Fix race condition in HttpHeaders parsing

src/libraries/System.Net.Http/src/System/Net/Http/Headers/HttpHeaders.cs

@@ -347,17 +347,14 @@ private IEnumerator<KeyValuePair<string, IEnumerable<string>>> GetEnumeratorCore
  // during enumeration so that we can parse the raw value in order to a) return
  // the correct set of parsed values, and b) update the instance for subsequent enumerations
  // to reflect that parsing.
- info = new HeaderStoreItemInfo() { RawValue = entry.Value };
 
- if (EntriesAreLiveView)
- {
- entries[i].Value = info;
- }
- else
- {
- Debug.Assert(Contains(entry.Key));
- ((Dictionary<HeaderDescriptor, object>)_headerStore!)[entry.Key] = info;
- }
+#nullable disable // https://github.com/dotnet/roslyn/issues/73928
+ ref object storeValueRef = ref EntriesAreLiveView
+ ? ref entries[i].Value
+ : ref CollectionsMarshal.GetValueRefOrNullRef((Dictionary<HeaderDescriptor, object>)_headerStore, entry.Key);
+
+ info = ReplaceWithHeaderStoreItemInfo(ref storeValueRef, entry.Value);
+#nullable restore
  }
 
  // Make sure we parse all raw values before returning the result. Note that this has to be
@@ -729,15 +726,10 @@ private bool TryGetAndParseHeaderInfo(HeaderDescriptor key, [NotNullWhen(true)]
  if (!Unsafe.IsNullRef(ref storeValueRef))
  {
  object value = storeValueRef;
- if (value is HeaderStoreItemInfo hsi)
- {
- info = hsi;
- }
- else
- {
- Debug.Assert(value is string);
- storeValueRef = info = new HeaderStoreItemInfo() { RawValue = value };
- }
+
+ info = value is HeaderStoreItemInfo hsi
+ ? hsi
+ : ReplaceWithHeaderStoreItemInfo(ref storeValueRef, value);
 
  ParseRawHeaderValues(key, info);
  return true;
@@ -747,6 +739,31 @@ private bool TryGetAndParseHeaderInfo(HeaderDescriptor key, [NotNullWhen(true)]
  return false;
  }
 
+ /// <summary>
+ /// Replaces <paramref name="storeValueRef"/> with a new <see cref="HeaderStoreItemInfo"/>,
+ /// or returns the existing <see cref="HeaderStoreItemInfo"/> if a different thread beat us to it.
+ /// </summary>
+ /// <remarks>
+ /// This helper should be used any time we're upgrading a storage slot from an unparsed string to a HeaderStoreItemInfo *while reading*.
+ /// Concurrent writes to the header collection are UB, so we don't need to worry about race conditions when doing the replacement there.
+ /// </remarks>
+ [MethodImpl(MethodImplOptions.AggressiveInlining)]
+ private static HeaderStoreItemInfo ReplaceWithHeaderStoreItemInfo(ref object storeValueRef, object value)
+ {
+ Debug.Assert(value is string);
+
+ var info = new HeaderStoreItemInfo() { RawValue = value };
+ object previousValue = Interlocked.CompareExchange(ref storeValueRef, info, value);
+
+ if (ReferenceEquals(previousValue, value))
+ {
+ return info;
+ }
+
+ // Rare race condition: Another thread replaced the value with a HeaderStoreItemInfo.
+ return (HeaderStoreItemInfo)previousValue;
+ }
+
  private static void ParseRawHeaderValues(HeaderDescriptor descriptor, HeaderStoreItemInfo info)
  {
  // Unlike TryGetHeaderInfo() this method tries to parse all non-validated header values (if any)

src/libraries/System.Net.Http/tests/UnitTests/Headers/HttpHeadersTest.cs

@@ -3,6 +3,7 @@
 
 using System.Collections;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.Globalization;
 using System.Linq;
 using System.Net.Http.Headers;
@@ -2502,6 +2503,51 @@ static HttpRequestHeaders CreateHeaders()
  }
  }
 
+ [Theory]
+ [InlineData(true, true)]
+ [InlineData(true, false)]
+ [InlineData(false, true)]
+ [InlineData(false, false)]
+ public async Task ConcurrentReads_ReturnTheSameParsedValues(bool useDictionary, bool useTypedProperty)
+ {
+ HttpContentHeaders dummyValues = new ByteArrayContent([]).Headers;
+ if (useDictionary)
+ {
+ for (int i = 0; i < HttpHeaders.ArrayThreshold; i++)
+ {
+ Assert.True(dummyValues.TryAddWithoutValidation($"foo-{i}", "Foo"));
+ }
+ }
+
+ Stopwatch s = Stopwatch.StartNew();
+
+ while (s.ElapsedMilliseconds < 100)
+ {
+ HttpContentHeaders headers = new ByteArrayContent([]).Headers;
+
+ headers.AddHeaders(dummyValues);
+
+ Assert.True(headers.TryAddWithoutValidation("Content-Type", "application/json; charset=utf-8"));
+
+ if (useTypedProperty)
+ {
+ Task<MediaTypeHeaderValue> task = Task.Run(() => headers.ContentType);
+ MediaTypeHeaderValue contentType1 = headers.ContentType;
+ MediaTypeHeaderValue contentType2 = await task;
+
+ Assert.Same(contentType1, contentType2);
+ }
+ else
+ {
+ Task task = Task.Run(() => headers.Count()); // Force enumeration
+ MediaTypeHeaderValue contentType1 = headers.ContentType;
+ await task;
+
+ Assert.Same(contentType1, headers.ContentType);
+ }
+ }
+ }
+
  [Fact]
  public void TryAddInvalidHeader_ShouldThrowFormatException()
  {

src/libraries/System.Net.Http/src/System/Net/Http/Headers/HttpHeaders.cs

@@ -347,17 +347,14 @@ private IEnumerator<KeyValuePair<string, IEnumerable<string>>> GetEnumeratorCore
  // during enumeration so that we can parse the raw value in order to a) return
  // the correct set of parsed values, and b) update the instance for subsequent enumerations
  // to reflect that parsing.
- info = new HeaderStoreItemInfo() { RawValue = entry.Value };
 
- if (EntriesAreLiveView)
- {
- entries[i].Value = info;
- }
- else
- {
- Debug.Assert(Contains(entry.Key));
- ((Dictionary<HeaderDescriptor, object>)_headerStore!)[entry.Key] = info;
- }
+#nullable disable // https://github.com/dotnet/roslyn/issues/73928
+ ref object storeValueRef = ref EntriesAreLiveView
+ ? ref entries[i].Value
+ : ref CollectionsMarshal.GetValueRefOrNullRef((Dictionary<HeaderDescriptor, object>)_headerStore, entry.Key);
+
+ info = ReplaceWithHeaderStoreItemInfo(ref storeValueRef, entry.Value);
+#nullable restore
  }
 
  // Make sure we parse all raw values before returning the result. Note that this has to be
@@ -729,15 +726,10 @@ private bool TryGetAndParseHeaderInfo(HeaderDescriptor key, [NotNullWhen(true)]
  if (!Unsafe.IsNullRef(ref storeValueRef))
  {
  object value = storeValueRef;
- if (value is HeaderStoreItemInfo hsi)
- {
- info = hsi;
- }
- else
- {
- Debug.Assert(value is string);
- storeValueRef = info = new HeaderStoreItemInfo() { RawValue = value };
- }
+
+ info = value is HeaderStoreItemInfo hsi
+ ? hsi
+ : ReplaceWithHeaderStoreItemInfo(ref storeValueRef, value);
 
  ParseRawHeaderValues(key, info);
  return true;
@@ -747,6 +739,31 @@ private bool TryGetAndParseHeaderInfo(HeaderDescriptor key, [NotNullWhen(true)]
  return false;
  }
 
+ /// <summary>
+ /// Replaces <paramref name="storeValueRef"/> with a new <see cref="HeaderStoreItemInfo"/>,
+ /// or returns the existing <see cref="HeaderStoreItemInfo"/> if a different thread beat us to it.
+ /// </summary>
+ /// <remarks>
+ /// This helper should be used any time we're upgrading a storage slot from an unparsed string to a HeaderStoreItemInfo *while reading*.
+ /// Concurrent writes to the header collection are UB, so we don't need to worry about race conditions when doing the replacement there.
+ /// </remarks>
+ [MethodImpl(MethodImplOptions.AggressiveInlining)]
+ private static HeaderStoreItemInfo ReplaceWithHeaderStoreItemInfo(ref object storeValueRef, object value)
+ {
+ Debug.Assert(value is string);
+
+ var info = new HeaderStoreItemInfo() { RawValue = value };
+ object previousValue = Interlocked.CompareExchange(ref storeValueRef, info, value);
+
+ if (ReferenceEquals(previousValue, value))
+ {
+ return info;
+ }
+
+ // Rare race condition: Another thread replaced the value with a HeaderStoreItemInfo.
+ return (HeaderStoreItemInfo)previousValue;
+ }
+
  private static void ParseRawHeaderValues(HeaderDescriptor descriptor, HeaderStoreItemInfo info)
  {
  // Unlike TryGetHeaderInfo() this method tries to parse all non-validated header values (if any)

src/libraries/System.Net.Http/tests/UnitTests/Headers/HttpHeadersTest.cs

@@ -3,6 +3,7 @@
 
 using System.Collections;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.Globalization;
 using System.Linq;
 using System.Net.Http.Headers;
@@ -2502,6 +2503,51 @@ static HttpRequestHeaders CreateHeaders()
  }
  }
 
+ [Theory]
+ [InlineData(true, true)]
+ [InlineData(true, false)]
+ [InlineData(false, true)]
+ [InlineData(false, false)]
+ public async Task ConcurrentReads_ReturnTheSameParsedValues(bool useDictionary, bool useTypedProperty)
+ {
+ HttpContentHeaders dummyValues = new ByteArrayContent([]).Headers;
+ if (useDictionary)
+ {
+ for (int i = 0; i < HttpHeaders.ArrayThreshold; i++)
+ {
+ Assert.True(dummyValues.TryAddWithoutValidation($"foo-{i}", "Foo"));
+ }
+ }
+
+ Stopwatch s = Stopwatch.StartNew();
+
+ while (s.ElapsedMilliseconds < 100)
+ {
+ HttpContentHeaders headers = new ByteArrayContent([]).Headers;
+
+ headers.AddHeaders(dummyValues);
+
+ Assert.True(headers.TryAddWithoutValidation("Content-Type", "application/json; charset=utf-8"));
+
+ if (useTypedProperty)
+ {
+ Task<MediaTypeHeaderValue> task = Task.Run(() => headers.ContentType);
+ MediaTypeHeaderValue contentType1 = headers.ContentType;
+ MediaTypeHeaderValue contentType2 = await task;
+
+ Assert.Same(contentType1, contentType2);
+ }
+ else
+ {
+ Task task = Task.Run(() => headers.Count()); // Force enumeration
+ MediaTypeHeaderValue contentType1 = headers.ContentType;
+ await task;
+
+ Assert.Same(contentType1, headers.ContentType);
+ }
+ }
+ }
+
  [Fact]
  public void TryAddInvalidHeader_ShouldThrowFormatException()
  {

src/libraries/System.Net.Http/src/System/Net/Http/Headers/HttpHeaders.cs

@@ -347,17 +347,14 @@ private IEnumerator<KeyValuePair<string, IEnumerable<string>>> GetEnumeratorCore
  // during enumeration so that we can parse the raw value in order to a) return
  // the correct set of parsed values, and b) update the instance for subsequent enumerations
  // to reflect that parsing.
- info = new HeaderStoreItemInfo() { RawValue = entry.Value };
 
- if (EntriesAreLiveView)
- {
- entries[i].Value = info;
- }
- else
- {
- Debug.Assert(Contains(entry.Key));
- ((Dictionary<HeaderDescriptor, object>)_headerStore!)[entry.Key] = info;
- }
+#nullable disable // https://github.com/dotnet/roslyn/issues/73928
+ ref object storeValueRef = ref EntriesAreLiveView
+ ? ref entries[i].Value
+ : ref CollectionsMarshal.GetValueRefOrNullRef((Dictionary<HeaderDescriptor, object>)_headerStore, entry.Key);
+
+ info = ReplaceWithHeaderStoreItemInfo(ref storeValueRef, entry.Value);
+#nullable restore
  }
 
  // Make sure we parse all raw values before returning the result. Note that this has to be
@@ -729,15 +726,10 @@ private bool TryGetAndParseHeaderInfo(HeaderDescriptor key, [NotNullWhen(true)]
  if (!Unsafe.IsNullRef(ref storeValueRef))
  {
  object value = storeValueRef;
- if (value is HeaderStoreItemInfo hsi)
- {
- info = hsi;
- }
- else
- {
- Debug.Assert(value is string);
- storeValueRef = info = new HeaderStoreItemInfo() { RawValue = value };
- }
+
+ info = value is HeaderStoreItemInfo hsi
+ ? hsi
+ : ReplaceWithHeaderStoreItemInfo(ref storeValueRef, value);
 
  ParseRawHeaderValues(key, info);
  return true;
@@ -747,6 +739,31 @@ private bool TryGetAndParseHeaderInfo(HeaderDescriptor key, [NotNullWhen(true)]
  return false;
  }
 
+ /// <summary>
+ /// Replaces <paramref name="storeValueRef"/> with a new <see cref="HeaderStoreItemInfo"/>,
+ /// or returns the existing <see cref="HeaderStoreItemInfo"/> if a different thread beat us to it.
+ /// </summary>
+ /// <remarks>
+ /// This helper should be used any time we're upgrading a storage slot from an unparsed string to a HeaderStoreItemInfo *while reading*.
+ /// Concurrent writes to the header collection are UB, so we don't need to worry about race conditions when doing the replacement there.
+ /// </remarks>
+ [MethodImpl(MethodImplOptions.AggressiveInlining)]
+ private static HeaderStoreItemInfo ReplaceWithHeaderStoreItemInfo(ref object storeValueRef, object value)
+ {
+ Debug.Assert(value is string);
+
+ var info = new HeaderStoreItemInfo() { RawValue = value };
+ object previousValue = Interlocked.CompareExchange(ref storeValueRef, info, value);
+
+ if (ReferenceEquals(previousValue, value))
+ {
+ return info;
+ }
+
+ // Rare race condition: Another thread replaced the value with a HeaderStoreItemInfo.
+ return (HeaderStoreItemInfo)previousValue;
+ }
+
  private static void ParseRawHeaderValues(HeaderDescriptor descriptor, HeaderStoreItemInfo info)
  {
  // Unlike TryGetHeaderInfo() this method tries to parse all non-validated header values (if any)

src/libraries/System.Net.Http/tests/UnitTests/Headers/HttpHeadersTest.cs

@@ -3,6 +3,7 @@
 
 using System.Collections;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.Globalization;
 using System.Linq;
 using System.Net.Http.Headers;
@@ -2502,6 +2503,51 @@ static HttpRequestHeaders CreateHeaders()
  }
  }
 
+ [Theory]
+ [InlineData(true, true)]
+ [InlineData(true, false)]
+ [InlineData(false, true)]
+ [InlineData(false, false)]
+ public async Task ConcurrentReads_ReturnTheSameParsedValues(bool useDictionary, bool useTypedProperty)
+ {
+ HttpContentHeaders dummyValues = new ByteArrayContent([]).Headers;
+ if (useDictionary)
+ {
+ for (int i = 0; i < HttpHeaders.ArrayThreshold; i++)
+ {
+ Assert.True(dummyValues.TryAddWithoutValidation($"foo-{i}", "Foo"));
+ }
+ }
+
+ Stopwatch s = Stopwatch.StartNew();
+
+ while (s.ElapsedMilliseconds < 100)
+ {
+ HttpContentHeaders headers = new ByteArrayContent([]).Headers;
+
+ headers.AddHeaders(dummyValues);
+
+ Assert.True(headers.TryAddWithoutValidation("Content-Type", "application/json; charset=utf-8"));
+
+ if (useTypedProperty)
+ {
+ Task<MediaTypeHeaderValue> task = Task.Run(() => headers.ContentType);
+ MediaTypeHeaderValue contentType1 = headers.ContentType;
+ MediaTypeHeaderValue contentType2 = await task;
+
+ Assert.Same(contentType1, contentType2);
+ }
+ else
+ {
+ Task task = Task.Run(() => headers.Count()); // Force enumeration
+ MediaTypeHeaderValue contentType1 = headers.ContentType;
+ await task;
+
+ Assert.Same(contentType1, headers.ContentType);
+ }
+ }
+ }
+
  [Fact]
  public void TryAddInvalidHeader_ShouldThrowFormatException()
  {

src/libraries/System.Net.Http/src/System/Net/Http/Headers/HttpHeaders.cs

@@ -347,17 +347,14 @@ private IEnumerator<KeyValuePair<string, IEnumerable<string>>> GetEnumeratorCore
  // during enumeration so that we can parse the raw value in order to a) return
  // the correct set of parsed values, and b) update the instance for subsequent enumerations
  // to reflect that parsing.
- info = new HeaderStoreItemInfo() { RawValue = entry.Value };
 
- if (EntriesAreLiveView)
- {
- entries[i].Value = info;
- }
- else
- {
- Debug.Assert(Contains(entry.Key));
- ((Dictionary<HeaderDescriptor, object>)_headerStore!)[entry.Key] = info;
- }
+#nullable disable // https://github.com/dotnet/roslyn/issues/73928
+ ref object storeValueRef = ref EntriesAreLiveView
+ ? ref entries[i].Value
+ : ref CollectionsMarshal.GetValueRefOrNullRef((Dictionary<HeaderDescriptor, object>)_headerStore, entry.Key);
+
+ info = ReplaceWithHeaderStoreItemInfo(ref storeValueRef, entry.Value);
+#nullable restore
  }
 
  // Make sure we parse all raw values before returning the result. Note that this has to be
@@ -729,15 +726,10 @@ private bool TryGetAndParseHeaderInfo(HeaderDescriptor key, [NotNullWhen(true)]
  if (!Unsafe.IsNullRef(ref storeValueRef))
  {
  object value = storeValueRef;
- if (value is HeaderStoreItemInfo hsi)
- {
- info = hsi;
- }
- else
- {
- Debug.Assert(value is string);
- storeValueRef = info = new HeaderStoreItemInfo() { RawValue = value };
- }
+
+ info = value is HeaderStoreItemInfo hsi
+ ? hsi
+ : ReplaceWithHeaderStoreItemInfo(ref storeValueRef, value);
 
  ParseRawHeaderValues(key, info);
  return true;
@@ -747,6 +739,31 @@ private bool TryGetAndParseHeaderInfo(HeaderDescriptor key, [NotNullWhen(true)]
  return false;
  }
 
+ /// <summary>
+ /// Replaces <paramref name="storeValueRef"/> with a new <see cref="HeaderStoreItemInfo"/>,
+ /// or returns the existing <see cref="HeaderStoreItemInfo"/> if a different thread beat us to it.
+ /// </summary>
+ /// <remarks>
+ /// This helper should be used any time we're upgrading a storage slot from an unparsed string to a HeaderStoreItemInfo *while reading*.
+ /// Concurrent writes to the header collection are UB, so we don't need to worry about race conditions when doing the replacement there.
+ /// </remarks>
+ [MethodImpl(MethodImplOptions.AggressiveInlining)]
+ private static HeaderStoreItemInfo ReplaceWithHeaderStoreItemInfo(ref object storeValueRef, object value)
+ {
+ Debug.Assert(value is string);
+
+ var info = new HeaderStoreItemInfo() { RawValue = value };
+ object previousValue = Interlocked.CompareExchange(ref storeValueRef, info, value);
+
+ if (ReferenceEquals(previousValue, value))
+ {
+ return info;
+ }
+
+ // Rare race condition: Another thread replaced the value with a HeaderStoreItemInfo.
+ return (HeaderStoreItemInfo)previousValue;
+ }
+
  private static void ParseRawHeaderValues(HeaderDescriptor descriptor, HeaderStoreItemInfo info)
  {
  // Unlike TryGetHeaderInfo() this method tries to parse all non-validated header values (if any)

src/libraries/System.Net.Http/tests/UnitTests/Headers/HttpHeadersTest.cs

@@ -3,6 +3,7 @@
 
 using System.Collections;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.Globalization;
 using System.Linq;
 using System.Net.Http.Headers;
@@ -2502,6 +2503,51 @@ static HttpRequestHeaders CreateHeaders()
  }
  }
 
+ [Theory]
+ [InlineData(true, true)]
+ [InlineData(true, false)]
+ [InlineData(false, true)]
+ [InlineData(false, false)]
+ public async Task ConcurrentReads_ReturnTheSameParsedValues(bool useDictionary, bool useTypedProperty)
+ {
+ HttpContentHeaders dummyValues = new ByteArrayContent([]).Headers;
+ if (useDictionary)
+ {
+ for (int i = 0; i < HttpHeaders.ArrayThreshold; i++)
+ {
+ Assert.True(dummyValues.TryAddWithoutValidation($"foo-{i}", "Foo"));
+ }
+ }
+
+ Stopwatch s = Stopwatch.StartNew();
+
+ while (s.ElapsedMilliseconds < 100)
+ {
+ HttpContentHeaders headers = new ByteArrayContent([]).Headers;
+
+ headers.AddHeaders(dummyValues);
+
+ Assert.True(headers.TryAddWithoutValidation("Content-Type", "application/json; charset=utf-8"));
+
+ if (useTypedProperty)
+ {
+ Task<MediaTypeHeaderValue> task = Task.Run(() => headers.ContentType);
+ MediaTypeHeaderValue contentType1 = headers.ContentType;
+ MediaTypeHeaderValue contentType2 = await task;
+
+ Assert.Same(contentType1, contentType2);
+ }
+ else
+ {
+ Task task = Task.Run(() => headers.Count()); // Force enumeration
+ MediaTypeHeaderValue contentType1 = headers.ContentType;
+ await task;
+
+ Assert.Same(contentType1, headers.ContentType);
+ }
+ }
+ }
+
  [Fact]
  public void TryAddInvalidHeader_ShouldThrowFormatException()
  {

src/libraries/System.Net.Http/src/System/Net/Http/Headers/HttpHeaders.cs

@@ -347,17 +347,14 @@ private IEnumerator<KeyValuePair<string, IEnumerable<string>>> GetEnumeratorCore
  // during enumeration so that we can parse the raw value in order to a) return
  // the correct set of parsed values, and b) update the instance for subsequent enumerations
  // to reflect that parsing.
- info = new HeaderStoreItemInfo() { RawValue = entry.Value };
 
- if (EntriesAreLiveView)
- {
- entries[i].Value = info;
- }
- else
- {
- Debug.Assert(Contains(entry.Key));
- ((Dictionary<HeaderDescriptor, object>)_headerStore!)[entry.Key] = info;
- }
+#nullable disable // https://github.com/dotnet/roslyn/issues/73928
+ ref object storeValueRef = ref EntriesAreLiveView
+ ? ref entries[i].Value
+ : ref CollectionsMarshal.GetValueRefOrNullRef((Dictionary<HeaderDescriptor, object>)_headerStore, entry.Key);
+
+ info = ReplaceWithHeaderStoreItemInfo(ref storeValueRef, entry.Value);
+#nullable restore
  }
 
  // Make sure we parse all raw values before returning the result. Note that this has to be
@@ -729,15 +726,10 @@ private bool TryGetAndParseHeaderInfo(HeaderDescriptor key, [NotNullWhen(true)]
  if (!Unsafe.IsNullRef(ref storeValueRef))
  {
  object value = storeValueRef;
- if (value is HeaderStoreItemInfo hsi)
- {
- info = hsi;
- }
- else
- {
- Debug.Assert(value is string);
- storeValueRef = info = new HeaderStoreItemInfo() { RawValue = value };
- }
+
+ info = value is HeaderStoreItemInfo hsi
+ ? hsi
+ : ReplaceWithHeaderStoreItemInfo(ref storeValueRef, value);
 
  ParseRawHeaderValues(key, info);
  return true;
@@ -747,6 +739,31 @@ private bool TryGetAndParseHeaderInfo(HeaderDescriptor key, [NotNullWhen(true)]
  return false;
  }
 
+ /// <summary>
+ /// Replaces <paramref name="storeValueRef"/> with a new <see cref="HeaderStoreItemInfo"/>,
+ /// or returns the existing <see cref="HeaderStoreItemInfo"/> if a different thread beat us to it.
+ /// </summary>
+ /// <remarks>
+ /// This helper should be used any time we're upgrading a storage slot from an unparsed string to a HeaderStoreItemInfo *while reading*.
+ /// Concurrent writes to the header collection are UB, so we don't need to worry about race conditions when doing the replacement there.
+ /// </remarks>
+ [MethodImpl(MethodImplOptions.AggressiveInlining)]
+ private static HeaderStoreItemInfo ReplaceWithHeaderStoreItemInfo(ref object storeValueRef, object value)
+ {
+ Debug.Assert(value is string);
+
+ var info = new HeaderStoreItemInfo() { RawValue = value };
+ object previousValue = Interlocked.CompareExchange(ref storeValueRef, info, value);
+
+ if (ReferenceEquals(previousValue, value))
+ {
+ return info;
+ }
+
+ // Rare race condition: Another thread replaced the value with a HeaderStoreItemInfo.
+ return (HeaderStoreItemInfo)previousValue;
+ }
+
  private static void ParseRawHeaderValues(HeaderDescriptor descriptor, HeaderStoreItemInfo info)
  {
  // Unlike TryGetHeaderInfo() this method tries to parse all non-validated header values (if any)

src/libraries/System.Net.Http/tests/UnitTests/Headers/HttpHeadersTest.cs

@@ -3,6 +3,7 @@
 
 using System.Collections;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.Globalization;
 using System.Linq;
 using System.Net.Http.Headers;
@@ -2502,6 +2503,51 @@ static HttpRequestHeaders CreateHeaders()
  }
  }
 
+ [Theory]
+ [InlineData(true, true)]
+ [InlineData(true, false)]
+ [InlineData(false, true)]
+ [InlineData(false, false)]
+ public async Task ConcurrentReads_ReturnTheSameParsedValues(bool useDictionary, bool useTypedProperty)
+ {
+ HttpContentHeaders dummyValues = new ByteArrayContent([]).Headers;
+ if (useDictionary)
+ {
+ for (int i = 0; i < HttpHeaders.ArrayThreshold; i++)
+ {
+ Assert.True(dummyValues.TryAddWithoutValidation($"foo-{i}", "Foo"));
+ }
+ }
+
+ Stopwatch s = Stopwatch.StartNew();
+
+ while (s.ElapsedMilliseconds < 100)
+ {
+ HttpContentHeaders headers = new ByteArrayContent([]).Headers;
+
+ headers.AddHeaders(dummyValues);
+
+ Assert.True(headers.TryAddWithoutValidation("Content-Type", "application/json; charset=utf-8"));
+
+ if (useTypedProperty)
+ {
+ Task<MediaTypeHeaderValue> task = Task.Run(() => headers.ContentType);
+ MediaTypeHeaderValue contentType1 = headers.ContentType;
+ MediaTypeHeaderValue contentType2 = await task;
+
+ Assert.Same(contentType1, contentType2);
+ }
+ else
+ {
+ Task task = Task.Run(() => headers.Count()); // Force enumeration
+ MediaTypeHeaderValue contentType1 = headers.ContentType;
+ await task;
+
+ Assert.Same(contentType1, headers.ContentType);
+ }
+ }
+ }
+
  [Fact]
  public void TryAddInvalidHeader_ShouldThrowFormatException()
  {

src/libraries/System.Net.Http/src/System/Net/Http/Headers/HttpHeaders.cs

@@ -347,17 +347,14 @@ private IEnumerator<KeyValuePair<string, IEnumerable<string>>> GetEnumeratorCore
  // during enumeration so that we can parse the raw value in order to a) return
  // the correct set of parsed values, and b) update the instance for subsequent enumerations
  // to reflect that parsing.
- info = new HeaderStoreItemInfo() { RawValue = entry.Value };
 
- if (EntriesAreLiveView)
- {
- entries[i].Value = info;
- }
- else
- {
- Debug.Assert(Contains(entry.Key));
- ((Dictionary<HeaderDescriptor, object>)_headerStore!)[entry.Key] = info;
- }
+#nullable disable // https://github.com/dotnet/roslyn/issues/73928
+ ref object storeValueRef = ref EntriesAreLiveView
+ ? ref entries[i].Value
+ : ref CollectionsMarshal.GetValueRefOrNullRef((Dictionary<HeaderDescriptor, object>)_headerStore, entry.Key);
+
+ info = ReplaceWithHeaderStoreItemInfo(ref storeValueRef, entry.Value);
+#nullable restore
  }
 
  // Make sure we parse all raw values before returning the result. Note that this has to be
@@ -729,15 +726,10 @@ private bool TryGetAndParseHeaderInfo(HeaderDescriptor key, [NotNullWhen(true)]
  if (!Unsafe.IsNullRef(ref storeValueRef))
  {
  object value = storeValueRef;
- if (value is HeaderStoreItemInfo hsi)
- {
- info = hsi;
- }
- else
- {
- Debug.Assert(value is string);
- storeValueRef = info = new HeaderStoreItemInfo() { RawValue = value };
- }
+
+ info = value is HeaderStoreItemInfo hsi
+ ? hsi
+ : ReplaceWithHeaderStoreItemInfo(ref storeValueRef, value);
 
  ParseRawHeaderValues(key, info);
  return true;
@@ -747,6 +739,31 @@ private bool TryGetAndParseHeaderInfo(HeaderDescriptor key, [NotNullWhen(true)]
  return false;
  }
 
+ /// <summary>
+ /// Replaces <paramref name="storeValueRef"/> with a new <see cref="HeaderStoreItemInfo"/>,
+ /// or returns the existing <see cref="HeaderStoreItemInfo"/> if a different thread beat us to it.
+ /// </summary>
+ /// <remarks>
+ /// This helper should be used any time we're upgrading a storage slot from an unparsed string to a HeaderStoreItemInfo *while reading*.
+ /// Concurrent writes to the header collection are UB, so we don't need to worry about race conditions when doing the replacement there.
+ /// </remarks>
+ [MethodImpl(MethodImplOptions.AggressiveInlining)]
+ private static HeaderStoreItemInfo ReplaceWithHeaderStoreItemInfo(ref object storeValueRef, object value)
+ {
+ Debug.Assert(value is string);
+
+ var info = new HeaderStoreItemInfo() { RawValue = value };
+ object previousValue = Interlocked.CompareExchange(ref storeValueRef, info, value);
+
+ if (ReferenceEquals(previousValue, value))
+ {
+ return info;
+ }
+
+ // Rare race condition: Another thread replaced the value with a HeaderStoreItemInfo.
+ return (HeaderStoreItemInfo)previousValue;
+ }
+
  private static void ParseRawHeaderValues(HeaderDescriptor descriptor, HeaderStoreItemInfo info)
  {
  // Unlike TryGetHeaderInfo() this method tries to parse all non-validated header values (if any)

src/libraries/System.Net.Http/tests/UnitTests/Headers/HttpHeadersTest.cs

@@ -3,6 +3,7 @@
 
 using System.Collections;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.Globalization;
 using System.Linq;
 using System.Net.Http.Headers;
@@ -2502,6 +2503,51 @@ static HttpRequestHeaders CreateHeaders()
  }
  }
 
+ [Theory]
+ [InlineData(true, true)]
+ [InlineData(true, false)]
+ [InlineData(false, true)]
+ [InlineData(false, false)]
+ public async Task ConcurrentReads_ReturnTheSameParsedValues(bool useDictionary, bool useTypedProperty)
+ {
+ HttpContentHeaders dummyValues = new ByteArrayContent([]).Headers;
+ if (useDictionary)
+ {
+ for (int i = 0; i < HttpHeaders.ArrayThreshold; i++)
+ {
+ Assert.True(dummyValues.TryAddWithoutValidation($"foo-{i}", "Foo"));
+ }
+ }
+
+ Stopwatch s = Stopwatch.StartNew();
+
+ while (s.ElapsedMilliseconds < 100)
+ {
+ HttpContentHeaders headers = new ByteArrayContent([]).Headers;
+
+ headers.AddHeaders(dummyValues);
+
+ Assert.True(headers.TryAddWithoutValidation("Content-Type", "application/json; charset=utf-8"));
+
+ if (useTypedProperty)
+ {
+ Task<MediaTypeHeaderValue> task = Task.Run(() => headers.ContentType);
+ MediaTypeHeaderValue contentType1 = headers.ContentType;
+ MediaTypeHeaderValue contentType2 = await task;
+
+ Assert.Same(contentType1, contentType2);
+ }
+ else
+ {
+ Task task = Task.Run(() => headers.Count()); // Force enumeration
+ MediaTypeHeaderValue contentType1 = headers.ContentType;
+ await task;
+
+ Assert.Same(contentType1, headers.ContentType);
+ }
+ }
+ }
+
  [Fact]
  public void TryAddInvalidHeader_ShouldThrowFormatException()
  {

src/libraries/System.Net.Http/src/System/Net/Http/Headers/HttpHeaders.cs

@@ -347,17 +347,14 @@ private IEnumerator<KeyValuePair<string, IEnumerable<string>>> GetEnumeratorCore
  // during enumeration so that we can parse the raw value in order to a) return
  // the correct set of parsed values, and b) update the instance for subsequent enumerations
  // to reflect that parsing.
- info = new HeaderStoreItemInfo() { RawValue = entry.Value };
 
- if (EntriesAreLiveView)
- {
- entries[i].Value = info;
- }
- else
- {
- Debug.Assert(Contains(entry.Key));
- ((Dictionary<HeaderDescriptor, object>)_headerStore!)[entry.Key] = info;
- }
+#nullable disable // https://github.com/dotnet/roslyn/issues/73928
+ ref object storeValueRef = ref EntriesAreLiveView
+ ? ref entries[i].Value
+ : ref CollectionsMarshal.GetValueRefOrNullRef((Dictionary<HeaderDescriptor, object>)_headerStore, entry.Key);
+
+ info = ReplaceWithHeaderStoreItemInfo(ref storeValueRef, entry.Value);
+#nullable restore
  }
 
  // Make sure we parse all raw values before returning the result. Note that this has to be
@@ -729,15 +726,10 @@ private bool TryGetAndParseHeaderInfo(HeaderDescriptor key, [NotNullWhen(true)]
  if (!Unsafe.IsNullRef(ref storeValueRef))
  {
  object value = storeValueRef;
- if (value is HeaderStoreItemInfo hsi)
- {
- info = hsi;
- }
- else
- {
- Debug.Assert(value is string);
- storeValueRef = info = new HeaderStoreItemInfo() { RawValue = value };
- }
+
+ info = value is HeaderStoreItemInfo hsi
+ ? hsi
+ : ReplaceWithHeaderStoreItemInfo(ref storeValueRef, value);
 
  ParseRawHeaderValues(key, info);
  return true;
@@ -747,6 +739,31 @@ private bool TryGetAndParseHeaderInfo(HeaderDescriptor key, [NotNullWhen(true)]
  return false;
  }
 
+ /// <summary>
+ /// Replaces <paramref name="storeValueRef"/> with a new <see cref="HeaderStoreItemInfo"/>,
+ /// or returns the existing <see cref="HeaderStoreItemInfo"/> if a different thread beat us to it.
+ /// </summary>
+ /// <remarks>
+ /// This helper should be used any time we're upgrading a storage slot from an unparsed string to a HeaderStoreItemInfo *while reading*.
+ /// Concurrent writes to the header collection are UB, so we don't need to worry about race conditions when doing the replacement there.
+ /// </remarks>
+ [MethodImpl(MethodImplOptions.AggressiveInlining)]
+ private static HeaderStoreItemInfo ReplaceWithHeaderStoreItemInfo(ref object storeValueRef, object value)
+ {
+ Debug.Assert(value is string);
+
+ var info = new HeaderStoreItemInfo() { RawValue = value };
+ object previousValue = Interlocked.CompareExchange(ref storeValueRef, info, value);
+
+ if (ReferenceEquals(previousValue, value))
+ {
+ return info;
+ }
+
+ // Rare race condition: Another thread replaced the value with a HeaderStoreItemInfo.
+ return (HeaderStoreItemInfo)previousValue;
+ }
+
  private static void ParseRawHeaderValues(HeaderDescriptor descriptor, HeaderStoreItemInfo info)
  {
  // Unlike TryGetHeaderInfo() this method tries to parse all non-validated header values (if any)

src/libraries/System.Net.Http/tests/UnitTests/Headers/HttpHeadersTest.cs

@@ -3,6 +3,7 @@
 
 using System.Collections;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.Globalization;
 using System.Linq;
 using System.Net.Http.Headers;
@@ -2502,6 +2503,51 @@ static HttpRequestHeaders CreateHeaders()
  }
  }
 
+ [Theory]
+ [InlineData(true, true)]
+ [InlineData(true, false)]
+ [InlineData(false, true)]
+ [InlineData(false, false)]
+ public async Task ConcurrentReads_ReturnTheSameParsedValues(bool useDictionary, bool useTypedProperty)
+ {
+ HttpContentHeaders dummyValues = new ByteArrayContent([]).Headers;
+ if (useDictionary)
+ {
+ for (int i = 0; i < HttpHeaders.ArrayThreshold; i++)
+ {
+ Assert.True(dummyValues.TryAddWithoutValidation($"foo-{i}", "Foo"));
+ }
+ }
+
+ Stopwatch s = Stopwatch.StartNew();
+
+ while (s.ElapsedMilliseconds < 100)
+ {
+ HttpContentHeaders headers = new ByteArrayContent([]).Headers;
+
+ headers.AddHeaders(dummyValues);
+
+ Assert.True(headers.TryAddWithoutValidation("Content-Type", "application/json; charset=utf-8"));
+
+ if (useTypedProperty)
+ {
+ Task<MediaTypeHeaderValue> task = Task.Run(() => headers.ContentType);
+ MediaTypeHeaderValue contentType1 = headers.ContentType;
+ MediaTypeHeaderValue contentType2 = await task;
+
+ Assert.Same(contentType1, contentType2);
+ }
+ else
+ {
+ Task task = Task.Run(() => headers.Count()); // Force enumeration
+ MediaTypeHeaderValue contentType1 = headers.ContentType;
+ await task;
+
+ Assert.Same(contentType1, headers.ContentType);
+ }
+ }
+ }
+
  [Fact]
  public void TryAddInvalidHeader_ShouldThrowFormatException()
  {

src/libraries/System.Net.Http/src/System/Net/Http/Headers/HttpHeaders.cs

@@ -347,17 +347,14 @@ private IEnumerator<KeyValuePair<string, IEnumerable<string>>> GetEnumeratorCore
  // during enumeration so that we can parse the raw value in order to a) return
  // the correct set of parsed values, and b) update the instance for subsequent enumerations
  // to reflect that parsing.
- info = new HeaderStoreItemInfo() { RawValue = entry.Value };
 
- if (EntriesAreLiveView)
- {
- entries[i].Value = info;
- }
- else
- {
- Debug.Assert(Contains(entry.Key));
- ((Dictionary<HeaderDescriptor, object>)_headerStore!)[entry.Key] = info;
- }
+#nullable disable // https://github.com/dotnet/roslyn/issues/73928
+ ref object storeValueRef = ref EntriesAreLiveView
+ ? ref entries[i].Value
+ : ref CollectionsMarshal.GetValueRefOrNullRef((Dictionary<HeaderDescriptor, object>)_headerStore, entry.Key);
+
+ info = ReplaceWithHeaderStoreItemInfo(ref storeValueRef, entry.Value);
+#nullable restore
  }
 
  // Make sure we parse all raw values before returning the result. Note that this has to be
@@ -729,15 +726,10 @@ private bool TryGetAndParseHeaderInfo(HeaderDescriptor key, [NotNullWhen(true)]
  if (!Unsafe.IsNullRef(ref storeValueRef))
  {
  object value = storeValueRef;
- if (value is HeaderStoreItemInfo hsi)
- {
- info = hsi;
- }
- else
- {
- Debug.Assert(value is string);
- storeValueRef = info = new HeaderStoreItemInfo() { RawValue = value };
- }
+
+ info = value is HeaderStoreItemInfo hsi
+ ? hsi
+ : ReplaceWithHeaderStoreItemInfo(ref storeValueRef, value);
 
  ParseRawHeaderValues(key, info);
  return true;
@@ -747,6 +739,31 @@ private bool TryGetAndParseHeaderInfo(HeaderDescriptor key, [NotNullWhen(true)]
  return false;
  }
 
+ /// <summary>
+ /// Replaces <paramref name="storeValueRef"/> with a new <see cref="HeaderStoreItemInfo"/>,
+ /// or returns the existing <see cref="HeaderStoreItemInfo"/> if a different thread beat us to it.
+ /// </summary>
+ /// <remarks>
+ /// This helper should be used any time we're upgrading a storage slot from an unparsed string to a HeaderStoreItemInfo *while reading*.
+ /// Concurrent writes to the header collection are UB, so we don't need to worry about race conditions when doing the replacement there.
+ /// </remarks>
+ [MethodImpl(MethodImplOptions.AggressiveInlining)]
+ private static HeaderStoreItemInfo ReplaceWithHeaderStoreItemInfo(ref object storeValueRef, object value)
+ {
+ Debug.Assert(value is string);
+
+ var info = new HeaderStoreItemInfo() { RawValue = value };
+ object previousValue = Interlocked.CompareExchange(ref storeValueRef, info, value);
+
+ if (ReferenceEquals(previousValue, value))
+ {
+ return info;
+ }
+
+ // Rare race condition: Another thread replaced the value with a HeaderStoreItemInfo.
+ return (HeaderStoreItemInfo)previousValue;
+ }
+
  private static void ParseRawHeaderValues(HeaderDescriptor descriptor, HeaderStoreItemInfo info)
  {
  // Unlike TryGetHeaderInfo() this method tries to parse all non-validated header values (if any)

src/libraries/System.Net.Http/tests/UnitTests/Headers/HttpHeadersTest.cs

@@ -3,6 +3,7 @@
 
 using System.Collections;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.Globalization;
 using System.Linq;
 using System.Net.Http.Headers;
@@ -2502,6 +2503,51 @@ static HttpRequestHeaders CreateHeaders()
  }
  }
 
+ [Theory]
+ [InlineData(true, true)]
+ [InlineData(true, false)]
+ [InlineData(false, true)]
+ [InlineData(false, false)]
+ public async Task ConcurrentReads_ReturnTheSameParsedValues(bool useDictionary, bool useTypedProperty)
+ {
+ HttpContentHeaders dummyValues = new ByteArrayContent([]).Headers;
+ if (useDictionary)
+ {
+ for (int i = 0; i < HttpHeaders.ArrayThreshold; i++)
+ {
+ Assert.True(dummyValues.TryAddWithoutValidation($"foo-{i}", "Foo"));
+ }
+ }
+
+ Stopwatch s = Stopwatch.StartNew();
+
+ while (s.ElapsedMilliseconds < 100)
+ {
+ HttpContentHeaders headers = new ByteArrayContent([]).Headers;
+
+ headers.AddHeaders(dummyValues);
+
+ Assert.True(headers.TryAddWithoutValidation("Content-Type", "application/json; charset=utf-8"));
+
+ if (useTypedProperty)
+ {
+ Task<MediaTypeHeaderValue> task = Task.Run(() => headers.ContentType);
+ MediaTypeHeaderValue contentType1 = headers.ContentType;
+ MediaTypeHeaderValue contentType2 = await task;
+
+ Assert.Same(contentType1, contentType2);
+ }
+ else
+ {
+ Task task = Task.Run(() => headers.Count()); // Force enumeration
+ MediaTypeHeaderValue contentType1 = headers.ContentType;
+ await task;
+
+ Assert.Same(contentType1, headers.ContentType);
+ }
+ }
+ }
+
  [Fact]
  public void TryAddInvalidHeader_ShouldThrowFormatException()
  {