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diff --git a/src/mongo/util/shared_buffer_test.cpp b/src/mongo/util/shared_buffer_test.cpp
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+/**
+ *    Copyright (C) 2019-present MongoDB, Inc.
+ *
+ *    This program is free software: you can redistribute it and/or modify
+ *    it under the terms of the Server Side Public License, version 1,
+ *    as published by MongoDB, Inc.
+ *
+ *    This program is distributed in the hope that it will be useful,
+ *    but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *    Server Side Public License for more details.
+ *
+ *    You should have received a copy of the Server Side Public License
+ *    along with this program. If not, see
+ *    <http://www.mongodb.com/licensing/server-side-public-license>.
+ *
+ *    As a special exception, the copyright holders give permission to link the
+ *    code of portions of this program with the OpenSSL library under certain
+ *    conditions as described in each individual source file and distribute
+ *    linked combinations including the program with the OpenSSL library. You
+ *    must comply with the Server Side Public License in all respects for
+ *    all of the code used other than as permitted herein. If you modify file(s)
+ *    with this exception, you may extend this exception to your version of the
+ *    file(s), but you are not obligated to do so. If you do not wish to do so,
+ *    delete this exception statement from your version. If you delete this
+ *    exception statement from all source files in the program, then also delete
+ *    it in the license file.
+ */
+
+#include "mongo/platform/basic.h"
+
+#include "mongo/base/string_data.h"
+#include "mongo/util/shared_buffer.h"
+#include "mongo/util/shared_buffer_fragment.h"
+
+#include "mongo/unittest/unittest.h"
+
+namespace mongo {
+namespace {
+
+using SharedBufferTest = unittest::Test;
+
+TEST_F(SharedBufferTest, ReallocOrCopyNull) {
+    SharedBuffer buf;
+    ASSERT_EQ(buf.capacity(), 0u);
+    ASSERT(!buf);
+    ASSERT(!buf.isShared());
+    buf.reallocOrCopy(10);
+    ASSERT(buf);
+    ASSERT(!buf.isShared());
+    ASSERT_EQ(buf.capacity(), 10u);
+}
+
+TEST_F(SharedBufferTest, ReallocOrCopyNullShared) {
+    // null SharedBuffers are never considered "shared", even when copied.
+    SharedBuffer buf;
+    const SharedBuffer sharer = buf;
+    ASSERT_EQ(buf.capacity(), 0u);
+    ASSERT(!buf);
+    ASSERT(!buf.isShared());
+    buf.reallocOrCopy(10);
+    ASSERT(buf);
+    ASSERT(!buf.isShared());
+    ASSERT_EQ(buf.capacity(), 10u);
+    ASSERT_EQ(sharer.capacity(), 0u);
+}
+
+SharedBuffer makeBuffer() {
+    SharedBuffer buf = SharedBuffer::allocate(4);
+    memcpy(buf.get(), "foo", 4);
+    return buf;
+}
+
+TEST_F(SharedBufferTest, ReallocOrCopyGrow) {
+    SharedBuffer buf = makeBuffer();
+    ASSERT_EQ(buf.capacity(), 4u);
+    ASSERT(buf);
+    ASSERT(!buf.isShared());
+    buf.reallocOrCopy(10);
+    ASSERT(buf);
+    ASSERT(!buf.isShared());
+    ASSERT_EQ(buf.capacity(), 10u);
+    ASSERT_EQ("foo"_sd, buf.get());
+}
+
+TEST_F(SharedBufferTest, ReallocOrCopyGrowShared) {
+    SharedBuffer buf = makeBuffer();
+    const SharedBuffer sharer = buf;
+    ASSERT_EQ(buf.capacity(), 4u);
+    ASSERT(buf);
+    ASSERT(buf.isShared());
+    buf.reallocOrCopy(10);
+    ASSERT(buf);
+    ASSERT(!buf.isShared());
+    ASSERT_EQ(buf.capacity(), 10u);
+    ASSERT_EQ(sharer.capacity(), 4u);
+    ASSERT_EQ("foo"_sd, buf.get());
+    ASSERT_EQ("foo"_sd, sharer.get());
+    ASSERT_NE(buf.get(), sharer.get());
+}
+
+TEST_F(SharedBufferTest, ReallocOrCopyShrink) {
+    SharedBuffer buf = makeBuffer();
+    ASSERT_EQ(buf.capacity(), 4u);
+    ASSERT(buf);
+    ASSERT(!buf.isShared());
+    // The buffer is already at least 1 byte.
+    buf.reallocOrCopy(1);
+    ASSERT(buf);
+    // We copy it anyway.
+    ASSERT(!buf.isShared());
+    ASSERT_EQ(buf.capacity(), 1u);
+    ASSERT_EQ('f', buf.get()[0]);
+}
+
+TEST_F(SharedBufferTest, ReallocOrCopyShrinkShared) {
+    SharedBuffer buf = makeBuffer();
+    const SharedBuffer sharer = buf;
+    ASSERT_EQ(buf.capacity(), 4u);
+    ASSERT(buf);
+    ASSERT(buf.isShared());
+    // The buffer is already at least 1 byte.
+    buf.reallocOrCopy(1);
+    ASSERT(buf);
+    // We copy it anyway.
+    ASSERT(!buf.isShared());
+    ASSERT_EQ(buf.capacity(), 1u);
+    ASSERT_EQ(sharer.capacity(), 4u);
+    ASSERT_EQ('f', buf.get()[0]);
+    ASSERT_EQ("foo"_sd, sharer.get());
+    ASSERT_NE(buf.get(), sharer.get());
+}
+
+TEST_F(SharedBufferTest, SharedBufferFragmentBuilder) {
+    constexpr size_t kBlockSize = 16;
+    SharedBufferFragmentBuilder builder(kBlockSize);
+
+    auto verifyFragment = [](const SharedBufferFragment& fragment, uint8_t expected) {
+        for (size_t i = 0; i < fragment.size(); ++i)
+            ASSERT(memcmp(fragment.get() + i, &expected, 1) == 0);
+    };
+
+    builder.start(kBlockSize / 2);
+    ASSERT_EQ(builder.capacity(), kBlockSize);
+    uint8_t one = 1;
+    memset(builder.get(), one, kBlockSize / 2);
+    auto fragment1 = builder.finish(kBlockSize / 2);
+    ASSERT_EQ(fragment1.size(), kBlockSize / 2);
+    verifyFragment(fragment1, one);
+
+    builder.start(kBlockSize / 2);
+    ASSERT_EQ(builder.capacity(), kBlockSize / 2);
+    // We can use less than we ask for
+    uint8_t two = 2;
+    memset(builder.get(), two, kBlockSize / 4);
+    auto fragment2 = builder.finish(kBlockSize / 4);
+    ASSERT_EQ(fragment2.size(), kBlockSize / 4);
+    // Buffers should not overlap and be next to each other
+    ASSERT_EQ(fragment1.get() + fragment1.size(), fragment2.get());
+    verifyFragment(fragment2, two);
+
+    // Verify that anything written is transfered when we grow
+    builder.start(builder.capacity());
+    ASSERT_EQ(builder.capacity(), kBlockSize / 4);
+    uint8_t three = 3;
+    size_t written = kBlockSize / 4;
+    // Write current capacity
+    memset(builder.get(), three, written);
+    builder.grow(kBlockSize);
+    // Write the rest
+    memset(builder.get() + written, three, builder.capacity() - written);
+    auto fragment3 = builder.finish(kBlockSize);
+    for (size_t i = 0; i < (kBlockSize / 4); ++i)
+        ASSERT(memcmp(fragment3.get() + i, &three, 1) == 0);
+    ASSERT_EQ(builder.capacity(), kBlockSize);
+    verifyFragment(fragment3, three);
+
+    builder.start(builder.capacity());
+    auto ptr = builder.get();
+    builder.discard();
+    builder.start(builder.capacity());
+    ASSERT_EQ(builder.get(), ptr);
+
+    // No buffers should have been overwritten by others
+    verifyFragment(fragment1, one);
+    verifyFragment(fragment2, two);
+    verifyFragment(fragment3, three);
+}
+
+TEST_F(SharedBufferTest, ManualFreeSharedBufferFragmentMemUsage1) {
+    constexpr size_t kBlockSize = 16;
+    SharedBufferFragmentBuilder builder(kBlockSize,
+                                        SharedBufferFragmentBuilder::DoubleGrowStrategy(
+                                            SharedBufferFragmentBuilder::kDefaultMaxBlockSize));
+
+    {
+        builder.start(kBlockSize / 2);
+        ASSERT_EQ(kBlockSize, builder.capacity());
+        auto fragment1 = builder.finish(kBlockSize / 2);
+        ASSERT_EQ(kBlockSize / 2, fragment1.size());
+        ASSERT_EQ(kBlockSize, builder.memUsage());
+
+        builder.start(kBlockSize / 2);
+        ASSERT_EQ(kBlockSize / 2, builder.capacity());
+        auto fragment2 = builder.finish(kBlockSize / 2);
+        ASSERT_EQ(kBlockSize / 2, fragment2.size());
+        ASSERT_EQ(kBlockSize, builder.memUsage());
+    }
+
+    ASSERT_EQ(kBlockSize, builder.memUsage());
+    builder.freeUnused();
+    // At a mimimum we will have a buffer of at least one block size in use.
+    ASSERT_EQ(kBlockSize, builder.memUsage());
+}
+
+TEST_F(SharedBufferTest, ManualFreeSharedBufferFragmentMemUsage2) {
+    constexpr size_t kBlockSize = 16;
+    SharedBufferFragmentBuilder builder(kBlockSize,
+                                        SharedBufferFragmentBuilder::DoubleGrowStrategy(
+                                            SharedBufferFragmentBuilder::kDefaultMaxBlockSize));
+
+    // This test allocates fragments on a block boundaries.
+    {
+        builder.start(kBlockSize);
+        ASSERT_EQ(kBlockSize, builder.capacity());
+        auto fragment1 = builder.finish(kBlockSize);
+        ASSERT_EQ(kBlockSize, fragment1.size());
+        ASSERT_EQ(kBlockSize, builder.memUsage());
+
+        builder.start(kBlockSize);
+        // Second buffer is allocated with twice the capacity.
+        ASSERT_EQ(2 * kBlockSize, builder.capacity());
+        auto fragment2 = builder.finish(kBlockSize);
+        ASSERT_EQ(kBlockSize, fragment2.size());
+        ASSERT_EQ(3 * kBlockSize, builder.memUsage());
+    }
+
+    ASSERT_EQ(3 * kBlockSize, builder.memUsage());
+    builder.freeUnused();
+    // Remaining buffer is the last allocated block of 2x block size;
+    ASSERT_EQ(2 * kBlockSize, builder.memUsage());
+}
+
+TEST_F(SharedBufferTest, ManualFreeSharedBufferFragmentMemUsage3) {
+    constexpr size_t kBlockSize = 16;
+    SharedBufferFragmentBuilder builder(kBlockSize,
+                                        SharedBufferFragmentBuilder::DoubleGrowStrategy(
+                                            SharedBufferFragmentBuilder::kDefaultMaxBlockSize));
+
+    size_t expectedMem = 0;
+    ASSERT_EQ(expectedMem, builder.memUsage());
+
+    // This test allocates a fragment that isn't on a block boundary.
+    {
+        builder.start(1);
+        ASSERT_EQ(kBlockSize, builder.capacity());
+        auto fragment1 = builder.finish(1);
+        ASSERT_EQ(1, fragment1.size());
+        ASSERT_EQ(kBlockSize, builder.memUsage());
+
+        builder.start(kBlockSize);
+        // We will realloc with double the size.
+        ASSERT_EQ(2 * kBlockSize, builder.capacity());
+        auto fragment2 = builder.finish(kBlockSize);
+        ASSERT_EQ(kBlockSize, fragment2.size());
+        // We have one buffer of size kBlockSize and another of 2x.
+        ASSERT_EQ(3 * kBlockSize, builder.memUsage());
+    }
+
+    ASSERT_EQ(3 * kBlockSize, builder.memUsage());
+    builder.freeUnused();
+    // The last buffer we allocated was 2x the block size, so this is what remains.
+    ASSERT_EQ(2 * kBlockSize, builder.memUsage());
+}
+
+TEST_F(SharedBufferTest, ManualFreeSharedBufferFragmentMemUsageGrow) {
+    constexpr size_t kBlockSize = 16;
+    SharedBufferFragmentBuilder builder(kBlockSize,
+                                        SharedBufferFragmentBuilder::DoubleGrowStrategy(
+                                            SharedBufferFragmentBuilder::kDefaultMaxBlockSize));
+    {
+        builder.start(1);
+        ASSERT_EQ(kBlockSize, builder.capacity());
+        auto fragment1 = builder.finish(1);
+        ASSERT_EQ(1, fragment1.size());
+        ASSERT_EQ(kBlockSize, builder.memUsage());
+
+        builder.start(1);
+        ASSERT_EQ(kBlockSize - 1, builder.capacity());
+        // We will realloc a buffer of 2x the block size.
+        builder.grow(kBlockSize);
+        auto fragment2 = builder.finish(kBlockSize);
+        ASSERT_EQ(kBlockSize, fragment2.size());
+        ASSERT_EQ(3 * kBlockSize, builder.memUsage());
+    }
+
+    ASSERT_EQ(3 * kBlockSize, builder.memUsage());
+    builder.freeUnused();
+    // The last buffer we allocated was 2x the block size, so this is what remains.
+    ASSERT_EQ(2 * kBlockSize, builder.memUsage());
+}
+
+TEST_F(SharedBufferTest, ManualFreeSharedBufferFragmentMemReUse) {
+    constexpr size_t kBlockSize = 16;
+    SharedBufferFragmentBuilder builder(kBlockSize,
+                                        SharedBufferFragmentBuilder::DoubleGrowStrategy(
+                                            SharedBufferFragmentBuilder::kDefaultMaxBlockSize));
+
+    {
+        builder.start(kBlockSize);
+        ASSERT_EQ(kBlockSize, builder.capacity());
+        auto fragment1 = builder.finish(kBlockSize);
+        ASSERT_EQ(kBlockSize, fragment1.size());
+        ASSERT_EQ(kBlockSize, builder.memUsage());
+    }
+
+    {
+        // Expect that there is no increase in memory usage when we allocate a new buffer of the
+        // same size.
+        builder.start(kBlockSize);
+        ASSERT_EQ(kBlockSize, builder.capacity());
+        auto fragment2 = builder.finish(kBlockSize);
+        ASSERT_EQ(kBlockSize, fragment2.size());
+        ASSERT_EQ(kBlockSize, builder.memUsage());
+    }
+
+    {
+        // Expect that the memory usage only doubles and no more when we allocate a new buffer
+        // greater than the current capacity.
+        builder.start(2 * kBlockSize);
+        ASSERT_EQ(2 * kBlockSize, builder.capacity());
+        auto fragment = builder.finish(2 * kBlockSize);
+        ASSERT_EQ(2 * kBlockSize, builder.memUsage());
+    }
+}
+
+TEST_F(SharedBufferTest, ManualFreeSharedBufferFragmentLotsOfGrows) {
+    constexpr size_t kBlockSize = 16;
+    SharedBufferFragmentBuilder builder(kBlockSize,
+                                        SharedBufferFragmentBuilder::DoubleGrowStrategy(
+                                            SharedBufferFragmentBuilder::kDefaultMaxBlockSize));
+
+    // Ensure that when we grow and we are the exclusive user of the builder, we don't need to
+    // allocate new buffers.
+    {
+        builder.start(kBlockSize);
+        ASSERT_EQ(kBlockSize, builder.capacity());
+        ASSERT_EQ(kBlockSize, builder.memUsage());
+
+        builder.grow(2 * kBlockSize);
+        ASSERT_EQ(2 * kBlockSize, builder.capacity());
+        ASSERT_EQ(2 * kBlockSize, builder.memUsage());
+
+        builder.grow(3 * kBlockSize);
+        // We double the buffer size internally every time we realloc.
+        ASSERT_EQ(4 * kBlockSize, builder.capacity());
+        ASSERT_EQ(4 * kBlockSize, builder.memUsage());
+
+        builder.grow(4 * kBlockSize);
+        ASSERT_EQ(4 * kBlockSize, builder.capacity());
+        ASSERT_EQ(4 * kBlockSize, builder.memUsage());
+
+        builder.grow(5 * kBlockSize);
+        ASSERT_EQ(8 * kBlockSize, builder.capacity());
+        ASSERT_EQ(8 * kBlockSize, builder.memUsage());
+
+        auto fragment1 = builder.finish(5 * kBlockSize);
+        ASSERT_EQ(fragment1.size(), 5 * kBlockSize);
+
+        // If we start a new fragment, we expect that it's capacity is what remains in the buffer.
+        builder.start(kBlockSize);
+        ASSERT_EQ(3 * kBlockSize, builder.capacity());
+        ASSERT_EQ(8 * kBlockSize, builder.memUsage());
+
+        auto fragment2 = builder.finish(kBlockSize);
+        ASSERT_EQ(kBlockSize, fragment2.size());
+    }
+
+    // This has no effect on memory usage since the last allocated buffer is 8x the block size. We
+    // can't reclaim the unused space right now, but the next fragment we build should be able to.
+    builder.freeUnused();
+    ASSERT_EQ(2 * kBlockSize, builder.capacity());
+    ASSERT_EQ(8 * kBlockSize, builder.memUsage());
+
+    {
+        builder.start(kBlockSize);
+        // Since there are no active fragments, we expect to have the full capacity of the
+        // underlying buffer.
+        ASSERT_EQ(8 * kBlockSize, builder.capacity());
+        ASSERT_EQ(8 * kBlockSize, builder.memUsage());
+
+        builder.grow(8 * kBlockSize);
+        ASSERT_EQ(8 * kBlockSize, builder.capacity());
+        ASSERT_EQ(8 * kBlockSize, builder.memUsage());
+
+        auto fragment1 = builder.finish(8 * kBlockSize);
+        ASSERT_EQ(fragment1.size(), 8 * kBlockSize);
+    }
+
+    builder.freeUnused();
+    ASSERT_EQ(8 * kBlockSize, builder.memUsage());
+}
+
+}  // namespace
+}  // namespace mongo