Fix File.ReadAllBytes{Async} for virtual file system files

Some file systems, like procfs, can return 0 for a file's length, even though it actually contains on-demand computed contents.  This breaks File.ReadAllBytes{Async}, which currently assumes that a length of 0 means the file is empty.  This commit fixes it by changing the assumption to mean that a length of 0 means we can't depend on the Length and instead need to read until EOF.

src/System.IO.FileSystem/src/System/IO/File.cs

@@ -15,6 +15,7 @@ namespace System.IO
     // routines such as Delete, etc.
     public static class File
     {
+        private const int MaxByteArrayLength = 0x7FFFFFC7;
         private static Encoding s_UTF8NoBOM;
 
         internal const int DefaultBufferSize = 4096;
@@ -324,7 +325,15 @@ public static byte[] ReadAllBytes(string path)
             {
                 long fileLength = fs.Length;
                 if (fileLength > int.MaxValue)
+                {
                     throw new IOException(SR.IO_FileTooLong2GB);
+                }
+                else if (fileLength == 0)
+                {
+                    // Some file systems (e.g. procfs on Linux) return 0 for length even when there's content.
+                    // Thus we need to assume 0 doesn't mean empty.
+                    return ReadAllBytesUnknownLength(fs);
+                }
 
                 int index = 0;
                 int count = (int)fileLength;
@@ -341,6 +350,50 @@ public static byte[] ReadAllBytes(string path)
             }
         }
 
+        private static byte[] ReadAllBytesUnknownLength(FileStream fs)
+        {
+            byte[] rentedArray = null;
+            Span<byte> buffer = stackalloc byte[512];
+            try
+            {
+                int bytesRead = 0;
+                while (true)
+                {
+                    if (bytesRead == buffer.Length)
+                    {
+                        uint newLength = (uint)buffer.Length * 2;
+                        if (newLength > MaxByteArrayLength)
+                        {
+                            newLength = (uint)Math.Max(MaxByteArrayLength, buffer.Length + 1);
+                        }
+
+                        byte[] tmp = ArrayPool<byte>.Shared.Rent((int)newLength);
+                        buffer.CopyTo(tmp);
+                        if (rentedArray != null)
+                        {
+                            ArrayPool<byte>.Shared.Return(rentedArray);
+                        }
+                        buffer = rentedArray = tmp;
+                    }
+
+                    Debug.Assert(bytesRead < buffer.Length);
+                    int n = fs.Read(buffer.Slice(bytesRead));
+                    if (n == 0)
+                    {
+                        return buffer.Slice(0, bytesRead).ToArray();
+                    }
+                    bytesRead += n;
+                }
+            }
+            finally
+            {
+                if (rentedArray != null)
+                {
+                    ArrayPool<byte>.Shared.Return(rentedArray);
+                }
+            }
+        }
+
         public static void WriteAllBytes(string path, byte[] bytes)
         {
             if (path == null)
@@ -709,31 +762,23 @@ private static async Task<string> InternalReadAllTextAsync(string path, Encoding
                 return Task.FromCanceled<byte[]>(cancellationToken);
             }
 
-            FileStream fs = new FileStream(
-                path, FileMode.Open, FileAccess.Read, FileShare.Read, DefaultBufferSize,
+            var fs = new FileStream(
+                path, FileMode.Open, FileAccess.Read, FileShare.Read, bufferSize: 1, // bufferSize == 1 used to avoid unnecessary buffer in FileStream
                 FileOptions.Asynchronous | FileOptions.SequentialScan);
 
             bool returningInternalTask = false;
             try
             {
                 long fileLength = fs.Length;
-                if (cancellationToken.IsCancellationRequested)
-                {
-                    return Task.FromCanceled<byte[]>(cancellationToken);
-                }
-
                 if (fileLength > int.MaxValue)
                 {
                     return Task.FromException<byte[]>(new IOException(SR.IO_FileTooLong2GB));
                 }
 
-                if (fileLength == 0)
-                {
-                    return Task.FromResult(Array.Empty<byte>());
-                }
-
                 returningInternalTask = true;
-                return InternalReadAllBytesAsync(fs, (int)fileLength, cancellationToken);
+                return fileLength > 0 ?
+                    InternalReadAllBytesAsync(fs, (int)fileLength, cancellationToken) :
+                    InternalReadAllBytesUnknownLengthAsync(fs, cancellationToken);
             }
             finally
             {
@@ -765,6 +810,44 @@ private static async Task<byte[]> InternalReadAllBytesAsync(FileStream fs, int c
             }
         }
 
+        private static async Task<byte[]> InternalReadAllBytesUnknownLengthAsync(FileStream fs, CancellationToken cancellationToken)
+        {
+            byte[] rentedArray = ArrayPool<byte>.Shared.Rent(512);
+            try
+            {
+                int bytesRead = 0;
+                while (true)
+                {
+                    if (bytesRead == rentedArray.Length)
+                    {
+                        uint newLength = (uint)rentedArray.Length * 2;
+                        if (newLength > MaxByteArrayLength)
+                        {
+                            newLength = (uint)Math.Max(MaxByteArrayLength, rentedArray.Length + 1);
+                        }
+
+                        byte[] tmp = ArrayPool<byte>.Shared.Rent((int)newLength);
+                        Buffer.BlockCopy(rentedArray, 0, tmp, 0, bytesRead);
+                        ArrayPool<byte>.Shared.Return(rentedArray);
+                        rentedArray = tmp;
+                    }
+
+                    Debug.Assert(bytesRead < rentedArray.Length);
+                    int n = await fs.ReadAsync(rentedArray.AsMemory(bytesRead), cancellationToken).ConfigureAwait(false);
+                    if (n == 0)
+                    {
+                        return rentedArray.AsSpan().Slice(0, bytesRead).ToArray();
+                    }
+                    bytesRead += n;
+                }
+            }
+            finally
+            {
+                fs.Dispose();
+                ArrayPool<byte>.Shared.Return(rentedArray);
+            }
+        }
+
         public static Task WriteAllBytesAsync(string path, byte[] bytes, CancellationToken cancellationToken = default(CancellationToken))
         {
             if (path == null)

src/System.IO.FileSystem/tests/File/ReadWriteAllBytes.cs

@@ -4,6 +4,7 @@
 
 using System.Runtime.InteropServices;
 using System.Text;
+using System.Threading.Tasks;
 using Xunit;
 
 namespace System.IO.Tests
@@ -120,5 +121,85 @@ public void WriteToReadOnlyFile()
                 File.SetAttributes(path, FileAttributes.Normal);
             }
         }
+
+        [Fact]
+        public async Task ReadAllBytes_EmptyFile_ReturnsEmptyArray()
+        {
+            string path = GetTestFilePath();
+            File.Create(path).Dispose();
+            Assert.Equal(0, File.ReadAllBytes(path).Length);
+            Assert.Equal(0, (await File.ReadAllBytesAsync(path)).Length);
+        }
+
+        [Theory]
+        [PlatformSpecific(TestPlatforms.Linux)]
+        [InlineData("/proc/cmdline")]
+        [InlineData("/proc/version")]
+        [InlineData("/proc/filesystems")]
+        public void ReadAllBytes_ReadAllText_ProcFs_Equal(string path)
+        {
+            byte[] bytes = null;
+            string text = null;
+
+            const int NumTries = 3; // some of these could theoretically change between reads, so allow retries just in case
+            for (int i = 1; i <= NumTries; i++)
+            {
+                try
+                {
+                    bytes = File.ReadAllBytes(path);
+                    text = File.ReadAllText(path);
+                    Assert.Equal(text, Encoding.UTF8.GetString(bytes));
+                }
+                catch when (i < NumTries) { }
+            }
+        }
+
+        [Theory]
+        [PlatformSpecific(TestPlatforms.Linux)]
+        [InlineData("/proc/cmdline")]
+        [InlineData("/proc/version")]
+        [InlineData("/proc/filesystems")]
+        [InlineData("/proc/cpuinfo")]
+        public async Task ReadAllBytesAsync_ReadAllTextAsync_ProcFs_Equal(string path)
+        {
+            byte[] bytes = null;
+            string text = null;
+
+            const int NumTries = 3; // some of these could theoretically change between reads, so allow retries just in case
+            for (int i = 1; i <= NumTries; i++)
+            {
+                try
+                {
+                    bytes = await File.ReadAllBytesAsync(path);
+                    text = await File.ReadAllTextAsync(path);
+                    Assert.Equal(text, Encoding.UTF8.GetString(bytes));
+                }
+                catch when (i < NumTries) { }
+            }
+        }
+
+        [Theory]
+        [PlatformSpecific(TestPlatforms.Linux)]
+        public async Task ReadAllBytes_ProcFs_Uptime_ContainsTwoNumbers(bool useAsync)
+        {
+            const string UptimePath = "/proc/uptime";
+            string text = Encoding.UTF8.GetString(useAsync ?
+                await File.ReadAllBytesAsync(UptimePath) :
+                File.ReadAllBytes(UptimePath));
+            string[] parts = text.Split(' ', StringSplitOptions.RemoveEmptyEntries);
+            Assert.Equal(2, parts.Length);
+            Assert.True(double.TryParse(parts[0].Trim(), out _));
+            Assert.True(double.TryParse(parts[1].Trim(), out _));
+        }
+
+        [Theory]
+        [PlatformSpecific(TestPlatforms.Linux)]
+        [InlineData("/proc/meminfo")]
+        [InlineData("/proc/stat")]
+        [InlineData("/proc/cpuinfo")]
+        public async Task ReadAllBytes_ProcFs_NotEmpty(string path)
+        {
+            Assert.InRange((await File.ReadAllBytesAsync(path)).Length, 1, int.MaxValue);
+        }
     }
 }