Same code starting wkhtmltopdf process works on netcoreapp3.1 but fails on net5.0 on Linux

[gurustron]

Description

Repro.

Following code starting process and reading stdout works fine in netcoreapp3.1:

    internal class Program
    {
        private static void Main(string[] args)
        {
            Console.WriteLine("Hello World!");
            using (var process = new Process
            {
                StartInfo = new ProcessStartInfo
                {
                    FileName = "wkhtmltopdf",
                    Arguments =
                        "-q -B 8 -L 8 -R 8 -T 8 --print-media-type --enable-local-file-access -s Letter -O Portrait - -",
                    UseShellExecute = false,
                    RedirectStandardOutput = true,
                    RedirectStandardError = true,
                    RedirectStandardInput = true,
                    CreateNoWindow = true
                }
            })
            {
                process.Start();

                byte[] array;

                var html = "test";
                using (var standardInput = process.StandardInput)
                {
                    standardInput.WriteLine(html);
                }

                using (var memoryStream = new MemoryStream())
                {
                    using (var baseStream = process.StandardOutput.BaseStream)
                    {
                        var buffer = new byte[4096];
                        int count;
                        while ((count = baseStream.Read(buffer, 0, buffer.Length)) > 0)
                            memoryStream.Write(buffer, 0, count);
                    }

                    var end = process.StandardError.ReadToEnd();
                    if (memoryStream.Length == 0L)
                        throw new Exception(end);
                    process.WaitForExit();
                    array = memoryStream.ToArray();
                }

                Console.WriteLine(array.Length);
            }
        }
    }

but fails in net5.0 with:

System.Exception: QPainter::begin(): Returned false
Exit with code 1, due to unknown error.

Configuration

• dotnet --version: 5.0.101
• Ubuntu 20.04.1 LTS
• wkhtmltox_0.12.6-1.bionic_amd64

Regression?

Works in netcoreapp3.1

Other information

related:

1. Linux version cannot write on STDOUT when wkhtmltopdf used as a child-process. wkhtmltopdf/wkhtmltopdf#3119
2. Failure on .NET 5.0 fpanaccia/Wkhtmltopdf.NetCore-deprecated#46 (comment)
3. QPainter::begin(): Returned false Exit with code 1, due to unknown error. .net 5 docker linux fpanaccia/Wkhtmltopdf.NetCore.Example-deprecated#14

Tagging subscribers to this area: @eiriktsarpalis
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Issue Details

---

Description

https://github.com/gurustron/repro-process-wkhtml-net-5.

Next code starting process and reading stdout works fine in netcoreapp3.1:

    internal class Program
    {
        private static void Main(string[] args)
        {
            Console.WriteLine("Hello World!");
            using (var process = new Process
            {
                StartInfo = new ProcessStartInfo
                {
                    FileName = "wkhtmltopdf",
                    Arguments =
                        "-q -B 8 -L 8 -R 8 -T 8 --print-media-type --enable-local-file-access -s Letter -O Portrait - -",
                    UseShellExecute = false,
                    RedirectStandardOutput = true,
                    RedirectStandardError = true,
                    RedirectStandardInput = true,
                    CreateNoWindow = true
                }
            })
            {
                process.Start();

                byte[] array;

                var html = "test";
                using (var standardInput = process.StandardInput)
                {
                    standardInput.WriteLine(html);
                }

                using (var memoryStream = new MemoryStream())
                {
                    using (var baseStream = process.StandardOutput.BaseStream)
                    {
                        var buffer = new byte[4096];
                        int count;
                        while ((count = baseStream.Read(buffer, 0, buffer.Length)) > 0)
                            memoryStream.Write(buffer, 0, count);
                    }

                    var end = process.StandardError.ReadToEnd();
                    if (memoryStream.Length == 0L)
                        throw new Exception(end);
                    process.WaitForExit();
                    array = memoryStream.ToArray();
                }

                Console.WriteLine(array.Length);
            }
        }
    }

but fails in net5.0 with:

System.Exception: QPainter::begin(): Returned false
Exit with code 1, due to unknown error.

Configuration

• dotnet --version: 5.0.101
• Ubuntu 20.04.1 LTS
• wkhtmltox_0.12.6-1.bionic_amd64

Regression?

Works in netcoreapp3.1

Other information

related:

1. Linux version cannot write on STDOUT when wkhtmltopdf used as a child-process. wkhtmltopdf/wkhtmltopdf#3119
2. Failure on .NET 5.0 fpanaccia/Wkhtmltopdf.NetCore-deprecated#46 (comment)
3. QPainter::begin(): Returned false Exit with code 1, due to unknown error. .net 5 docker linux fpanaccia/Wkhtmltopdf.NetCore.Example-deprecated#14

Author:	gurustron
Assignees:	-
Labels:	area-System.Diagnostics.Process, untriaged
Milestone:	-

[yakeer]

Following, facing the same issue.

[imb590]

wkhtmltopdf, when instructed to output in stdout, tries to open the file /dev/stdout and that fails with ENXIO.

/dev/stdout is a symlink to /proc/PID/fd/1, which is a socket for the process started from .NET 5 with System.Diagnostics.Process with RedirectStandardInput = true. In .NET Core 3.1, it was a pipe, and Linux syscall open works fine with that; the change was made in #34861.

As far as I understand, there is no way to start a process from .NET 5 through System.Diagnostics.Process with input redirection that works with wkhtmltopdf's way of opening /dev/stdout.

[yakeer]

wkhtmltopdf, when instructed to output in stdout, tries to open the file /dev/stdout and that fails with ENXIO.

/dev/stdout is a symlink to /proc/PID/fd/1, which is a socket for the process started from .NET 5 with System.Diagnostics.Process with RedirectStandardInput = true. In .NET Core 3.1, it was a pipe, and Linux syscall open works fine with that; the change was made in #34861.

As far as I understand, there is no way to start a process from .NET 5 through System.Diagnostics.Process with input redirection that works with wkhtmltopdf's way of opening /dev/stdout.

so basically, as far as I understand, there is no current way to support wkhtmltopdf in .NET 5 in the current situation?

That's a big thing for me, and as far as I believe for other developers that are exporting PDF's in their .NET applications.

[eiriktsarpalis]

Agree with @MBashov's analysis. However the error in the linked reproduction is caused by the fact that it is disposing the StandardInput object prematurely, which results in the corresponding socket being closed. Once I removed the unneeded using statement, the error went away.

Edit: fixed typo.

[gurustron]

@eiriktsarpalis changing code to:

                using (var memoryStream = new MemoryStream())
                {
                    var baseStream = process.StandardOutput.BaseStream;
                    // using (var baseStream = process.StandardOutput.BaseStream)
                    //{
                        var buffer = new byte[4096];
                        int count;
                        while ((count = baseStream.Read(buffer, 0, buffer.Length)) > 0)
                            memoryStream.Write(buffer, 0, count);


                        var end = process.StandardError.ReadToEnd();
                        if (memoryStream.Length == 0L)
                            throw new Exception(end);
                        process.WaitForExit();
                        array = memoryStream.ToArray();
                    //}
                }

still throws the same error for me. And it works in .NET Core 3.1 without any changes.

[eiriktsarpalis]

Apologies, my original comment should have said "StandardInput", namely making the following change:

-                using (var standardInput = process.StandardInput)
-                {
-                    standardInput.WriteLine(html);
-                }
+                process.StandardInput.WriteLine(html);

In general, I would say there are too many using statements in this code. Just applying it to the Process object should suffice.

[gurustron]

@eiriktsarpalis thank you! this works for 5.0. Still wonder what actually changed since 3.1 cause it worked there with disposes =)

Agreed on the disposes - code was written by someone else and I have not read it thoroughly enough.

[eiriktsarpalis]

Great to know! In that case I'm going to close this issue. Thank you for your contribution.

[gurustron]

@eiriktsarpalis

I was running wrong project. Removing the using from standardInput leads to repro project just hanging indefinitely on first baseStream.Read(buffer, 0, buffer.Length) (which kind of makes sense, using Close instead of disposing leads to the same exception).

can you please reopen the issue?