Hello Fediverse
After several years of not blogging, I've started writing the occasional post, but this time on the Fediverse. Visit my new blog on wordsmith.social:
After several years of not blogging, I've started writing the occasional post, but this time on the Fediverse. Visit my new blog on wordsmith.social:
Posted by Glyn at 5:32 PM 0 comments
Following last week's public launch of the Go-2 project, this post describes the first release and invites contributions. Go-2 is a simplified, extended subset of Google's Go programming language which addresses many of Go's short-comings long recognized by the community. Some of these gaps were highlighted recently in a blog by Gary Willoughby, particularly the lack of generics.
Steve Powell and I are experimenting with a minimal DI convention for Go which simplifies how we construct values of Go interfaces and use mocks of dependencies during testing.
Posted by Glyn at 3:41 PM 0 comments
Labels: dependency injection, golang
In order to use libcontainer as the basis for a new Warden backend, it needs upgrading. The requirements for a Container API over and above what libcontainer already provides are:
1. Multiple user processes: It must be possible to run multiple user processes in the same Container, either serially or in parallel. All user processes must run in the Container’s namespaces and control groups (so that they are all subject to the same isolation requirements and resource limits). User processes are peers in the sense that they share a common parent and one may terminate without terminating any others.
2. Container life cycle: It must be possible to create a new Container with no user processes. A Container should continue to exist after user processes have terminated.
3. Dynamic reconfiguration: It must be possible to reconfigure the Container after it has been created. There are some fundamental limitations in what can be reconfigured, but these must be kept to a bare minimum. In particular, it must be possible to reconfigure the Container’s control groups and network settings.
4. Container file system: It must be possible to share a single read-only root file system across multiple Container instances. A Container’s file system must be updatable and any updates made in one Container instance must not be visible to other Container instances.
5. Copying and streaming files: It must be possible to copy files and directories between the host and Container file systems. It must also be possible to stream files from the Container’s file system back to the host’s, for example to tail a log file.
Some of these requirements turn out to be beyond the scope of libcontainer as envisaged by its maintainers. In particular, since libcontainer's only consumer is Docker and Docker deals with file system layers, libcontainer avoids managing the read-write layer necessary to satisfy requirement 4. Requirement 5 is closely related to requirement 4.
Similarly, naming and managing collections of Container instances is deemed to be out of scope for libcontainer. So we envisage building an "ideal" Container API at a higher level than libcontainer and reusing libcontainer to provide the core function for a single Container:
Posted by Glyn at 4:07 PM 0 comments
Labels: CloudFoundry, docker, libcontainer
Posted by Glyn at 1:23 PM 10 comments
Labels: CloudFoundry, docker, libcontainer, warden
After discussing Better error handling idioms in Go and casting around a while, nothing turned up which was ideally suited to our project, so a colleague and I implemented the gerror package which captures stack traces when errors are created and enables errors to be identified without relying on the error message content.
So how does it look to a user? The first code to use it is a fileutils package which implements a file copy function in pure Go (no "shelling out" to cp). Let's take a look at a typical piece of error handling:
src, err := os.Open(source)
if err != nil {
return gerror.NewFromError(ErrOpeningSourceDir, err)
}
What does this achieve over and above the normal Go idiom of simply returning err if it is non-nil?
Firstly, it captures a stack trace in the error which appears when the error is logged (see below for an example). This gives the full context of the error which, in a large project, avoids guesswork and saves time locating the source of the error.
Secondly, it associates a "tag", in the form of an error identifier, with the error. Callers can use the tag if they want to check for particular errors programmatically:
gerr := fileutils.Copy(destPath, srcPath)
if gerr.EqualTag(fileutils.ErrOpeningSourceDir) {
...
}
Thirdly, the resultant error conforms to the builtin error interface and so can be returned or passed around wherever an error is expected.
Fourthly, by defining a function's error return type to be gerror.Gerror, the compiler prevents a "vanilla" error being returned accidentally from the function, which is useful when we want to ensure that all errors have stack traces and tags.
So how are error identifier tags defined? It's easy, as this code from fileutils shows:
type ErrorId int
const (
ErrFileNotFound ErrorId = iota
ErrOpeningSourceDir
...
)
Note that this has an advantage over the approach of using variables to refer to specific errors - variables can be overwritten (see this example of issue 7885), whereas constants cannot.
When errors are constructed, the gerror package stores the tag and its type. Both the tag and its type are included in the error string (returned, as usual, by the Error method) and are used when checking for equality in the EqualTag method.
The tag type is logically of the form package.Type which could be ambiguous if two packages had the same name, but the stack trace avoids the ambiguity. For example the following stack trace of a "file not found" error makes it clear that the tag type fileutils.ErrorId refers to the type in the package github.com/cf-guardian/guardian/kernel/fileutils:
0 fileutils.ErrorId: Error caused by: lstat /tmp/fileutils_test-027950024/src.file: no such file or directory
goroutine 8 [running]:
github.com/cf-guardian/guardian/gerror.NewFromError(0xe49a0, 0x0, 0x3484c8, 0xc21000aa80, 0x3484c8, ...)
/Users/gnormington/go/src/github.com/cf-guardian/guardian/gerror/gerror.go:68 +0x8d
github.com/cf-guardian/guardian/kernel/fileutils.fileMode(0xc21000a960, 0x26, 0xc21000a9c0, 0x29, 0x0)
/Users/gnormington/go/src/github.com/cf-guardian/guardian/kernel/fileutils/fileutils.go:196 +0x6b
github.com/cf-guardian/guardian/kernel/fileutils.doCopy(0xc21000a9c0, 0x29, 0xc21000a960, 0x26, 0xc21000a960, ...)
/Users/gnormington/go/src/github.com/cf-guardian/guardian/kernel/fileutils/fileutils.go:68 +0x87
github.com/cf-guardian/guardian/kernel/fileutils.Copy(0xc21000a9c0, 0x29, 0xc21000a960, 0x26, 0x29, ...)
/Users/gnormington/go/src/github.com/cf-guardian/guardian/kernel/fileutils/fileutils.go:61 +0x14f
...
The gerror package is available as open source on github and is licensed under the Apache v2 license. It's really a starting point and others are free to use it "as is" or adapt it to their own needs. If you have an improvement you think we might like, please read our contribution guidelines and send us a pull request.
Posted by Glyn at 2:25 PM 0 comments
Labels: diagnostics, error, FFDC, golang
How often have you seen, or written, Go code like this?
"open someFile: No such file or directory"
2009/11/10 23:00:00 "open someFile: No such file or directory"
goroutine 1 [running]:
main.main()
/tmpfs/gosandbox-xxx/prog.go:15 +0xe0
runtime.main()
/tmp/sandbox/go/src/pkg/runtime/proc.c:220 +0x1c0
runtime.goexit()
/tmp/sandbox/go/src/pkg/runtime/proc.c:1394
It is the error implementation's responsibility to summarize the context.but doesn't address the difficulty of large codebases where the immediate program context isn't always sufficient to diagnose problems.
Posted by Glyn at 10:23 AM 9 comments
Labels: diagnostics, error, FFDC, golang, logging