Update copy logic to use dedicated threads. (#11272)

Context
The existing implementation does synchronous file copying on threadpool threads which can lead to starvation. Switching to dedicated threads to do synchronous copying helps keep the threadpool threads available for other tasks.

Also updated some checks to help avoid hitting the file system if possible.

Author: Erarndt

src/Framework/NativeMethods.cs

@@ -1522,9 +1522,18 @@ private static unsafe int GetCurrentDirectoryWin32(int nBufferLength, char* lpBu
     [SupportedOSPlatform("windows")]
     internal static unsafe string GetFullPath(string path)
     {
-        int bufferSize = GetFullPathWin32(path, 0, null, IntPtr.Zero);
-        char* buffer = stackalloc char[bufferSize];
-        int fullPathLength = GetFullPathWin32(path, bufferSize, buffer, IntPtr.Zero);
+        char* buffer = stackalloc char[MAX_PATH];
+        int fullPathLength = GetFullPathWin32(path, MAX_PATH, buffer, IntPtr.Zero);
+
+        // if user is using long paths we could need to allocate a larger buffer
+        if (fullPathLength > MAX_PATH)
+        {
+            char* newBuffer = stackalloc char[fullPathLength];
+            fullPathLength = GetFullPathWin32(path, fullPathLength, newBuffer, IntPtr.Zero);
+
+            buffer = newBuffer;
+        }
+
         // Avoid creating new strings unnecessarily
         return AreStringsEqual(buffer, fullPathLength, path) ? path : new string(buffer, startIndex: 0, length: fullPathLength);
     }

src/Tasks.UnitTests/Copy_Tests.cs

@@ -79,11 +79,6 @@ public static IEnumerable<object[]> GetNullAndEmptyArrays() =>
             */
         };
 
-        private const int NoParallelismThreadCount = 1;
-        private const int DefaultParallelismThreadCount = int.MaxValue;
-
-        private int GetParallelismThreadCount(bool isUseSingleThreadedCopy) => isUseSingleThreadedCopy ? NoParallelismThreadCount : DefaultParallelismThreadCount;
-
         /// <summary>
         /// Temporarily save off the value of MSBUILDALWAYSOVERWRITEREADONLYFILES, so that we can run
         /// the tests isolated from the current state of the environment, but put it back how it belongs
@@ -509,7 +504,7 @@ public void DontCopyOverSameFile(bool isUseHardLinks, bool isUseSymbolicLinks, b
                     UseSymboliclinksIfPossible = isUseSymbolicLinks,
                 };
 
-                t.Execute(m.CopyFile, GetParallelismThreadCount(isUseSingleThreadedCopy));
+                t.Execute(m.CopyFile, !isUseSingleThreadedCopy);
 
                 // Expect for there to have been no copies.
                 Assert.Equal(0, m.copyCount);
@@ -557,7 +552,7 @@ public void QuestionCopyFile(bool isUseHardLinks, bool isUseSymbolicLinks, bool
                     FailIfNotIncremental = true,
                 };
 
-                Assert.False(t.Execute(m.CopyFile, GetParallelismThreadCount(isUseSingleThreadedCopy)));
+                Assert.False(t.Execute(m.CopyFile, !isUseSingleThreadedCopy));
 
                 // Expect for there to have been no copies.
                 Assert.Equal(0, m.copyCount);
@@ -617,7 +612,7 @@ public void QuestionCopyFileSameContent(bool isUseHardLinks, bool isUseSymbolicL
                     SkipUnchangedFiles = true,
                     FailIfNotIncremental = true,
                 };
-                Assert.True(t.Execute(m.CopyFile, GetParallelismThreadCount(isUseSingleThreadedCopy)));
+                Assert.True(t.Execute(m.CopyFile, !isUseSingleThreadedCopy));
 
                 // Expect for there to have been no copies.
                 Assert.Equal(0, m.copyCount);
@@ -670,7 +665,7 @@ public void QuestionCopyFileNotSameContent(bool isUseHardLinks, bool isUseSymbol
                     FailIfNotIncremental = true,
                 };
 
-                Assert.False(t.Execute(m.CopyFile, GetParallelismThreadCount(isUseSingleThreadedCopy)));
+                Assert.False(t.Execute(m.CopyFile, !isUseSingleThreadedCopy));
 
                 // Expect for there to have been no copies.
                 Assert.Equal(0, m.copyCount);
@@ -2015,7 +2010,7 @@ public void CopyWithDuplicatesUsingFolder(bool isUseHardLinks, bool isUseSymboli
                     filesActuallyCopied.Add(new KeyValuePair<FileState, FileState>(source, dest));
                 }
                 return true;
-            }, GetParallelismThreadCount(isUseSingleThreadedCopy));
+            }, !isUseSingleThreadedCopy);
 
             Assert.True(success);
             Assert.Equal(2, filesActuallyCopied.Count);
@@ -2082,7 +2077,7 @@ public void CopyWithDuplicatesUsingFiles(bool isUseHardLinks, bool isUseSymbolic
                     filesActuallyCopied.Add(new KeyValuePair<FileState, FileState>(source, dest));
                 }
                 return true;
-            }, GetParallelismThreadCount(isUseSingleThreadedCopy));
+            }, !isUseSingleThreadedCopy);
 
             Assert.True(success);
             Assert.Equal(4, filesActuallyCopied.Count);
@@ -2352,7 +2347,7 @@ public void FailureWithNoRetries(bool isUseHardLinks, bool isUseSymbolicLinks, b
             };
 
             var copyFunctor = new CopyFunctor(2, false /* do not throw on failure */);
-            bool result = t.Execute(copyFunctor.Copy, GetParallelismThreadCount(isUseSingleThreadedCopy));
+            bool result = t.Execute(copyFunctor.Copy, !isUseSingleThreadedCopy);
 
             Assert.False(result);
             engine.AssertLogDoesntContain("MSB3026");
@@ -2419,7 +2414,7 @@ public void SuccessAfterOneRetry(bool isUseHardLinks, bool isUseSymbolicLinks, b
             };
 
             var copyFunctor = new CopyFunctor(2, false /* do not throw on failure */);
-            bool result = t.Execute(copyFunctor.Copy, GetParallelismThreadCount(isUseSingleThreadedCopy));
+            bool result = t.Execute(copyFunctor.Copy, !isUseSingleThreadedCopy);
 
             Assert.True(result);
             engine.AssertLogContains("MSB3026");
@@ -2446,7 +2441,7 @@ public void SuccessAfterOneRetryContinueToNextFile(bool isUseHardLinks, bool isU
             };
 
             var copyFunctor = new CopyFunctor(2, false /* do not throw on failure */);
-            bool result = t.Execute(copyFunctor.Copy, GetParallelismThreadCount(isUseSingleThreadedCopy));
+            bool result = t.Execute(copyFunctor.Copy, !isUseSingleThreadedCopy);
 
             Assert.True(result);
             engine.AssertLogContains("MSB3026");
@@ -2478,7 +2473,7 @@ public void TooFewRetriesReturnsFalse(bool isUseHardLinks, bool isUseSymbolicLin
             };
 
             var copyFunctor = new CopyFunctor(4, false /* do not throw */);
-            bool result = t.Execute(copyFunctor.Copy, GetParallelismThreadCount(isUseSingleThreadedCopy));
+            bool result = t.Execute(copyFunctor.Copy, !isUseSingleThreadedCopy);
 
             Assert.False(result);
             engine.AssertLogContains("MSB3026");
@@ -2507,7 +2502,7 @@ public void TooFewRetriesThrows(bool isUseHardLinks, bool isUseSymbolicLinks, bo
             };
 
             var copyFunctor = new CopyFunctor(3, true /* throw */);
-            bool result = t.Execute(copyFunctor.Copy, GetParallelismThreadCount(isUseSingleThreadedCopy));
+            bool result = t.Execute(copyFunctor.Copy, !isUseSingleThreadedCopy);
 
             Assert.False(result);
             engine.AssertLogContains("MSB3026");