Don't call palloc() while holding a spinlock, either.

Fix some more violations of the "only straight-line code inside a
spinlock" rule.  These are hazardous not only because they risk
holding the lock for an excessively long time, but because it's
possible for palloc to throw elog(ERROR), leaving a stuck spinlock
behind.

copy_replication_slot() had two separate places that did pallocs
while holding a spinlock.  We can make the code simpler and safer
by copying the whole ReplicationSlot struct into a local variable
while holding the spinlock, and then referencing that copy.
(While that's arguably more cycles than we really need to spend
holding the lock, the struct isn't all that big, and this way seems
far more maintainable than copying fields piecemeal.  Anyway this
is surely much cheaper than a palloc.)  That bug goes back to v12.

InvalidateObsoleteReplicationSlots() not only did a palloc while
holding a spinlock, but for extra sloppiness then leaked the memory
--- probably for the lifetime of the checkpointer process, though
I didn't try to verify that.  Fortunately that silliness is new
in HEAD.

pg_get_replication_slots() had a cosmetic violation of the rule,
in that it only assumed it's safe to call namecpy() while holding
a spinlock.  Still, that's a hazard waiting to bite somebody, and
there were some other cosmetic coding-rule violations in the same
function, so clean it up.  I back-patched this as far as v10; the
code exists before that but it looks different, and this didn't
seem important enough to adapt the patch further back.

Discussion: https://postgr.es/m/20200602.161518.1399689010416646074[email protected]

diff --git a/src/backend/replication/slotfuncs.c b/src/backend/replication/slotfuncs.c
index 3a7cea54d59dea8ce1249093e944eaf62b53460a..947644b5dd6c8ade421f8828db7c006a7a985b04 100644 (file)
--- a/src/backend/replication/slotfuncs.c
+++ b/src/backend/replication/slotfuncs.c
@@ -277,87 +277,73 @@ pg_get_replication_slots(PG_FUNCTION_ARGS)
    for (slotno = 0; slotno < max_replication_slots; slotno++)
    {
        ReplicationSlot *slot = &ReplicationSlotCtl->replication_slots[slotno];
+       ReplicationSlot slot_contents;
        Datum       values[PG_GET_REPLICATION_SLOTS_COLS];
        bool        nulls[PG_GET_REPLICATION_SLOTS_COLS];
-
-       ReplicationSlotPersistency persistency;
-       TransactionId xmin;
-       TransactionId catalog_xmin;
-       XLogRecPtr  restart_lsn;
-       XLogRecPtr  confirmed_flush_lsn;
-       pid_t       active_pid;
-       Oid         database;
-       NameData    slot_name;
-       NameData    plugin;
        int         i;
 
        if (!slot->in_use)
            continue;
 
+       /* Copy slot contents while holding spinlock, then examine at leisure */
        SpinLockAcquire(&slot->mutex);
-
-       xmin = slot->data.xmin;
-       catalog_xmin = slot->data.catalog_xmin;
-       database = slot->data.database;
-       restart_lsn = slot->data.restart_lsn;
-       confirmed_flush_lsn = slot->data.confirmed_flush;
-       namecpy(&slot_name, &slot->data.name);
-       namecpy(&plugin, &slot->data.plugin);
-       active_pid = slot->active_pid;
-       persistency = slot->data.persistency;
-
+       slot_contents = *slot;
        SpinLockRelease(&slot->mutex);
 
+       memset(values, 0, sizeof(values));
        memset(nulls, 0, sizeof(nulls));
 
        i = 0;
-       values[i++] = NameGetDatum(&slot_name);
+       values[i++] = NameGetDatum(&slot_contents.data.name);
 
-       if (database == InvalidOid)
+       if (slot_contents.data.database == InvalidOid)
            nulls[i++] = true;
        else
-           values[i++] = NameGetDatum(&plugin);
+           values[i++] = NameGetDatum(&slot_contents.data.plugin);
 
-       if (database == InvalidOid)
+       if (slot_contents.data.database == InvalidOid)
            values[i++] = CStringGetTextDatum("physical");
        else
            values[i++] = CStringGetTextDatum("logical");
 
-       if (database == InvalidOid)
+       if (slot_contents.data.database == InvalidOid)
            nulls[i++] = true;
        else
-           values[i++] = database;
+           values[i++] = ObjectIdGetDatum(slot_contents.data.database);
 
-       values[i++] = BoolGetDatum(persistency == RS_TEMPORARY);
-       values[i++] = BoolGetDatum(active_pid != 0);
+       values[i++] = BoolGetDatum(slot_contents.data.persistency == RS_TEMPORARY);
+       values[i++] = BoolGetDatum(slot_contents.active_pid != 0);
 
-       if (active_pid != 0)
-           values[i++] = Int32GetDatum(active_pid);
+       if (slot_contents.active_pid != 0)
+           values[i++] = Int32GetDatum(slot_contents.active_pid);
        else
            nulls[i++] = true;
 
-       if (xmin != InvalidTransactionId)
-           values[i++] = TransactionIdGetDatum(xmin);
+       if (slot_contents.data.xmin != InvalidTransactionId)
+           values[i++] = TransactionIdGetDatum(slot_contents.data.xmin);
        else
            nulls[i++] = true;
 
-       if (catalog_xmin != InvalidTransactionId)
-           values[i++] = TransactionIdGetDatum(catalog_xmin);
+       if (slot_contents.data.catalog_xmin != InvalidTransactionId)
+           values[i++] = TransactionIdGetDatum(slot_contents.data.catalog_xmin);
        else
            nulls[i++] = true;
 
-       if (restart_lsn != InvalidXLogRecPtr)
-           values[i++] = LSNGetDatum(restart_lsn);
+       if (slot_contents.data.restart_lsn != InvalidXLogRecPtr)
+           values[i++] = LSNGetDatum(slot_contents.data.restart_lsn);
        else
            nulls[i++] = true;
 
-       if (confirmed_flush_lsn != InvalidXLogRecPtr)
-           values[i++] = LSNGetDatum(confirmed_flush_lsn);
+       if (slot_contents.data.confirmed_flush != InvalidXLogRecPtr)
+           values[i++] = LSNGetDatum(slot_contents.data.confirmed_flush);
        else
            nulls[i++] = true;
 
+       Assert(i == PG_GET_REPLICATION_SLOTS_COLS);
+
        tuplestore_putvalues(tupstore, tupdesc, values, nulls);
    }
+
    LWLockRelease(ReplicationSlotControlLock);
 
    tuplestore_donestoring(tupstore);
@@ -616,6 +602,8 @@ copy_replication_slot(FunctionCallInfo fcinfo, bool logical_slot)
    Name        src_name = PG_GETARG_NAME(0);
    Name        dst_name = PG_GETARG_NAME(1);
    ReplicationSlot *src = NULL;
+   ReplicationSlot first_slot_contents;
+   ReplicationSlot second_slot_contents;
    XLogRecPtr  src_restart_lsn;
    bool        src_islogical;
    bool        temporary;
@@ -655,13 +643,10 @@ copy_replication_slot(FunctionCallInfo fcinfo, bool logical_slot)
 
        if (s->in_use && strcmp(NameStr(s->data.name), NameStr(*src_name)) == 0)
        {
+           /* Copy the slot contents while holding spinlock */
            SpinLockAcquire(&s->mutex);
-           src_islogical = SlotIsLogical(s);
-           src_restart_lsn = s->data.restart_lsn;
-           temporary = s->data.persistency == RS_TEMPORARY;
-           plugin = logical_slot ? pstrdup(NameStr(s->data.plugin)) : NULL;
+           first_slot_contents = *s;
            SpinLockRelease(&s->mutex);
-
            src = s;
            break;
        }
@@ -674,6 +659,11 @@ copy_replication_slot(FunctionCallInfo fcinfo, bool logical_slot)
                (errcode(ERRCODE_UNDEFINED_OBJECT),
                 errmsg("replication slot \"%s\" does not exist", NameStr(*src_name))));
 
+   src_islogical = SlotIsLogical(&first_slot_contents);
+   src_restart_lsn = first_slot_contents.data.restart_lsn;
+   temporary = (first_slot_contents.data.persistency == RS_TEMPORARY);
+   plugin = logical_slot ? NameStr(first_slot_contents.data.plugin) : NULL;
+
    /* Check type of replication slot */
    if (src_islogical != logical_slot)
        ereport(ERROR,
@@ -738,18 +728,20 @@ copy_replication_slot(FunctionCallInfo fcinfo, bool logical_slot)
 
        /* Copy data of source slot again */
        SpinLockAcquire(&src->mutex);
-       copy_effective_xmin = src->effective_xmin;
-       copy_effective_catalog_xmin = src->effective_catalog_xmin;
+       second_slot_contents = *src;
+       SpinLockRelease(&src->mutex);
 
-       copy_xmin = src->data.xmin;
-       copy_catalog_xmin = src->data.catalog_xmin;
-       copy_restart_lsn = src->data.restart_lsn;
-       copy_confirmed_flush = src->data.confirmed_flush;
+       copy_effective_xmin = second_slot_contents.effective_xmin;
+       copy_effective_catalog_xmin = second_slot_contents.effective_catalog_xmin;
+
+       copy_xmin = second_slot_contents.data.xmin;
+       copy_catalog_xmin = second_slot_contents.data.catalog_xmin;
+       copy_restart_lsn = second_slot_contents.data.restart_lsn;
+       copy_confirmed_flush = second_slot_contents.data.confirmed_flush;
 
        /* for existence check */
-       copy_name = pstrdup(NameStr(src->data.name));
-       copy_islogical = SlotIsLogical(src);
-       SpinLockRelease(&src->mutex);
+       copy_name = NameStr(second_slot_contents.data.name);
+       copy_islogical = SlotIsLogical(&second_slot_contents);
 
        /*
         * Check if the source slot still exists and is valid. We regard it as

diff --git a/src/include/replication/slot.h b/src/include/replication/slot.h
index 8fbddea78fd3270748819833c72ae4d1dceacd0a..b3013abe348dde39f72576bfa13b3809bd291f4b 100644 (file)
--- a/src/include/replication/slot.h
+++ b/src/include/replication/slot.h
@@ -151,8 +151,8 @@ typedef struct ReplicationSlot
    XLogRecPtr  candidate_restart_lsn;
 } ReplicationSlot;
 
-#define SlotIsPhysical(slot) (slot->data.database == InvalidOid)
-#define SlotIsLogical(slot) (slot->data.database != InvalidOid)
+#define SlotIsPhysical(slot) ((slot)->data.database == InvalidOid)
+#define SlotIsLogical(slot) ((slot)->data.database != InvalidOid)
 
 /*
  * Shared memory control area for all of replication slots.