Deploying buildbot workers on Windows Server 2016

At LiveCode, we use a buildbot system to perform our continuous integration and release builds. Recently, we moved from building our Windows binaries in a Linux container using Wine to building on a native Windows system running in an Azure virtual machine.

Deploying buildbot on Windows is not totally straightforward, and the documentation for installing it is quite hard to follow. It's quite important to us that our build infrastructure is reproducible, so we wanted to have a procedure that could bring up a buildbot worker on a newly-allocated server quickly and with as little manual intervention as possible.

This blog post provides step-by-step instructions for installing buildbot 0.8.12 on Windows Server 2016 Datacenter Edition, with explanations of what's going on at each step. The target configuration is a buildbot worker that runs as an unprivileged user and communicates with the buildbot master over an SSL tunnel. All of the commands are written using PowerShell. It's recommended to run them via the 'PowerShell ISE' application, running as a user in the 'Administrators' group. The full script is available as a GitHub Gist.

Although this describes installing buildbot 0.8.12, there's no reason it shouldn't work for buildbot 0.9.x. If you try it, please let me know how you get on in the comments.

Note: Don't run these commands unless you've checked them very carefully first. They're adapted from the scripts used for our buildbot deployment, and may not work as you expect. You should use them as the basis of your own installation script and test it thoroughly before using it in production.

Support functions

First, ensure that the script stops immediately if any error is thrown, and that "verbose" messages are displayed.

$VerbosePreference = 'Continue'
$ErrorActionPreference = 'Stop'

By default, PowerShell doesn't convert non-zero exit codes from subprocess into errors, so define a helper function that you can use to accomplish this. By default, CheckLastExitCode will throw an error on a non-zero exit code, but if there are other exit codes that should be considered successful, you can pass in an array of permitted exit codes, e.g. CheckLastExitCode(@(0,10)).

function CheckLastExitCode {
    param ([int[]]$SuccessCodes = @(0))
    if ($SuccessCodes -notcontains $LASTEXITCODE) {
        throw "Command failed (exit code $LASTEXITCODE)"
    }
}

For this to work, you'll need to implement a Fetch-BuildbotResource function that obtains a named resource file and places it in a given output location. Fill in the blanks (possibly with some sort of Invoke-WebRequest):

function Fetch-BuildbotResource {
    param([string]$Path,
          [string]$OutFile)
    # Your code goes here
}

It's also a good idea to activate Windows. The virtual machines provisioned by Azure may not have been activated; this command will do so automatically.

cscript.exe C:\Windows\System32\slmgr.vbs /ato

Finally, define variables with the root path for the buildbot installation and the IP or DNS address of the buildbot master, and create the buildbot worker's root directory

$k_buildbot_root = 'C:\buildbot'
$k_buildbot_master = 'buildbot.example.org'

New-Item -Path $k_buildbot_root -ItemType Container -Force | Out-Null

Installing programs with Chocolatey

Chocolatey is a package manager for Windows that can automatically install a variety of applications and services in much the same way as the Linux `apt-get`, `dnf` or `yum` programs. Here, you can use it for installing Python (for running buildbot) and for installing the stunnel SSL tunnel service.

Install Chocolatey by the time-honoured process of "downloading a random script from the Internet and running it as a superuser".

$env:ChocolateyInstall = 'C:\ProgramData\chocolatey'

# Install Chocolatey, if not already present
if (!(Test-Path -LiteralPath $env:ChocolateyInstall -PathType Container)) {
    Invoke-WebRequest 'https://chocolatey.org/install.ps1' -UseBasicParsing | Invoke-Expression
}

Next, use Chocolatey to install stunnel and Python 2.7:

Write-Verbose 'Installing Python and stunnel'
choco install --yes stunnel python2
CheckLastExitCode

Installing Python modules and buildbot

It's easiest to install buildbot and its dependencies using the pip Python package manager.

Write-Verbose 'Installing Python modules'
$t_pip = 'C:\Python27\Scripts\pip.exe'
& $t_pip install pypiwin32 buildbot-slave==0.8.12
CheckLastExitCode

The pypiwin32 package installs some DLLs that are required for buildbot to run as a service, but when installed with pip, these DLLs are not automatically registered in the Windows registry. This caused me at least a day of wondering why my buildbot service was failing to start with the super informative message:

'Buildbot Worker (Buildbot)' cannot be started due to the following error: Cannot start service Buildbot on computer '.'.

(╯°□°）╯︵ ┻━┻)

— Dr Peter Brett (@PeterTBBrett) February 13, 2017

Luckily, pypiwin32 installs a script that will set everything up properly.

Write-Verbose 'Registering pywin32 DLLs'
$t_python = C:\Python27\python.exe
& $t_python C:\Python27\Scripts\pywin32_postinstall.py -install

SSL tunnel service

You'll need to configure stunnel to run on your buildbot master, and listen on port 9988. I recommend configuring the buildbot master's stunnel with a certificate, and then making sure workers always fully authenticate the certificate when connecting to it. This will prevent people from obtaining your workers' login credentials by impersonating the buildbot master machine.

Write-Verbose 'Installing buildbot-stunnel service'
$t_stunnel = 'C:\Program Files (x86)\stunnel\bin\stunnel.exe'
$t_stunnel_conf = Join-Path $k_buildbot_root 'stunnel.conf'
$t_stunnel_crt  = Join-Path $k_buildbot_root 'buildbot.crt'

# Fetch the client certificate that will be used to authenticate
# the buildbot master
Fetch-BuildbotResource `
    -Path 'buildbot/stunnel/master.crt' -Outfile $t_stunnel_crt

# Create the stunnel configuration file
Set-Content -Path $t_stunnel_conf -Value @"
[buildbot]
client = yes
accept = 127.0.0.1:9989
cafile = $t_stunnel_crt
verify = 3 
connect = $k_buildbot_master:9988
"@

# Register the stunnel service, if not already present
if (!(Get-Service buildbot-stunnel -ErrorAction Ignore)) {
    New-Service -Name buildbot-stunnel `
        -BinaryPathName "$t_stunnel -service $t_stunnel_conf" `
        -DisplayName 'Buildbot Secure Tunnel' `
        -StartupType Automatic
}

The buildbot worker instance

Creating and configuring the worker instance, and setting up buildbot to run as a Windows service, are the most complicated part of the installation process. Before dealing with the Windows service, instantiate a worker with the info it needs to connect to the buildbot master.

First, set up a bunch of values that will be needed later. The worker's name will just be the name of the server it's running on, and it will be configured to use a randomly-generated password.

Write-Verbose 'Initialising buildbot worker'

# Needed for password generation
Add-Type -AssemblyName System.Web

$t_buildbot_worker_script = 'C:\Python27\Scripts\buildslave'

$t_worker_dir = Join-Path $k_buildbot_root worker
$t_worker_name = "$env:COMPUTERNAME-$_"
$t_worker_password = `
    [System.Web.Security.Membership]::GeneratePassword(12,0)
$t_worker_admin = 'Example Organisation'

Run buildbot to actually instantiate the worker. We have to manually check the contents of the standard output from the setup process, because the exit status isn't a reliable indicator of success.

$t_log = Join-Path $k_buildbot_root setup.log
Start-Process -Wait -NoNewWindow -FilePath $t_python `
    -ArgumentList @($t_buildbot_worker_script, 'create-slave', `
        $t_worker_dir, 127.0.0.1, $t_worker_name,
        $t_worker_password) `
    -RedirectStandardOutput $t_log

# Check log file contents
$t_expected = "buildslave configured in $t_worker_dir"
if ((Get-Content $t_log)[-1] -ne $t_expected) {
    Get-Content $t_log | Write-Error
    throw "Build worker setup failed (exit code $LASTEXITCODE)"
}

It's helpful to provide some information about the host and who administrates it.

Set-Content -Path (Join-Path $t_worker_dir 'info\admin') `
    -Value $t_worker_admin
Set-Content -Path (Join-Path $t_worker_dir 'info\host') `
    -Value (Get-WmiObject -Class Win32_OperatingSystem).Caption

While testing our Windows-based buildbot workers, I found that I was getting "slave lost" errors during many build steps. I found that getting the workers to send really frequent "keep alive" messages to the build master prevented this from happening almost entirely. I used a 10 second period, but you might find that unnecessarily frequent.

$t_config = Join-Path $t_worker_dir buildbot.tac
Get-Content $t_config | `
    ForEach {$_ -replace '^keepalive\s*=\s*.*$', 'keepalive = 10'} | `
    Set-Content "$t_config.new"
Remove-Item $t_config
Move-Item "$t_config.new" $t_config

Configuring the buildbot service

Now for the final part: getting buildbot to run as a Windows service. It's a bad idea to run the worker as a privileged user, so this will create a 'BuildBot' user with a randomly-generated password, configure the service to use that account, and make sure it has full access to the worker's working directory.

Some of the commands used in this section expect passwords to be handled in the form of "secure strings" and some expect them to be handled in the clear. There's a fair degree of shuttling between the two representations.

Once again, begin by setting up some variables to use during these steps.

Write-Verbose 'Installing buildbot service'

$t_buildbot_service_script = 'C:\Python27\Scripts\buildbot_service.py'
$t_service_name = 'BuildBot'
$t_user_name = $t_service_name
$t_full_user_name = "$env:COMPUTERNAME\$t_service_name"

$t_user_password_clear = `
    [System.Web.Security.Membership]::GeneratePassword(12,0)
$t_user_password = `
    ConvertTo-SecureString $t_user_password_clear -AsPlainText -Force

Create the 'BuildBot' user:

$t_user = New-LocalUser -AccountNeverExpires `
    -PasswordNeverExpires `
    -UserMayNotChangePassword `
    -Name $t_user_name `
    -Password $t_user_password

You need to create the buildbot service by running the installation script provided by buildbot. Although there's a New-Service command in PowerShell, the pywin32 support for services written in Python expects a variety of registry keys to be set up correctly, and it won't work properly if they're not.

& $t_python $t_buildbot_service_script `
    --username $t_full_user_name `
    --password $t_user_password_clear `
    --startup auto install
CheckLastExitCode

It's still necessary to tell the service where to find the worker directory. You can do this by creating a special registry that the service checks on startup to discover its workers.

$t_parameters_key = "HKLM:\SYSTEM\CurrentControlSet\Services\$t_service_name\Parameters"
New-Item -Path $t_parameters_key -Force
Set-ItemProperty -Path $t_parameters_key -Name "directories" `
    -Value $t_worker_dir

Although the service is configured to start as the 'BuildBot' user, that user doesn't yet have the permissions required to read and write in the worker directory.

$t_acl = Get-Acl $t_worker_dir
$t_access_rule = New-Object `
    System.Security.AccessControl.FileSystemAccessRule `
    -ArgumentList @($t_full_user_name, 'FullControl', `
        'ContainerInherit,ObjectInherit', 'None', 'Allow')
$t_acl.SetAccessRule($t_access_rule)
Set-Acl $t_worker_dir $t_acl

Granting 'Log on as a service' rights

Your work is nearly done! However, there's one task that I have not yet worked out how to automate, and still requires manual intervention: granting the 'Buildbot' user the right to log on as a service. Without granting this right, the buildbot service will fail to start with a permissions error.

1. Open the 'Local Security Policy' tool
2. Choose 'Local Policies' -> 'User Rights Assignment' in the tree
3. Double-click on 'Log on as a service' in the details pane
4. Click 'Add User or Group', and add 'BuildBot' to the list of accounts

Time to launch

Everything should now be correctly configured!

There's one final bit of work required: you need to add the worker's username and password to the buildbot master's list of authorised workers. If you need it, you can obtain the username and password for the worker using PowerShell:

Get-Content C:\buildbot\worker\buildbot.tac | `
    Where {$_ -match '^(slavename|passwd)' }

You can use the `Start-Service` command to start the stunnel and buildbot services:

Start-Service buildbot-stunnel
Start-Service buildbot

Conclusions

You can view the full script described in this blog post as a GitHub Gist.

On top of installing buildbot itself, you'll need to install the various toolchains that you require. If you're using Microsoft Visual Studio, the "build tools only" installers provided by Microsoft for MSVC 2010 and MSVC 2015 are really useful. Many other dependencies can be installed using Chocolatey.

Installing buildbot on Windows is currently a pain, and I hope that someone who knows more about Windows development than I do can help the buildbot team make it easier to get started.

How to stop mspdbsrv from breaking your continuous integration system

Over the last month, I've been working on getting the LiveCode build cluster to do Windows builds using Visual Studio 2015. We've been using Visual Studio 2010 since I originally set up the build service in mid-2015. This upgrade was prompted by needing support for some C++ language features used by the latest version of libskia.

Once the new Windows Server buildbot workers had their tools installed and were connected to the build service, I noticed a couple of pretty weird things going on:

• after one initial build, the build workers were repeated failing to clean the build tree in preparation for for the next build
• builds were getting "stuck" after completing successfully, and were then being detected as timed out and forcibly killed

Blocked build tree cleanup

The first problem was easy to track down. I guessed that the clean step was failing because some process still had an open file handle to one of the files or directories that the worker was trying to delete. I used the Windows 'Resource Monitor' application (resmon.exe), which can be launched from the 'Start' menu or from 'Task Manager', to find the offending process. The 'CPU' tab lets you search all open file handles on the system by filename, and I quickly discovered that mspdbsrv.exe was holding a file handle to one of the build directories.

What is mspdbsrv?

mspdbsrv is a helper service used by the Visual Studio C and C++ compiler, cl.exe; it collects debugging information for code that's being compiled and writes out .pdb databases. CL automatically spawns mspdbsrv if debug info is being generated and it connect to an existing instance. When the build completes, CL doesn't clean up any mspdbsrv that it spawned; it just leaves it running. There's no way to prevent CL from doing this.

So, it looked like the abandoned mspdbsrv instance had its current working directory set to one of the directories that the build worker was trying to delete, and on Windows you can't delete a directory if there's a process running there. So much for the first problem.

Build step timeouts

The second issue was more subtle -- but it also appeared to be due to the lingering mspdbsrv process! I noticed that mspdbsrv was actually holding a file handle to one of the buildbot worker's internal log files. It appears that buildbot doesn't close file handles when starting build processes, and these handles were being inherited by mspbdsrv, which was holding them open. As result, the buildbot worker (correctly) inferred that there were still unfinished build job processes running, and didn't report the build as completed.

Mismatched MSVC versions

When I thought through this a bit further, I realised there was another problem being caused by lingering mspdbsrv instances. Some of the builds being handled by the Windows build workers need to use MSVC 2015, and some still need to use MSVC 2010. Each type of build should use the corresponding version of mspdbsrv, but by default CL always connects to any available service process.

Steps towards a fix

So, what was the solution?

1. Run mspdbsrv explicitly as part of the build setup, and keep a handle to the process so that it can be terminated once the build has finished.

2. Launch mspdbsrv with a current working directory outside the build tree.
3. Force CL to use a specific mspdbsrv instance rather than just picking any available one.

LiveCode CI builds are now performed using a Python helper script. Here's a snippet that implements all of these requirements (note that it hardcodes the path to the MSVC 2010 mspbdsrv.exe:

import os
import subprocess
import uuid

# Find the 32-bit program files directory
def get_program_files_x86():
    return os.environ.get('ProgramFiles(x86)',
                          os.environ.get('ProgramFiles',
                                         'C:\\Program Files\\'))

# mspdbsrv is the service used by Visual Studio to collect debug
# data during compilation.  One instance is shared by all C++
# compiler instances and threads.  It poses a unique challenge in
# several ways:
#
# - If not running when the build job starts, the build job will
#   automatically spawn it as soon as it needs to emit debug symbols.
#   There's no way to prevent this from happening.
#
# - The build job _doesn't_ automatically clean it up when it finishes
#
# - By default, mspdbsrv inherits its parent process' file handles,
#   including (unfortunately) some log handles owned by Buildbot.  This
#   can prevent Buildbot from detecting that the compile job is finished
#
# - If a compile job starts and detects an instance of mspdbsrv already
#   running, by default it will reuse it.  So, if you have a compile
#   job A running, and start a second job B, job B will use job A's
#   instance of mspdbsrv.  If you kill mspdbsrv when job A finishes,
#   job B will die horribly.  To make matters worse, the version of
#   mspdbsrv should match the version of Visual Studio being used.
#
# This class works around these problems:
#
# - It sets the _MSPDBSRV_ENDPOINT_ to a value that's probably unique to
#   the build, to prevent other builds on the same machine from sharing
#   the same mspdbsrv endpoint
#
# - It launches mspdbsrv with _all_ file handles closed, so that it
#   can't block the build from being detected as finished.
#
# - It explicitly kills mspdbsrv after the build job has finished.
#
# - It wraps all of this into a context manager, so mspdbsrv gets killed
#   even if a Python exception causes a non-local exit.
class UniqueMspdbsrv(object):
    def __enter__(self):
        os.environ['_MSPDBSRV_ENDPOINT_'] = str(uuid.uuid4())

        mspdbsrv_exe = os.path.join(get_program_files_x86(),
            'Microsoft Visual Studio 10.0\\Common7\\IDE\\mspdbsrv.exe')
        args = [mspdbsrv_exe, '-start', '-shutdowntime', '-1']
        print(' '.join(args))
        self.proc = subprocess.Popen(args, cwd='\\', close_fds=True)
        return self

    def __exit__(self, type, value, traceback):
        self.proc.terminate()
        return False

You can then use this when implementing a build step:

with UniqueMspdbsrv() as mspdbsrv:
    # Do your build steps here (e.g. msbuild invocation)
    pass

# mspdbsrv automatically cleaned up by context manager

It took me a couple of days to figure out what was going on and to find an adequate solution. A lot of very tedious trawling through obscure bits of the Internet were required to find all of the required pieces; for example, Microsoft do not document arguments to mspdbsrv or the environment variables that it understands anywhere on MSDN.

Hopefully, if you are running into problems with your Jenkins or buildbot workers interacting weirdly with Microsoft Visual Studio C or C++ builds, this will save you some time!

When C uninitialised variables and misleading whitespace combine

Recently, LiveCode Builder has gained a namespace resolution operator .. It allows LCB modules to declare functions, constants, and variables which have the same name, by providing a way for modules that import them to distinguish between them.

During this work, we ran into a problem: the modified LCB compiler (lc-compile) worked correctly in "Debug" builds, but reliably crashed in "Release" builds. More peculiarly, we found that lc-compile crashes depended on which compiler was used: some builds using certain versions of GCC crashed reliably, while some builds using clang worked fine. We spent a lot of time staring at output from gdb and Valgrind, and came to the conclusion that maybe it was a compiler bug in GCC.

It turned out that we were wrong. When we switched to using clang to build full LiveCode releases, the mysterious crashes popped up again. Since this had now become a problem that was breaking the build, I decided to dig into it again. Originally, we'd not been able to duplicate the crash in very recent versions of GCC and clang, so my first step was to try and make lc-compile crash when compiled with GCC 6.

The problem seemed to revolve around some code in the following form:

class T;
typedef T* TPtr;

// (1) function returning true iff r_value was set
bool maybeFetch(TPtr& r_value);

void f()
{
    TPtr t_value;
    if (maybeFetch(t_value))
    {
        // (2) dereference t_value
    }
}

lc-compile was sometimes, but not reliably, crashing at point (2).

Initially, when I compiled with GCC 6, I was not able to induce a crash. However, I did receive a warning that t_value might be used without being initialised. I therefore modified the implementation of f() to initialise t_value at its declaration:

void f()
{
    TPtr t_value = nullptr;
    // ...
}

With that modification, the crash became reliably reproducible in all build modes using all of the compilers I had available. This drew my suspicion to the maybeFetch() function (1). The function's API contract requires it to return true if (and only if) it sets its out parameter r_value, and return false otherwise.

So, I had a look at it, and it looked fine. What else could be going wrong?

Much of lc-compile is implemented using a domain-specific language called Gentle, which generates bison and flex grammars, which are in turn used to generate some megabytes of C code that's hard to read and harder to debug.

I disappeared into this code for quite a while, and couldn't find anything to suggest that the Gentle grammar was wrong, or that the generated code was the cause of the segfault. What I did find suggested that there were problems with the values being provided by the maybeFetch() function.

Because explicit initialisation made the crashes reliable and reproducible, I came to the conclusion that maybeFetch() was sometimes returning true without setting its out parameter. So, what was maybeFetch() doing?

A simplified form of maybeFetch() as I found it was:

bool maybeFetch(TPtr& r_value)
{
    for (TPtr t_loop_var = /* loop form ... */)
    {
        if (condition(t_loop_var))
            *r_value = t_loop_var;
            return true;
    }
    return false;
}

Needless to say, when I saw the problem it was moment of slightly bemused hilarity. This function had been reviewed several times by various team members, and all of us had missed the missing block braces { ... } hidden by misleading indentation.

if (condition(t_loop_var))
{ // missing open brace
    *r_value = t_loop_var;
    return true;
} // missing close brace

Once these braces had been inserted, all of the problems went away.

What lessons could be taken away from this?

1. The bug itself eluded review because of misleading indentation. GCC 6 provides a "misleading indentation" warning which would have immediately flagged up this warning if it had been enabled. We do not use GCC 6 for LiveCode builds; even if we did, we wouldn't be able to enable the "misleading indentation" warning to good effect because the LiveCode C++ sources don't currently use a consistent indentation style. This problem could maybe be avoided if LiveCode builds enforced a specific indentation style (in which case the bug would have been obvious in review), or if we regularly did builds with GCC 6 and -Werror=misleading-indentation.
2. The effect of the bug was an API contract violation, where the relationship between the return value and the value of an out parameter wasn't satisfied. The problem could have been avoided if the API contract was expressed in a way that the compiler could check. C++17 adds std::optional, which combines the idea of "is there a value or not" with returning the value itself. If the function took the form std::optional maybeFetch() then it would have been impossible for it to claim to return a value without actually returning one.
3. Finally, the problem was obfuscated by failing to initialise stack variables. Although, if maybeFetch() was working correctly, the pointer on the stack would get initialised before use, in this case, it didn't. Diagnosing the problem may have been much easier if we routinely initialised stack variables to suitable uninformative values at the point of declaration, even if we think they _should_ get initialised via a function's out parameter before use.

This was a very easy mistake to make, and an easy issue to miss in a code review, but it was very costly to clean up. I hope that we'll be able to make some changes to our development processes and our coding style to try and avoid things like this happening in the future.

Update: My colleague points out another contributing factor to making the error hard to spot: the condition(t_loop_var) was a composite condition spread across multiple lines, i.e.

if (conditionA(t_loop_var) ||
    (conditionB(t_loop_var) &&
     conditionC(t_loop_var)))

    *r_value = t_loop_var;
    return true;

This code layout maybe makes it slightly less obvious where the body of the if lies.

Using C library functions from LiveCode Builder

This blog post is part of an ongoing series about writing LiveCode Builder applications without the LiveCode engine.

Currently, the LiveCode Builder (LCB) standard library is fairly minimal. This means that there are some types of task for which you'll want to go beyond the standard library.

In a previous post, I described how to use LiveCode's foundation library. This lets you access plenty of built-in LiveCode functionality that isn't directly exposed to LCB code yet.

Someone else's problem

Often someone's already wrapped the functions that you need in another program, especially on Linux. You can run that program as a subprocess to access it. In LiveCode Script, you could use the shell function to run an external program. Unfortunately, the LCB standard library doesn't have an equivalent feature yet!

On the other hand, the standard C library's system(3) function can be used to run a shell command. Its prototype is:

int system(const char *command);

In this post, I'll describe how LCB's foreign function interface lets you call it.

Declaring a foreign handler

As last time, you can use the foreign handler syntax to declare the C library function. The com.livecode.foreign provides some important C types.

use com.livecode.foreign

foreign handler _system(in pCommand as ZStringNative) \
      returns CInt binds to "system"

Some things to bear in mind here:

• I've named the foreign handler _system because the all-lowercase identifier system is reserved for syntax tokens
• The ZStringNative type automatically converts a LCB string into a null-terminated string into whatever encoding LiveCode thinks is the system's "native" encoding.
• Because the C library is always linked into the LiveCode program when it's started, you don't need to specify a library name in the binds to clause; you can just use the name of the system(3) function.

Understanding the results

So, now you've declared the foreign handler, that's it! You can now just _system("rm -rf /opt/runrev") (or some other helpful operation). Right?

Well, not quite. If you want to know whether the shell command succeeded, you'll need to interpret the return value of the _system handler, and unfortunately, this isn't just the exit status of the command. From the system(3) man page:

The value returned is -1 on error (e.g., fork(2) failed), and the return status of the command otherwise. This latter return status is in the format specified in wait(2). Thus, the exit code of the command will be WEXITSTATUS(status). In case /bin/sh could not be executed, the exit status will be that of a command that does exit(127).

So if the _system handler returns -1, then an error occurred. Otherwise, it's necessary to do something equivalent to the WIFEXITED C macro to check if the command ran normally. If it didn't, then some sort of abnormal condition occurred in the command (e.g. it was killed). Finally, the actual exit status is extracted by doing something equivalent to the WEXITSTATUS C macro.

On Linux, these two macros are defined as follows:

#define WIFEXITED(status)     __WIFEXITED (__WAIT_INT (status))
#define WEXITSTATUS(status)   __WEXITSTATUS (__WAIT_INT (status))
#define __WIFEXITED(status)   (__WTERMSIG(status) == 0)
#define __WEXITSTATUS(status) (((status) & 0xff00) >> 8)
#define __WTERMSIG(status)    ((status) & 0x7f)
#define __WAIT_INT(status)    (status)

Or, more succinctly:

#define WIFEXITED(status)   (((status) & 0x7f) == 0)
#define WEXITSTATUS(status) (((status) & 0xff00) >> 8)

This is enough to be able to fully define a function that runs a shell command and returns its exit status.

module org.example.system

use com.livecode.foreign

private foreign handler _system(in pCommand as ZStringNative) \
      returns CInt binds to "system"

/*
Run the shell command  and wait for it to finish.
Returns the exit status of if the command completed, and nothing
if an error occurred or the command exited abnormally.
*/
public handler System(in pCommand as String) \
      returns optional Number

   variable tStatus as Number
   put _system(pCommand) into tStatus

   -- Check for error
   if tStatus is -1 then
      return nothing
   end if

   -- Check for abnormal exit
   if (127 bitwise and tStatus) is not 0 then
      return nothing
   end if

   -- Return exit status
   return 255 bitwise and (tStatus shifted right by 8 bitwise)

end module

Tip of the iceberg

This post has hopefully demonstrated the potential of LiveCode Builder's FFI. Even if you use only the C standard library's functions, you gain access to almost everything that the operating system is capable of!

Using a C function from LCB involves reading the manual pages to find out how the function should be used, and how best to map its arguments and return values onto LCB types; often, reading C library header files to understand how particular values should be encoded or decoded; and finally, binding the library function and providing a wrapper that makes it comfortable use from LCB programs.

LiveCode Builder can do a lot more than just making widgets and — as I hope I've demonstrated — can do useful things without the rest of the LiveCode engine. Download LiveCode 8 and try some things out!

Compiling multi-module LiveCode Builder programs

This blog post is part of an ongoing series about writing LiveCode Builder applications without the LiveCode engine.

Multi-module programs

When writing a large program, it's often useful to break it down into more than one module. For example, you might want to make a module that's dedicated to loading and saving the program's data, which has quite a lot of internal complexity but exposes a very simple API with Load() and Save() handlers. This is handy for making sure that it's easy to find the source file where each piece of functionality is located.

However, it can become tricky to compile the program. Each module may depend on any number of other modules, and you have to compile them in the correct order or the compilation result may be incorrect. Also, if one module changes, you have to recompile all of the modules that depend on it. If you tried to do this all by hand, it would be nigh-on impossible to correctly compile your program once you got above about 10 source files.

Fortunately, there are two really useful tools that can make it all rather easy. GNU Make (the make command) can perform all the required build steps in the correct order (and even in parallel!). And to help you avoid writing Makefiles by hand, lc-compile has a useful --deps mode.

Most of the remainder of this blog post will assume some familiarity with make and common Unix command-line tools.

The --deps option for lc-compile

make lets you express dependencies between files. However, you already express the dependencies between LCB source files when you write a use declaration. For example:

use com.livecode.foreign

says that your module depends on the .lci (LiveCode Interface) file for the com.livecode.foreign module.

So, the LCB compiler (a) already knows all the dependencies between the source files of your project and (b) already knows how to find the files. To take advantage of this and to massively simplify the process of creating a Makefile for a LCB project, lc-compile provides a --deps mode. In --deps mode, lc-compile doesn't do any of the normal compilation steps; instead, it outputs a set of Make rules on standard output.

Consider the following trivial two-file program.

-- org.example.numargs.lcb

module org.example.numargs

public handler NumArgs()
   return the number of elements in the command arguments
end handler

end module

-- org.example.countargs.lcb

module org.example.countargs

use org.example.numargs

public handler Main()
   quit with status NumArgs()
end handler

end module

To generate the dependency rules, you run lc-compile with almost a normal command line — but you specify --deps make instead of an --output argument, and you list all of your source files instead of just one of them. See also my previous blog post about compiling and running pure LCB programs. For the "countargs" example program you could run:

$TOOLCHAIN/lc-compile --modulepath . --modulepath $TOOLCHAIN/modules/lci --deps make org.example.numargs.lcb org.example.countargs.lcb

This would print the following rules:

org.example.countargs.lci: org.example.numargs.lci org.example.countargs.lcb
org.example.numargs.lci: org.example.numargs.lcb

Integrating with make

You can integrate this info into a Makefile quite easily. There are two pieces that you need: 1) tell make to load the extra rules, and 2) tell make how to generate them. In particular, it's important to regenerate the rules whenever the Makefile itself is modified (e.g. to add an additional source file).

# List of source code files
SOURCES = org.example.countargs.lcb org.example.numargs.lcb

# Include all the generated dependency rules
include deps.mk

# Rules for regenerating dependency rules whenever
# the source code changes
deps.mk: $(SOURCES) Makefile
 $(TOOLCHAIN)/lc-compile --modulepath . --modulepath $(TOOLCHAIN)/modules/lci --deps make -- $(SOURCES) > $@

A complete Makefile

Putting this all together, I've created a complete Makefile for the example multi-file project. It has the usual make compile and make clean targets, and places all of the built artefacts in a subdirectory called _build.

################################################################
# Parameters

# Tools etc.
LC_SRC_DIR ?= ../livecode
LC_BUILD_DIR ?= $(LC_SRC_DIR)/build-linux-x86_64/livecode/out/Debug
LC_LCI_DIR = $(LC_BUILD_DIR)/modules/lci
LC_COMPILE ?= $(LC_BUILD_DIR)/lc-compile
LC_RUN ?= $(LC_BUILD_DIR)/lc-run

BUILDDIR = _build

LC_COMPILE_FLAGS += --modulepath $(BUILDDIR) --modulepath $(LC_LCI_DIR)

# List of source code files.
SOURCES = org.example.countargs.lcb org.example.numargs.lcb

# List of compiled module filenames.
MODULES = $(patsubst %.lcb,$(BUILDDIR)/%.lcm,$(SOURCES))

################################################################
# Top-level targets
all: compile

compile: $(MODULES)

clean:
 -rm -rf $(BUILDDIR)

.DEFAULT: all
.PHONY: all compile clean

################################################################
# Build dependencies rules
include $(BUILDDIR)/deps.mk

$(BUILDDIR):
 mkdir -p $(BUILDDIR)

$(BUILDDIR)/deps.mk: $(SOURCES) Makefile | $(BUILDDIR)
 $(LC_COMPILE) $(LC_COMPILE_FLAGS) --deps make -- $(SOURCES) > $@

################################################################
# Build rules
$(BUILDDIR)/%.lcm $(BUILDDIR)/%.lci: %.lcb | $(BUILDDIR)
 $(LC_COMPILE) $(LC_COMPILE_FLAGS) --output $@ -- $<

You should be able to use this directly in your own projects. All you need to do is to modify the list of source files in the SOURCES variable!

Note that you need to name your source files exactly the same as the corresponding interface files in order for this Makefile to work correctly. I'll leave adapting to the case where the source file and interface file are named differently as an exercise to the reader…

I hope you find this useful as a basis for writing new LiveCode Builder projects! Let me know how you get on.

Accessing the Foundation library with LiveCode Builder

This blog post is part of an ongoing series about writing LiveCode Builder applications without the LiveCode engine.

The LiveCode Foundation library

LiveCode includes a "foundation" library (called, unsurprisingly, libfoundation) which provides a lot of useful functions that work on all the platforms that LiveCode supports. This is used to make sure that LiveCode works in the same way no matter which operating system or processor you're using. libfoundation is compiled into both the LiveCode engine and LiveCode Builder's lc-run tool, so it's always available.

libfoundation is written in C and C++. The functions available in the library are declared in the foundation.h header file.

Among other capabilities, libfoundation handles encoding and decoding text. This provides an opportunity to fix one of the problems with the "hello world" program I described in a previous post.

Foreign function access to libfoundation

The "hello world" program read in a file and wrote it out to the standard output stream. Unlike "hello world" programs seen elsewhere, it *didn't* write out a string, e.g.:

write "Hello World!" to the output stream

This doesn't work because write needs to receive Data, and converting a String to Data requires encoding (using a suitable string encoding). And unfortunately, the LiveCode Builder library doesn't supply any text encoding/decoding syntax, although I'm working on it.

However, and fortunately for this blog post, libfoundation supplies a suitable function, MCStringEncode. Its C++ declaration looks like:

bool MCStringEncode(MCStringRef string, MCStringEncoding encoding, bool is_external_rep, MCDataRef& r_data);

You can use it in a LiveCode Builder program by declaring it as a foreign handler. The com.livecode.foreign module provides some helpful declarations for C and C++ types.

use com.livecode.foreign

foreign handler MCStringEncode(in Source as String, \
      in Encoding as CInt, in IsExternalRep as CBool, \
      out Encoded as Data) returns CBool binds to "<builtin>"

CInt and CBool are C & C++'s int and bool types, respectively.

Encoding a string with UTF-8

Next, you can write a LiveCode Builder handler that encodes a string using UTF-8 (an 8-bit Unicode encoding). Almost every operating system will Do The Right Thing if you write UTF-8 encoded text to standard output; the only ones that might complain are some versions of Windows and some weirdly-configured Linux systems.

handler EncodeUTF8(in pString as String) returns Data
   variable tEncoded as Data
   MCStringEncode(pString, 4 /* UTF-8 */, false, tEncoded)
   return tEncoded
end handler

The "4" in there is a magic number that comes from libfoundation's kMCStringEncodingUTF8 constant. Also, you should always pass false to the IsExternalRep argument (for historical reasons).

A better "hello world" program

Putting this all together, you can now write an improved "hello world" program that doesn't get its text from an external file.

module org.example.helloworld2

use com.livecode.foreign

foreign handler MCStringEncode(in Source as String, \
      in Encoding as CInt, in IsExternalRep as CBool, \
      out Encoded as Data) returns CBool binds to "<builtin>"

handler EncodeUTF8(in pString as String) returns Data
   variable tEncoded as Data
   MCStringEncode(pString, 4 /* UTF-8 */, false, tEncoded)
   return tEncoded
end handler

public handler Main()
   write EncodeUTF8("Hello World!\n") to the output stream
end handler

end module

If you compile and run this program, you'll now get the same "Hello World!" message -- but this time, it's taking some text, turning it into Data by encoding it, and writing it out, rather than just regurgitating some previously-encoded data.

Other neat stuff

There's other cool (and, often, terribly unsafe) stuff you can do with direct access to libfoundation functions, like allocate Pointers to new memory buffers and directly manipulate LiveCode types & values. However, most of libfoundation's capabilities are already available using normal LiveCode Builder syntax.

The real power of foreign handler declarations becomes apparent when accessing functions that aren't in libfoundation — and this may be the subject of a future blog post!

LiveCode Builder without the LiveCode bit

Since my last post almost two years ago, I've moved to Edinburgh. I now work for LiveCode as an open source software engineer.

Introducing LiveCode Builder

LiveCode 8, the upcoming release of the LiveCode HyperCard-like application development environment, introduces a new xTalk-like language for writing LiveCode extensions. It's called LiveCode Builder (or LCB). It shares much of the same syntax as the original LiveCode scripting language, but it's a compiled, strongly-typed language.

Most of the public discussion about LiveCode Builder has revolved around using it to extend LiveCode — either by creating new widgets to display in the user interface, or by writing libraries that add new capabilities to the scripting language. However, one topic that *hasn't* been discussed much is the fact that you can write complete applications using only LCB, and compile and run them without using the main LiveCode engine at all.

LiveCode Builder without the engine

This is actually pretty useful when writing simple command-line tools or services that don't need a user interface and for which the main LiveCode engine provides little value (for example, if you need your tool to start up really quickly). There are a couple of good examples that I've written during the last few months.

The LCB standard library's test suite uses a test runner written in LCB. This is quite a useful "smoke test" for the compiler, virtual machine, and standard library -- if any of them break, the test suite won't run at all!

More recently, I've written a bot that connects our GitHub repositories to our BuildBot continuous integration system. Every few minutes, it checks the status of all the outstanding pull requests, and either submits new build jobs or reports on completed ones. This is also written entirely in LCB. One of main advantages of using LCB for this were that LCB has a proper List type that can contain arrays as elements.

"Hello World" in LCB

A pure LCB program looks like this:

module org.example.helloworld

public handler Main()
   write the contents of file "hello.txt" to the output stream
end handler

end module

It has a top-level module, that contains a public handler called Main. Note that unlike in C or C++, the Main handler doesn't take any arguments (you can access the command-line arguments using `the command arguments`).

Next, you need to compile your application using the lc-compile tool. To do this, you need to locate the directory from the LiveCode installation that contains the `.lci` files -- these are LiveCode's equivalent to C or C++'s header files. For example, on my system, I could compile the example above using (let's assume I've saved it to a file called hello.lcb:

$ export TOOLCHAIN='/opt/runrev/livecodecommunity-8.0.0-dp-3 (x86_64)/Toolchain/
$ "$TOOLCHAIN/lc-compile" --modulepath . --modulepath "$TOOLCHAIN/modules/lci" --output hello.lcm hello.lcb

These commands generate two files: hello.lcm, containing LCB bytecode, and org.example.helloworld.lci containing the interface.

Finally, you can run the program using lc-run. This is a really minimal tool that provides only the LCB virtual machine and standard library.

$ echo "Hello world!" > hello.txt
$ "$TOOLCHAIN/lc-run" hello.lcm
Hello world!

Finding out more

To more information on the standard library syntax available in LCB, visit the "LiveCode Builder" section of the dictionary in the LiveCode IDE. Note that the "widget", "engine" and "canvas" syntax isn't currently available to pure LCB programs. You should also check out the "Extending LiveCode" guide.