avoid Lock acquisition on ErrorCount/WarningCount when no errors exist (dotnet#4157)

* avoid Lock acquisition on ErrorCount/WarningCount when no errors exist

  1. _hasErrorOrWarning marked volatile (line 278) -- the field was already read without a lock at line 1212, so this  makes the existing pattern and the new fast paths correct across all memory models.
  2. ErrorCount fast path -- returns 0 immediately when _hasErrorOrWarning is false, skipping the lock. Safe because  _hasErrorOrWarning is only set to false under the lock when _errors is also nulled.
  3. WarningCount fast path -- same treatment.

* Update src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/TdsParserStateObject.cs

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Update src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/TdsParserStateObject.cs

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Update TdsParserStateObject.cs

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: SimonCropp

src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/TdsParserStateObject.cs

@@ -275,7 +275,7 @@ private enum SnapshotStatus
         internal SqlErrorCollection _errors;
         internal SqlErrorCollection _warnings;
         internal object _errorAndWarningsLock = new object();
-        private bool _hasErrorOrWarning;
+        private volatile bool _hasErrorOrWarning;
 
         // local exceptions to cache warnings and errors that occurred prior to sending attention
         internal SqlErrorCollection _preAttentionErrors;
@@ -2672,6 +2672,12 @@ internal int ErrorCount
         {
             get
             {
+                if (!_hasErrorOrWarning &&
+                    Volatile.Read(ref _errors) == null &&
+                    Volatile.Read(ref _warnings) == null)
+                {
+                    return 0;
+                }
                 int count = 0;
                 lock (_errorAndWarningsLock)
                 {
@@ -2713,6 +2719,10 @@ internal int WarningCount
         {
             get
             {
+                if (!_hasErrorOrWarning && _warnings == null)
+                {
+                    return 0;
+                }
                 int count = 0;
                 lock (_errorAndWarningsLock)
                 {