microsoft/FASTER · error · FasterException

Attempting to allocate an already-allocated block

Error message

Attempting to allocate an already-allocated block

What it means

FasterException thrown by the DEBUG-only (CHECK_FREE) guard in BufferPool's SectorAlignedMemory state setter. Each pooled block carries a free/allocated bit; this fires when code calls Allocate() on a block whose kFreeBitMask indicates it is already allocated, i.e. a double-acquire of the same buffer. It indicates broken ownership tracking of pooled sector-aligned memory.

Solutions

1. Audit ownership so each SectorAlignedMemory is allocated by exactly one consumer at a time
2. Ensure every Allocate is paired with exactly one Free and the reference is not reused after freeing
3. Guard shared buffers with synchronization or per-thread pools instead of sharing one pool entry
4. Enable/keep CHECK_FREE in tests to catch double allocation before production

Example fix

// before
var buf = pool.Get(0);
Use(buf);
buf.Allocate(); // throws: already allocated
// after
var buf = pool.Get(0);
Use(buf);
buf.Free();
var buf2 = pool.Get(0); // allocate a fresh block instead

if (buffer.Free) buffer.Allocate(); else throw new InvalidOperationException("Buffer already allocated; fix ownership");

bool IsAllocatable(SectorAlignedMemory m) => m != null && m.Free;

When it happens

Trigger: Calling SectorAlignedMemory.Allocate() (via BufferPool.Get/GetBufferPool) on a block that is already marked allocated — typically handing the same SectorAlignedMemory object to two consumers, or calling Allocate twice without a matching Free in between.

Common situations: Refactoring code that returns buffers to the pool while keeping a reference to the old buffer; sharing a buffer across threads without synchronization; custom checkpoint/replay code re-acquiring a saved buffer reference; only surfaces in CHECK_FREE builds so it appears after enabling debug checks.

Understand the failure class

Background: "This is a bug, please report it": internal invariant violations, unreachable panics, and SNH errors explained — this error's family across 47 libraries.

Related errors

• Attempting to return an already-free block
• Cannot create new SpanByteAndMemory using serialized…
• elementIndex
• Out of order message within session
• Unexpected status of SubscribeKV

AI-assisted analysis of microsoft/FASTER@321d872eab (2026-09-15). Data as JSON: /api/errors/1bcfc176135e3fe3. Report an issue: GitHub.

Appendix: source

Thrown at cs/src/core/Utilities/BufferPool.cs:85

        internal SectorAlignedBufferPool pool;

#if CHECK_FREE
        internal bool Free
        {
            get => (level & kFreeBitMask) != 0;
            set 
            {
                if (value)
                {
                    if (Free)
                        throw new FasterException("Attempting to return an already-free block");
                    this.level |= kFreeBitMask;
                }
                else
                {
                    if (!Free)
                        throw new FasterException("Attempting to allocate an already-allocated block");
                    this.level &= ~kFreeBitMask;
                }
            }
        }
#endif // CHECK_FREE

        /// <summary>
        /// Default constructor
        /// </summary>
        public SectorAlignedMemory(int level = default)
        {
            this.level = level;
            // Assume ctor is called for allocation and leave Free unset
        }

        /// <summary>
        /// Create new instance of SectorAlignedMemory
        /// </summary>

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