MDEV-17133 dump thread reads from a past position

According to logs analysis the Dump thread attempted to read again data which
was already sent. The reason of regressed read turns out in an _my_b_cache_read()
early exit branch which missed to distinguish between total zero size read (e.g
ineffective read when Count argument is zero) from a case when the
requested amount of data is fully read out by sole accessing the cache's
file. In the latter case such then *effective* reading was not
reflected in the cache's state to screw the cache's state.

Fixed with a check introduced of whether the file reading was effective prior to
early exit. When this is the case conduct standard cache state change to
account the actual read size.

Notice the bug can show up also as an error to read binlog event e.g
through BINLOG_GTID_POS() (of MDEV-16886).

unittest/sql/mf_iocache-t.cc

@@ -285,11 +285,95 @@ void mdev14014()
   close_cached_file(&info);
 }
 
+void mdev17133()
+{
+  int   res, k;
+  const int eof_iter=4, read_iter= 4;
+  uchar buf_i[1024*256];      // read
+  uchar buf_o[sizeof(buf_i)]; // write
+  const size_t eof_block_size= sizeof(buf_o) / eof_iter;
+  const size_t read_size= eof_block_size / read_iter;
+  size_t total;
+
+  srand((uint) time(NULL));
+  memset(buf_i,    0, sizeof( buf_i));
+  memset(buf_o, FILL, sizeof(buf_o));
+
+  diag("MDEV-17133 Dump thread reads from the past");
+
+  init_io_cache_encryption();
+
+  res= open_cached_file(&info, 0, 0, CACHE_SIZE, 0);
+  ok(res == 0, "open_cached_file" INFO_TAIL);
+
+  res= my_b_write(&info, buf_o, sizeof(buf_o));
+  ok(res == 0, "buffer is written" INFO_TAIL);
+  res= my_b_tell(&info);
+  ok(res == sizeof(buf_o), "cache size as expected");
+
+  res= my_b_flush_io_cache(&info, 1);
+  ok(res == 0, "flush" INFO_TAIL);
+  res= reinit_io_cache(&info, READ_CACHE, 0, 0, 0);
+  ok(res == 0, "reinit READ_CACHE" INFO_TAIL);
+
+  // read the written data by chunks of variable size eof_iter times
+  for (k= eof_iter, info.end_of_file=0, total= 0; k; k--)
+  {
+    int i;
+    size_t curr_read_size;
+    info.end_of_file=
+      k == 1 ? sizeof(buf_o) :
+      MY_MIN(sizeof(buf_o),
+             info.end_of_file + eof_block_size +
+             // plus 25% of block for randomization to the average
+             (eof_block_size/4 - rand() % (eof_block_size/2)));
+
+    // read a chunk by blocks of variable size read_iter times
+    // the last block completes the current chunk
+    for (i= 0; i < read_iter; i++, total += curr_read_size)
+    {
+      char buf_check[eof_block_size];
+      uint a,b;
+
+      a= info.end_of_file - total;
+      b= read_size + read_size/4 - rand() % (read_size/2);
+      curr_read_size= (i == read_iter - 1) ? info.end_of_file - total :
+        MY_MIN(a, b);
+
+      DBUG_ASSERT(curr_read_size <= info.end_of_file - total);
+
+      res= my_b_read(&info, buf_i + total, MY_MIN(19, curr_read_size));
+      ok(res == 0, "read of 19");
+      // mark read bytes in the used part of the cache buffer
+      memset(info.buffer, 0, info.read_pos - info.buffer);
+
+      // random size 2nd read
+      res= my_b_read(&info, buf_i + total + MY_MIN(19, curr_read_size),
+                     19 >= curr_read_size ? 0 : curr_read_size - 19);
+      ok(res == 0, "rest of read %lu", curr_read_size - 19);
+      // mark read bytes in the used part of the cache buffer
+      memset(info.buffer, 0, info.read_pos - info.buffer);
+
+      // check that no marked bytes are read
+      memset(buf_check, FILL, curr_read_size);
+      ok(memcmp(buf_i + total, buf_check, curr_read_size) == 0,
+         "read correct data");
+    }
+    ok(info.pos_in_file + (info.read_end - info.buffer) == info.end_of_file,
+       "cache is read up to eof");
+    ok(total == info.end_of_file, "total matches eof");
+  }
+  ok(total == sizeof(buf_i), "read total size match");
+  ok(buf_i[sizeof(buf_i) - 1] == FILL, "data read correctly");
+
+  close_cached_file(&info);
+}
+
 
 int main(int argc __attribute__((unused)),char *argv[])
 {
   MY_INIT(argv[0]);
-  plan(51);
+  plan(114);
 
   /* temp files with and without encryption */
   encrypt_tmp_files= 1;
@@ -306,6 +390,7 @@ int main(int argc __attribute__((unused)),char *argv[])
   encrypt_tmp_files= 0;
 
   mdev14014();
+  mdev17133();
 
   my_end(0);
   return exit_status();