mdbx: добавление ошибки MDBX_BACKLOG_DEPLETED и соответствующей логики в page_alloc_slowpath().

mdbx.h

@@ -1926,6 +1926,12 @@ enum MDBX_error_t {
   /** Overlapping read and write transactions for the current thread */
   MDBX_TXN_OVERLAPPING = -30415,
 
+  /** Внутренняя ошибка возвращаемая в случае нехватки запаса свободных страниц
+   * при обновлении GC. Используется как вспомогательное средство для отладки.
+   * \note С точки зрения пользователя семантически
+   *       равнозначна \ref MDBX_PROBLEM. */
+  MDBX_BACKLOG_DEPLETED = -30414,
+
   /* The last of MDBX-added error codes */
   MDBX_LAST_ADDED_ERRCODE = MDBX_TXN_OVERLAPPING,
 

src/bits.md

@@ -5,7 +5,7 @@ N |   MASK  | ENV       | TXN          | DB       | PUT       | DBI        | NOD
 2 |0000 0004|ALLOC_NEW  |TXN_DIRTY     |DUPSORT   |           |DBI_FRESH   |F_DUPDATA|P_OVERFLOW|         |
 3 |0000 0008|ALLOC_SLOT |TXN_SPILLS    |INTEGERKEY|           |DBI_CREAT   |         |P_META    |         |
 4 |0000 0010|ALLOC_FAKE |TXN_HAS_CHILD |DUPFIXED  |NOOVERWRITE|DBI_VALID   |         |P_BAD     |         |
-5 |0000 0020|           |              |INTEGERDUP|NODUPDATA  |DBI_USRVALID|         |P_LEAF2   |         |
+5 |0000 0020|           |TXN_DRAINED_GC|INTEGERDUP|NODUPDATA  |DBI_USRVALID|         |P_LEAF2   |         |
 6 |0000 0040|           |              |REVERSEDUP|CURRENT    |DBI_DUPDATA |         |P_SUBP    |         |
 7 |0000 0080|           |              |          |ALLDUPS    |DBI_AUDITED |         |          |         |
 8 |0000 0100| _MAY_MOVE |              |          |           |            |         |          | <=      |

src/core.c

@@ -6750,10 +6750,12 @@ static __inline bool is_gc_usable(MDBX_txn *txn, const MDBX_cursor *mc,
       !(mc->mc_flags & C_GCU))
     return false;
 
-  /* avoid (recursive) search inside empty tree and while tree is
+  /* avoid search inside empty tree and while tree is updating,
-     updating, https://libmdbx.dqdkfa.ru/dead-github/issues/31 */
+     https://libmdbx.dqdkfa.ru/dead-github/issues/31 */
-  if (txn->mt_dbs[FREE_DBI].md_entries == 0)
+  if (unlikely(txn->mt_dbs[FREE_DBI].md_entries == 0)) {
+    txn->mt_flags |= MDBX_TXN_DRAINED_GC;
     return false;
+  }
 
   return true;
 }
@@ -7090,8 +7092,10 @@ static pgr_t page_alloc_slowpath(const MDBX_cursor *const mc, const size_t num,
 
   //---------------------------------------------------------------------------
 
-  if (unlikely(!is_gc_usable(txn, mc, flags)))
+  if (unlikely(!is_gc_usable(txn, mc, flags))) {
+    eASSERT(env, txn->mt_flags & MDBX_TXN_DRAINED_GC);
     goto no_gc;
+  }
 
   eASSERT(env, (flags & (MDBX_ALLOC_COALESCE | MDBX_ALLOC_LIFO |
                          MDBX_ALLOC_SHOULD_SCAN)) == 0);
@@ -7178,6 +7182,7 @@ next_gc:;
     if (unlikely(id >= detent))
       goto depleted_gc;
   }
+  txn->mt_flags &= ~MDBX_TXN_DRAINED_GC;
 
   /* Reading next GC record */
   MDBX_val data;
@@ -7326,9 +7331,12 @@ scan:
   }
 
 depleted_gc:
+  TRACE("%s: last id #%" PRIaTXN ", re-len %zu", "gc-depleted", id,
+        MDBX_PNL_GETSIZE(txn->tw.relist));
   ret.err = MDBX_NOTFOUND;
   if (flags & MDBX_ALLOC_SHOULD_SCAN)
     goto scan;
+  txn->mt_flags |= MDBX_TXN_DRAINED_GC;
 
   //-------------------------------------------------------------------------
 
@@ -7431,6 +7439,14 @@ depleted_gc:
 
 no_gc:
   eASSERT(env, pgno == 0);
+#ifndef MDBX_ENABLE_BACKLOG_DEPLETED
+#define MDBX_ENABLE_BACKLOG_DEPLETED 0
+#endif /* MDBX_ENABLE_BACKLOG_DEPLETED*/
+  if (MDBX_ENABLE_BACKLOG_DEPLETED &&
+      unlikely(!(txn->mt_flags & MDBX_TXN_DRAINED_GC))) {
+    ret.err = MDBX_BACKLOG_DEPLETED;
+    goto fail;
+  }
   if (flags & MDBX_ALLOC_RESERVE) {
     ret.err = MDBX_NOTFOUND;
     goto fail;
@@ -9915,6 +9931,8 @@ static int gcu_prepare_backlog(MDBX_txn *txn, gcu_context_t *ctx) {
         (size_t)txn->mt_dbs[FREE_DBI].md_leaf_pages,
         (size_t)txn->mt_dbs[FREE_DBI].md_overflow_pages,
         (size_t)txn->mt_dbs[FREE_DBI].md_entries);
+  tASSERT(txn,
+          err != MDBX_NOTFOUND || (txn->mt_flags & MDBX_TXN_DRAINED_GC) != 0);
   return (err != MDBX_NOTFOUND) ? err : MDBX_SUCCESS;
 }
 

src/internals.h

@@ -1013,9 +1013,11 @@ struct MDBX_txn {
   /* Additional flag for sync_locked() */
 #define MDBX_SHRINK_ALLOWED UINT32_C(0x40000000)
 
+#define MDBX_TXN_DRAINED_GC 0x20 /* GC was depleted up to oldest reader */
+
 #define TXN_FLAGS                                                              \
   (MDBX_TXN_FINISHED | MDBX_TXN_ERROR | MDBX_TXN_DIRTY | MDBX_TXN_SPILLS |     \
-   MDBX_TXN_HAS_CHILD | MDBX_TXN_INVALID)
+   MDBX_TXN_HAS_CHILD | MDBX_TXN_INVALID | MDBX_TXN_DRAINED_GC)
 
 #if (TXN_FLAGS & (MDBX_TXN_RW_BEGIN_FLAGS | MDBX_TXN_RO_BEGIN_FLAGS)) ||       \
     ((MDBX_TXN_RW_BEGIN_FLAGS | MDBX_TXN_RO_BEGIN_FLAGS | TXN_FLAGS) &         \