Remove cDAC fallback testing mode

eng/pipelines/diagnostics/sos-test-leg.yml

@@ -1,6 +1,6 @@
 # sos-test-leg.yml -- wraps the matrix + job + dep + artifact download +
 # standard postBuildSteps pattern used by the SOS-style test legs (cDAC,
-# cDAC_fallback, cDAC_verify, DAC) that run on windows_x64. Each of those legs
+# cDAC_verify, DAC) that run on windows_x64. Each of those legs
 # only differs in its `name` and `dacMode` value.
 #
 # AzDO tasks workaround:

eng/pipelines/runtime-diagnostics.yml

@@ -3,8 +3,8 @@
 # Pipeline overview:
 #
 # SOSTests windows_x64 Release. One shared coreclr+libs build (-c Debug
-# -rc release -lc release -clrinterpreter) consumed by 4 SOS
-# legs that run on top of it: cDAC, cDAC_fallback, cDAC_verify, DAC.
+# -rc release -lc release -clrinterpreter) consumed by 3 SOS
+# legs that run on top of it: cDAC, cDAC_verify, DAC.
 # Each leg sets testInterpreter: true so interpreter coverage
 # is exercised inline (no separate Interpreter leg).
 #
@@ -177,13 +177,8 @@ extends:
  name: cDAC
  dacMode: cdac
 
- # cDAC_fallback / cDAC_verify: the in-box DAC hosts the cDAC contract reader, with per-API fallback
- # to the legacy DAC (fallback) or no fallback but still verifying against it (verify).
- - template: /eng/pipelines/diagnostics/sos-test-leg.yml
- parameters:
- name: cDAC_fallback
- dacMode: cdacfallback
-
+ # cDAC_verify: the in-box DAC hosts the cDAC contract reader without fallback,
+ # while still verifying against the legacy DAC.
  - template: /eng/pipelines/diagnostics/sos-test-leg.yml
  parameters:
  name: cDAC_verify

src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Legacy/LegacyFallbackHelper.cs

@@ -11,15 +11,12 @@ namespace Microsoft.Diagnostics.DataContractReader.Legacy;
 
 /// <summary>
 /// Controls whether delegation-only APIs can fall back to the legacy DAC implementation.
-/// When <c>CDAC_NO_FALLBACK=1</c> is set, only explicitly allowlisted methods may delegate.
+/// Only explicitly allowlisted methods may delegate.
 /// All fallback attempts are logged to stderr for capture by the test infrastructure.
 /// </summary>
 internal static class LegacyFallbackHelper
 {
- private static readonly bool s_noFallback =
- Environment.GetEnvironmentVariable("CDAC_NO_FALLBACK") == "1";
-
- // Methods that are allowed to fall back even in no-fallback mode.
+ // Methods that are allowed to fall back.
  // Use the method name as it appears via [CallerMemberName].
  private static readonly HashSet<string> s_allowlist = new(StringComparer.Ordinal)
  {
@@ -36,18 +33,14 @@ internal static class LegacyFallbackHelper
 
  /// <summary>
  /// Returns <c>true</c> if the calling method is allowed to delegate to the legacy DAC.
- /// In normal mode (no <c>CDAC_NO_FALLBACK</c>), always returns <c>true</c>.
- /// In no-fallback mode, returns <c>true</c> only for allowlisted methods.
+ /// Returns <c>true</c> only for allowlisted methods.
  /// All fallback attempts (allowed and blocked) are logged to stderr.
  /// </summary>
  internal static bool CanFallback(
  [CallerMemberName] string name = "",
  [CallerFilePath] string file = "",
  [CallerLineNumber] int line = 0)
  {
- if (!s_noFallback)
- return true;
-
  if (s_allowlist.Contains(name) || s_fileAllowlist.Contains(Path.GetFileName(file)))
  {
  Console.Error.WriteLine($"[cDAC] Allowed fallback: {name} at {Path.GetFileName(file)}:{line}");