mdbx: rework nested transactions and write-transaction state.

This resolves https://github.com/leo-yuriev/libmdbx/issues/62

Change-Id: Icbd762ca69ab1ccf8c3440c71931d2ac14599a1b

src/elements/internals.h

@@ -658,7 +658,7 @@ typedef pgno_t *MDBX_PNL;
 #define MDBX_PNL_DISORDERED(first, last) ((first) <= (last))
 #endif
 
-/* List of txnid, only for MDBX_env.mt_lifo_reclaimed */
+/* List of txnid, only for MDBX_txn.tw.lifo_reclaimed */
 typedef txnid_t *MDBX_TXL;
 
 /* An Dirty-Page list item is an pgno/pointer pair. */
@@ -747,31 +747,42 @@ struct MDBX_txn {
    * aborts, the ID may be re-used by the next writer. */
   txnid_t mt_txnid;
   MDBX_env *mt_env; /* the DB environment */
-                    /* The list of reclaimed txns from freeDB */
-  MDBX_TXL mt_lifo_reclaimed;
-  /* The list of pages that became unused during this transaction. */
-  MDBX_PNL mt_retired_pages;
-  /* The list of loose pages that became unused and may be reused
-   * in this transaction, linked through NEXT_LOOSE_PAGE(page). */
-  MDBX_page *mt_loose_pages;
-  /* Number of loose pages (mt_loose_pages) */
-  unsigned mt_loose_count;
-  /* The sorted list of dirty pages we temporarily wrote to disk
-   * because the dirty list was full. page numbers in here are
-   * shifted left by 1, deleted slots have the LSB set. */
-  MDBX_PNL mt_spill_pages;
-  union {
-    /* For write txns: Modified pages. Sorted when not MDBX_WRITEMAP. */
-    MDBX_DPL mt_rw_dirtylist;
-    /* For read txns: This thread/txn's reader table slot, or NULL. */
-    MDBX_reader *mt_ro_reader;
-  };
   /* Array of records for each DB known in the environment. */
   MDBX_dbx *mt_dbxs;
   /* Array of MDBX_db records for each known DB */
   MDBX_db *mt_dbs;
   /* Array of sequence numbers for each DB handle */
   unsigned *mt_dbiseqs;
+  union {
+    struct {
+      /* For read txns: This thread/txn's reader table slot, or NULL. */
+      MDBX_reader *reader;
+    } to;
+    struct {
+      /* The list of reclaimed txns from GC */
+      MDBX_TXL lifo_reclaimed;
+      /* The list of pages that became unused during this transaction. */
+      MDBX_PNL retired_pages;
+      /* The list of loose pages that became unused and may be reused
+       * in this transaction, linked through NEXT_LOOSE_PAGE(page). */
+      MDBX_page *loose_pages;
+      /* Number of loose pages (tw.loose_pages) */
+      unsigned loose_count;
+      /* The sorted list of dirty pages we temporarily wrote to disk
+       * because the dirty list was full. page numbers in here are
+       * shifted left by 1, deleted slots have the LSB set. */
+      MDBX_PNL spill_pages;
+      /* dirtylist room: Array size - dirty pages visible to this txn.
+       * Includes ancestor txns' dirty pages not hidden by other txns'
+       * dirty/spilled pages. Thus commit(nested txn) has room to merge
+       * dirtylist into mt_parent after freeing hidden mt_parent pages. */
+      unsigned dirtyroom;
+      /* For write txns: Modified pages. Sorted when not MDBX_WRITEMAP. */
+      MDBX_DPL dirtylist;
+      pgno_t *reclaimed_pglist; /* Reclaimed freeDB pages */
+      txnid_t last_reclaimed;   /* ID of last used record */
+    } tw;
+  };
 
 /* Transaction DB Flags */
 #define DB_DIRTY MDBX_TBL_DIRTY /* DB was written in this txn */
@@ -790,11 +801,6 @@ struct MDBX_txn {
    * This number only ever increments until the txn finishes; we
    * don't decrement it when individual DB handles are closed. */
   MDBX_dbi mt_numdbs;
-  /* dirtylist room: Array size - dirty pages visible to this txn.
-   * Includes ancestor txns' dirty pages not hidden by other txns'
-   * dirty/spilled pages. Thus commit(nested txn) has room to merge
-   * dirtylist into mt_parent after freeing hidden mt_parent pages. */
-  unsigned mt_dirtyroom;
   size_t mt_owner; /* thread ID that owns this transaction */
   mdbx_canary mt_canary;
 };
@@ -843,7 +849,7 @@ struct MDBX_cursor {
 #define C_DEL 0x08                /* last op was a cursor_del */
 #define C_UNTRACK 0x10            /* Un-track cursor when closing */
 #define C_RECLAIMING 0x20         /* FreeDB lookup is prohibited */
-#define C_GCFREEZE 0x40           /* me_reclaimed_pglist must not be updated */
+#define C_GCFREEZE 0x40           /* reclaimed_pglist must not be updated */
   unsigned mc_flags;              /* see mdbx_cursor */
   MDBX_page *mc_pg[CURSOR_STACK]; /* stack of pushed pages */
   indx_t mc_ki[CURSOR_STACK];     /* stack of page indices */
@@ -884,13 +890,6 @@ typedef struct MDBX_cursor_couple {
       (mc)->mc_xcursor->mx_cursor.mc_pg[0] = NODEDATA(xr_node);                \
   } while (0)
 
-/* State of FreeDB old pages, stored in the MDBX_env */
-typedef struct MDBX_pgstate {
-  pgno_t *mf_reclaimed_pglist; /* Reclaimed freeDB pages, or NULL before use */
-  txnid_t mf_last_reclaimed;   /* ID of last used record, or 0 if
-                                  !mf_reclaimed_pglist */
-} MDBX_pgstate;
-
 /* The database environment. */
 struct MDBX_env {
 #define MDBX_ME_SIGNATURE UINT32_C(0x9A899641)
@@ -932,11 +931,8 @@ struct MDBX_env {
   uint16_t *me_dbflags;        /* array of flags from MDBX_db.md_flags */
   unsigned *me_dbiseqs;        /* array of dbi sequence numbers */
   volatile txnid_t *me_oldest; /* ID of oldest reader last time we looked */
-  MDBX_pgstate me_pgstate;     /* state of old pages from freeDB */
-#define me_last_reclaimed me_pgstate.mf_last_reclaimed
-#define me_reclaimed_pglist me_pgstate.mf_reclaimed_pglist
-  MDBX_page *me_dpages; /* list of malloc'd blocks for re-use */
-                        /* PNL of pages that became unused in a write txn */
+  MDBX_page *me_dpages;        /* list of malloc'd blocks for re-use */
+  /* PNL of pages that became unused in a write txn */
   MDBX_PNL me_retired_pages;
   /* MDBX_DP of pages written during a write txn. Length MDBX_DPL_TXNFULL. */
   MDBX_DPL me_dirtylist;
@@ -991,12 +987,6 @@ struct MDBX_env {
 #endif
 };
 
-/* Nested transaction */
-typedef struct MDBX_ntxn {
-  MDBX_txn mnt_txn;         /* the transaction */
-  MDBX_pgstate mnt_pgstate; /* parent transaction's saved freestate */
-} MDBX_ntxn;
-
 /*----------------------------------------------------------------------------*/
 /* Debug and Logging stuff */
 
@@ -1235,7 +1225,7 @@ MDBX_INTERNAL_FUNC void mdbx_rthc_thread_dtor(void *ptr);
 /* The number of overflow pages needed to store the given size. */
 #define OVPAGES(env, size) (bytes2pgno(env, PAGEHDRSZ - 1 + (size)) + 1)
 
-/* Link in MDBX_txn.mt_loose_pages list.
+/* Link in MDBX_txn.tw.loose_pages list.
  * Kept outside the page header, which is needed when reusing the page. */
 #define NEXT_LOOSE_PAGE(p) (*(MDBX_page **)((p) + 2))