Pool the dictionary buffer when training a Zstandard dictionary

ZstandardDictionary.Train allocated 'new byte[maxDictionarySize]' on every call. Dictionary sizes are typically tens to hundreds of KB (zstd recommends up to ~100 KB, but the API allows more), so each training call paid for a fresh GC allocation that often landed on the LOH.

Rent the buffer from ArrayPool<byte>.Shared instead. Create copies the trained slice into an exact-sized array before returning, so the rented buffer can be returned immediately. Use clearArray: true on Return because the trained dictionary is derived from caller-supplied samples and must not linger in the shared pool.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: alinpahontu2912

src/libraries/System.IO.Compression/src/System/IO/Compression/Zstandard/ZstandardDictionary.cs

@@ -127,23 +127,30 @@ public static ZstandardDictionary Train(ReadOnlySpan<byte> samples, ReadOnlySpan
 
                 ArgumentOutOfRangeException.ThrowIfLessThan(maxDictionarySize, 256, nameof(maxDictionarySize));
 
-                byte[] dictionaryBuffer = new byte[maxDictionarySize];
-
-                nuint dictSize;
-
-                unsafe
+                byte[] dictionaryBuffer = ArrayPool<byte>.Shared.Rent(maxDictionarySize);
+                try
                 {
-                    fixed (byte* samplesPtr = &MemoryMarshal.GetReference(samples))
-                    fixed (byte* dictPtr = dictionaryBuffer)
-                    fixed (nuint* lengthsAsNuintPtr = &MemoryMarshal.GetReference(lengthsAsNuint))
+                    nuint dictSize;
+
+                    unsafe
                     {
-                        dictSize = Interop.Zstd.ZDICT_trainFromBuffer(
-                                dictPtr, (nuint)maxDictionarySize,
-                                samplesPtr, lengthsAsNuintPtr, (uint)sampleLengths.Length);
+                        fixed (byte* samplesPtr = &MemoryMarshal.GetReference(samples))
+                        fixed (byte* dictPtr = dictionaryBuffer)
+                        fixed (nuint* lengthsAsNuintPtr = &MemoryMarshal.GetReference(lengthsAsNuint))
+                        {
+                            dictSize = Interop.Zstd.ZDICT_trainFromBuffer(
+                                    dictPtr, (nuint)maxDictionarySize,
+                                    samplesPtr, lengthsAsNuintPtr, (uint)sampleLengths.Length);
+                        }
+
+                        ZstandardUtils.ThrowIfError(dictSize);
+                        return Create(dictionaryBuffer.AsSpan(0, (int)dictSize));
                     }
-
-                    ZstandardUtils.ThrowIfError(dictSize);
-                    return Create(dictionaryBuffer.AsSpan(0, (int)dictSize));
+                }
+                finally
+                {
+                    // Clear before returning: the trained dictionary is derived from caller-supplied samples.
+                    ArrayPool<byte>.Shared.Return(dictionaryBuffer, clearArray: true);
                 }
             }
             finally