MemoryMappedFile fixes

This patch includes a few fixes:

	* CreateNew will actually create a memory map file, the previous logic
	  in MemoryMapImpl.Open was wrong, and would fail if the file did not
	  exist.   We now cope with this (bug #20052 was caused by this).

 	* Some exceptions were wrong, so this patch updates the
	  exceptions being thrown to be the right ones

	* On Mobile, we inverted the logic to ensure that capacity must be
	  bigger or equal than the file size (which is what the API needs to do),
	  previously it performed the opposite test.

Added tests that match the behavior on .NET for this.

mcs/class/System.Core/System.IO.MemoryMappedFiles/MemoryMappedFile.cs

@@ -25,7 +25,6 @@
 // OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
 // WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 //
-
 #if NET_4_0
 
 using System;
@@ -44,8 +43,21 @@ using System.Runtime.CompilerServices;
 
 namespace System.IO.MemoryMappedFiles
 {
+	internal static partial class MemoryMapImpl {
+		static Exception ArgumentCapacity ()
+		{
+			return new ArgumentException ("A positive capacity must be specified for a Memory Mapped File backed by an empty file.");
+		}
+
+		static Exception CapacitySmallerThanSize ()
+		{
+			return new ArgumentOutOfRangeException ("The capacity may not be smaller than the file size.");
+		}
+	}
+	
+	
 #if !MOBILE
-	internal static class MemoryMapImpl {
+	partial class MemoryMapImpl {
 		//
 		// Turns the FileMode into the first half of open(2) flags
 		//
@@ -121,25 +133,39 @@ namespace System.IO.MemoryMappedFiles
 		{
 			if (MonoUtil.IsUnix){
 				Stat buf;
-				if (Syscall.stat (path, out buf) == -1)
-					UnixMarshal.ThrowExceptionForLastError ();
 
-				if (capacity == 0) {
-					// Special files such as FIFOs, sockets, and devices can
-					// have a size of 0. Specifying a capacity for these
-					// also makes little sense, so don't do the check if the
-					// file is one of these.
-					if (buf.st_size == 0 &&
-						(buf.st_mode & (FilePermissions.S_IFCHR |
-						                FilePermissions.S_IFBLK |
-						                FilePermissions.S_IFIFO |
-						                FilePermissions.S_IFSOCK)) == 0) {
-						throw new ArgumentException ("A positive capacity must be specified for a Memory Mapped File backed by an empty file.");
-					}
+				int result = Syscall.stat (path, out buf);
 
-					capacity = buf.st_size;
-				} else if (capacity < buf.st_size) {
-					throw new ArgumentException ("The capacity may not be smaller than the file size.");
+				if (mode == FileMode.Truncate || mode == FileMode.Append || mode == FileMode.Open){
+					if (result == -1)
+						UnixMarshal.ThrowExceptionForLastError ();
+				}
+				if (mode == FileMode.CreateNew && result == 0)
+					throw new IOException ("The file already exists");
+				
+				if (result == 0){
+					if (capacity == 0) {
+						// Special files such as FIFOs, sockets, and devices can
+						// have a size of 0. Specifying a capacity for these
+						// also makes little sense, so don't do the check if the
+						// file is one of these.
+						if (buf.st_size == 0 &&
+						    (buf.st_mode & (FilePermissions.S_IFCHR |
+								    FilePermissions.S_IFBLK |
+								    FilePermissions.S_IFIFO |
+								    FilePermissions.S_IFSOCK)) == 0) {
+							throw ArgumentCapacity ();
+						}
+						
+						capacity = buf.st_size;
+					} else if (capacity < buf.st_size) {
+						throw CapacitySmallerThanSize ();
+					}
+				} else {
+					if (mode == FileMode.CreateNew){
+						if (capacity == 0)
+							throw ArgumentCapacity ();
+					}
 				}
 
 				int fd = Syscall.open (path, ToUnixMode (mode) | ToUnixMode (access), FilePermissions.DEFFILEMODE);
@@ -232,7 +258,7 @@ namespace System.IO.MemoryMappedFiles
 
 	}
 #else
-	internal static class MemoryMapImpl {
+	partial class MemoryMapImpl {
 		[DllImport ("libc")]
 		static extern int fsync (int fd);
 
@@ -383,11 +409,23 @@ namespace System.IO.MemoryMappedFiles
 		internal static int Open (string path, FileMode mode, ref long capacity, MemoryMappedFileAccess access)
 		{
 			long file_size = mono_filesize_from_path (path);
-			if (file_size < 0)
-				throw new FileNotFoundException (path);
 
-			if (capacity > file_size)
-				throw new ArgumentException ("capacity");
+			Console.WriteLine ("{0} is {1} big", path, file_size);
+			if (mode == FileMode.Truncate || mode == FileMode.Append || mode == FileMode.Open){
+				if (file_size < 0)
+					throw new FileNotFoundException (path);
+				if (capacity == 0)
+					capacity = file_size;
+				if (capacity < file_size)
+					throw CapacitySmallerThanSize ();
+			} else {
+				if (mode == FileMode.CreateNew){
+					if (file_size >= 0)
+						throw new IOException ("The file already exists");
+					if (capacity == 0)
+						throw ArgumentCapacity ();
+				}
+			}
 
 			int fd = open (path, ToUnixMode (mode) | ToUnixMode (access), DEFFILEMODE);
 

mcs/class/System.Core/Test/System.IO.MemoryMappedFiles/MemoryMappedFileTest.cs

@@ -92,6 +92,43 @@ namespace MonoTests.System.IO.MemoryMappedFiles {
 			}
 		}
 
+		[Test]
+		public void CreateNew ()
+		{
+			// This must succeed
+			MemoryMappedFile.CreateNew (Path.Combine (tempDir, "createNew.test"), 8192);
+		}
+
+		[Test]
+		[ExpectedException (typeof (IOException))]
+		public void CreateNew_OnExistingFile ()
+		{
+			// This must succeed
+			MemoryMappedFile.CreateNew (Path.Combine (tempDir, "createNew.test"), 8192);
+			
+			// This should fail, the file exists
+			MemoryMappedFile.CreateNew (Path.Combine (tempDir, "createNew.test"), 8192);
+		}
+
+		// Call this twice, it should always work
+		[Test]
+		public void CreateOrOpen_Multiple ()
+		{
+			MemoryMappedFile.CreateOrOpen (Path.Combine (tempDir, "createOrOpen.test"), 8192);
+			MemoryMappedFile.CreateOrOpen (Path.Combine (tempDir, "createOrOpen.test"), 8192);
+		}
+
+		[Test]
+		[ExpectedException(typeof(ArgumentOutOfRangeException))]
+		public void CreateFromFileWithSmallerCapacityThanFile ()
+		{
+			var f = Path.Combine (tempDir, "8192-file");
+			File.WriteAllBytes (f, new byte [8192]);
+
+			// We are requesting fewer bytes to map.
+			MemoryMappedFile.CreateFromFile (f, FileMode.Open, "myMap", 4192);
+		}
+	
 		[Test]
 		public void CreateFromFile_Null () {
 			AssertThrows<ArgumentNullException> (delegate () {