Add the ability to not block when opening a write transaction

mdbx.h

@@ -327,6 +327,10 @@ typedef int(MDBX_cmp_func)(const MDBX_val *a, const MDBX_val *b);
 /* Store multiple data items in one call. Only for MDBX_DUPFIXED. */
 #define MDBX_MULTIPLE 0x80000u
 
+/* Transaction Flags*/
+/* Do not block when starting a write transaction */
+#define MDBX_TRYTXN 0x10000000u
+
 /* Copy Flags */
 /* Compacting copy: Omit free space from copy, and renumber all
  * pages sequentially. */
@@ -420,8 +424,10 @@ typedef enum MDBX_cursor_op {
 #define MDBX_BAD_DBI (-30780)
 /* Unexpected problem - txn should abort */
 #define MDBX_PROBLEM (-30779)
+/* Unexpected problem - txn should abort */
+#define MDBX_BUSY (-30778)
 /* The last defined error code */
-#define MDBX_LAST_ERRCODE MDBX_PROBLEM
+#define MDBX_LAST_ERRCODE MDBX_BUSY
 
 /* The mdbx_put() or mdbx_replace() was called for key,
     that has more that one associated value. */
@@ -953,6 +959,9 @@ LIBMDBX_API int mdbx_env_set_assert(MDBX_env *env, MDBX_assert_func *func);
  *  - MDBX_RDONLY
  *      This transaction will not perform any write operations.
  *
+ *  - MDBX_TRYTXN
+ *      Do not block when starting a write transaction
+ *
  * [out] txn Address where the new MDBX_txn handle will be stored
  *
  * Returns A non-zero error value on failure and 0 on success, some
@@ -964,7 +973,8 @@ LIBMDBX_API int mdbx_env_set_assert(MDBX_env *env, MDBX_assert_func *func);
  *                         as well. See mdbx_env_set_mapsize().
  *  - MDBX_READERS_FULL  - a read-only transaction was requested and the reader
  *                         lock table is full. See mdbx_env_set_maxreaders().
- *  - MDBX_ENOMEM        - out of memory. */
+ *  - MDBX_ENOMEM        - out of memory.
+ *  - MDBX_BUSY          - a write transaction is already started. */
 LIBMDBX_API int mdbx_txn_begin(MDBX_env *env, MDBX_txn *parent, unsigned flags,
                                MDBX_txn **txn);
 

src/bits.h

@@ -553,7 +553,8 @@ struct MDBX_txn {
 
 /* Transaction Flags */
 /* mdbx_txn_begin() flags */
-#define MDBX_TXN_BEGIN_FLAGS (MDBX_NOMETASYNC | MDBX_NOSYNC | MDBX_RDONLY)
+#define MDBX_TXN_BEGIN_FLAGS_PERSISTENT (MDBX_NOMETASYNC | MDBX_NOSYNC | MDBX_RDONLY)
+#define MDBX_TXN_BEGIN_FLAGS (MDBX_TXN_BEGIN_FLAGS_PERSISTENT | MDBX_TRYTXN)
 #define MDBX_TXN_NOMETASYNC                                                    \
   MDBX_NOMETASYNC                   /* don't sync meta for this txn on commit */
 #define MDBX_TXN_NOSYNC MDBX_NOSYNC /* don't sync this txn on commit */

src/lck-posix.c

@@ -184,6 +184,13 @@ static int mdbx_robust_lock(MDBX_env *env, pthread_mutex_t *mutex) {
   return rc;
 }
 
+static int mdbx_robust_trylock(MDBX_env *env, pthread_mutex_t *mutex) {
+  int rc = pthread_mutex_trylock(mutex);
+  if (unlikely(rc != 0))
+    rc = mdbx_mutex_failed(env, mutex, rc);
+  return rc;
+}
+
 static int mdbx_robust_unlock(MDBX_env *env, pthread_mutex_t *mutex) {
   int rc = pthread_mutex_unlock(mutex);
   if (unlikely(rc != 0))
@@ -213,6 +220,13 @@ int mdbx_txn_lock(MDBX_env *env) {
   return MDBX_IS_ERROR(rc) ? rc : MDBX_SUCCESS;
 }
 
+int mdbx_txn_trylock(MDBX_env *env) {
+  mdbx_trace(">>");
+  int rc = mdbx_robust_trylock(env, &env->me_lck->mti_wmutex);
+  mdbx_trace("<< rc %d", rc);
+  return MDBX_IS_ERROR(rc) ? rc : MDBX_SUCCESS;
+}
+
 void mdbx_txn_unlock(MDBX_env *env) {
   mdbx_trace(">>");
   int rc = mdbx_robust_unlock(env, &env->me_lck->mti_wmutex);
@@ -313,7 +327,9 @@ static int __cold mdbx_mutex_failed(MDBX_env *env, pthread_mutex_t *mutex,
 #endif /* MDBX_USE_ROBUST */
 
   mdbx_error("mutex (un)lock failed, %s", mdbx_strerror(rc));
-  if (rc != EDEADLK) {
+  if (rc == EBUSY) {
+    rc = MDBX_BUSY;
+  } else if (rc != EDEADLK && rc != EBUSY) {
     env->me_flags |= MDBX_FATAL_ERROR;
     rc = MDBX_PANIC;
   }

src/lck-windows.c

@@ -132,6 +132,16 @@ int mdbx_txn_lock(MDBX_env *env) {
   return GetLastError();
 }
 
+int mdbx_txn_trylock(MDBX_env *env) {
+  if (flock(env->me_fd, LCK_EXCLUSIVE | LCK_DONTWAIT, LCK_BODY))
+    return MDBX_SUCCESS;
+  int rc = GetLastError();
+  if (rc == ERROR_LOCK_VIOLATION) {
+    rc = MDBX_BUSY;
+  }
+  return rc;
+}
+
 void mdbx_txn_unlock(MDBX_env *env) {
   if (!funlock(env->me_fd, LCK_BODY))
     mdbx_panic("%s failed: errcode %u", mdbx_func_, GetLastError());

src/mdbx.c

@@ -708,6 +708,7 @@ static const char *__mdbx_strerr(int errnum) {
       "DUPFIXED size",
       "MDBX_BAD_DBI: The specified DBI handle was closed/changed unexpectedly",
       "MDBX_PROBLEM: Unexpected problem - txn should abort",
+      "MDBX_BUSY: Another write transation is started",
   };
 
   if (errnum >= MDBX_KEYEXIST && errnum <= MDBX_LAST_ERRCODE) {
@@ -2562,7 +2563,7 @@ static int mdbx_txn_renew0(MDBX_txn *txn, unsigned flags) {
   } else {
     /* Not yet touching txn == env->me_txn0, it may be active */
     mdbx_jitter4testing(false);
-    rc = mdbx_txn_lock(env);
+    rc = F_ISSET(flags, MDBX_TRYTXN) ? mdbx_txn_trylock(env) : mdbx_txn_lock(env);
     if (unlikely(rc))
       return rc;
 
@@ -2664,6 +2665,7 @@ int mdbx_txn_begin(MDBX_env *env, MDBX_txn *parent, unsigned flags,
                    MDBX_txn **ret) {
   MDBX_txn *txn;
   MDBX_ntxn *ntxn;
+  //unsigned pflags;
   int rc, size, tsize;
 
   if (unlikely(!env || !ret))
@@ -2718,7 +2720,7 @@ int mdbx_txn_begin(MDBX_env *env, MDBX_txn *parent, unsigned flags,
   txn->mt_dbxs = env->me_dbxs; /* static */
   txn->mt_dbs = (MDBX_db *)((char *)txn + tsize);
   txn->mt_dbflags = (uint8_t *)txn + size - env->me_maxdbs;
-  txn->mt_flags = flags;
+  txn->mt_flags = flags & MDBX_TXN_BEGIN_FLAGS_PERSISTENT;
   txn->mt_env = env;
 
   if (parent) {

src/osal.h

@@ -506,6 +506,7 @@ int mdbx_rdt_lock(MDBX_env *env);
 void mdbx_rdt_unlock(MDBX_env *env);
 
 int mdbx_txn_lock(MDBX_env *env);
+int mdbx_txn_trylock(MDBX_env *env);
 void mdbx_txn_unlock(MDBX_env *env);
 
 int mdbx_rpid_set(MDBX_env *env);