.
+
+
   @retval non-NULL The table list element for the table that
                    represents the duplicate. 
   @retval NULL     No duplicates found.

storage/myisam/mi_statrec.c

@@ -273,7 +273,12 @@ int _mi_read_rnd_static_record(MI_INFO *info, uchar *buf,
     DBUG_RETURN(error);
   }
 
-	/* Read record with cacheing */
+  /*
+    Read record with caching. If my_b_read() returns TRUE, less than the
+    requested bytes have been read. In this case rec_cache.error is
+    either -1 for a read error, or contains the number of bytes copied
+    into the buffer.
+  */
   error=my_b_read(&info->rec_cache,(uchar*) buf,share->base.reclength);
   if (info->s->base.pack_reclength != info->s->base.reclength && !error)
   {