MDEV-39788: Remove added line in master.info format

sql/rpl_info_file.h

@@ -131,6 +131,14 @@ namespace Int_IO_CACHE
   Other files may include these files directly.
   [C++20 modules](https://en.cppreference.com/w/cpp/language/modules.html)
   can supercede the header-only design as well as headers' `#include` guards.
+
+  @note
+    It's an old inconsistency that @ref Master_info_file's line count includes
+    the line-count "value" while @ref Relay_log_info_file's excludes it.
+    This is reason that Info_file::load_from_file() and
+    Info_file::save_to_file() defines the line-count lines differently -
+    It is so the two subclasses can implement either interpretation
+    on top of the indifferent common code in this superclass.
 */
 struct Info_file
 {
@@ -250,15 +258,15 @@ struct Info_file
     (Re)load the MySQL line-based section from the @ref file
     @param value_list
       List of wrapped member pointers to values. The first element must be a
-      file name @ref String_value to be unambiguous with the line count line.
+      file name @ref String_value to be unambiguous with the line-count line.
     @param default_line_count
       We cannot simply read lines until EOF as all versions
       of MySQL/MariaDB may generate more lines than needed.
       Therefore, starting with MySQL/MariaDB 4.1.x for @ref Master_info_file and
-      5.6.x for @ref Relay_log_info_file, the first line of the file is number
-      of one-line-per-value lines in the file, including this line count itself.
+      5.6.x/10.0.x for @ref Relay_log_info_file, the first line of the file is
+      the number of one-line-per-value lines (excluding itself) in the file.
       This parameter specifies the number of effective lines before those
-      versions (i.e., not counting the line count line if it was to have one),
+      versions (i.e., not counting the line-count line if it was to have one),
       where the first line is a filename with extension
       (either contains a `.` or is entirely empty) rather than an integer.
     @return `false` if the file has parsed successfully or `true` if error
@@ -272,14 +280,15 @@ struct Info_file
     @param value_list List of wrapped member pointers to values.
     @param total_line_count
       The number of lines to describe the file as on the first line of the file.
-      If this is larger than `value_list.size()`, suffix the file with empty
-      lines until the line count (including the line count line) is this many.
-      This reservation provides compatibility with MySQL,
-      who has added more old-style lines while MariaDB innovated.
+      If this is larger than `size`, suffix the file with empty
+      lines until the line count (*including* the line-count line) is this many.
+      (No correction is done if it is less than `size` + 1 for the line-count.)
+      This reservation provides compatibility
+      with older versions of MySQL and MariaDB.
   */
   template<size_t size> void save_to_file(
     const Mem_fn (&value_list)[size],
-    size_t total_line_count= size + /* line count line */ 1
+    size_t total_line_count= size
   ) { return save_to_file(value_list, size, total_line_count); }
 
 private:
@@ -336,7 +345,6 @@ struct Info_file
 
   void save_to_file(const Mem_fn *values, size_t size, size_t total_line_count)
   {
-    DBUG_ASSERT(total_line_count > size);
     my_b_seek(&file, 0);
     /*
       If the new contents take less space than the previous file contents,
@@ -355,10 +363,10 @@ struct Info_file
     }
     /*
       Pad additional reserved lines:
-      (1 for the line count line + line count) inclusive -> max line inclusive
-       = line count exclusive <- max line inclusive
+      (1 for the line count line + line count) exclusive -> max line inclusive
+       = line count exclusive <- max line exclusive
     */
-    for (; total_line_count > size; --total_line_count)
+    while (--total_line_count > size)
       my_b_write_byte(&file, '\n');
   }
 

sql/rpl_master_info_file.h

@@ -88,6 +88,11 @@ inline uint64_t master_retry_count= 100000;
 /// }@
 
 
+/**
+  @note The line count includes the line-count line.
+    Master_info_file::load_from_file() implements this
+    using @ref Master_info_file::LINE_COUNT_FIX.
+*/
 struct Master_info_file: Info_file
 {
 
@@ -540,6 +545,21 @@ struct Master_info_file: Info_file
   /// }@
 
 
+protected:
+  /**
+    Info_file::load_from_file() does not include
+    the line-count "value" in the line count.
+    The Master_info_file::load_from_file() overload utilizes the presence
+    of this no-op pseudo-value to emulating counting the line-count "value" -
+    The driver loop will increment the line counter while this reads nothing.
+  */
+  struct: Persistent
+  {
+    bool load_from(IO_CACHE *file) override { return false; } ///< No-op
+    void save_to(IO_CACHE *file) override {} ///< No-op
+  } LINE_COUNT_FIX;
+public:
+
   inline static const Mem_fn VALUE_LIST[] {
     &Master_info_file::master_log_file,
     &Master_info_file::master_log_pos,
@@ -561,7 +581,9 @@ struct Master_info_file: Info_file
     nullptr, // MySQL `master_uuid`, which MariaDB ignores.
     &Master_info_file::master_retry_count,
     &Master_info_file::master_ssl_crl,
-    &Master_info_file::master_ssl_crlpath
+    &Master_info_file::master_ssl_crlpath,
+    // This must be last, so its blank line blend into trailing blank line(s).
+    &Master_info_file::LINE_COUNT_FIX
   };
 
   /**

sql/rpl_relay_log_info_file.h

@@ -20,6 +20,7 @@
 
 #include "rpl_info_file.h"
 
+/// @note The line count excludes the line-count line.
 struct Relay_log_info_file: Info_file
 {
   /**