5127f1ef8b
This adds supports for "unmanaged" providers, or providers with process lifecycles not controlled by Terraform. These providers are assumed to be started before Terraform is launched, and are assumed to shut themselves down after Terraform has finished running. To do this, we must update the go-plugin dependency to v1.3.0, which added support for the "test mode" plugin serving that powers all this. As a side-effect of not needing to manage the process lifecycle anymore, Terraform also no longer needs to worry about the provider's binary, as it won't be used for anything anymore. Because of this, we can disable the init behavior that concerns itself with downloading that provider's binary, checking its version, and otherwise managing the binary. This is all managed on a per-provider basis, so managed providers that Terraform downloads, starts, and stops can be used in the same commands as unmanaged providers. The TF_REATTACH_PROVIDERS environment variable is added, and is a JSON encoding of the provider's address to the information we need to connect to it. This change enables two benefits: first, delve and other debuggers can now be attached to provider server processes, and Terraform can connect. This allows for attaching debuggers to provider processes, which before was difficult to impossible. Second, it allows the SDK test framework to host the provider in the same process as the test driver, while running a production Terraform binary against the provider. This allows for Go's built-in race detector and test coverage tooling to work as expected in provider tests. Unmanaged providers are expected to work in the exact same way as managed providers, with one caveat: Terraform kills provider processes and restarts them once per graph walk, meaning multiple times during most Terraform CLI commands. As unmanaged providers can't be killed by Terraform, and have no visibility into graph walks, unmanaged providers are likely to have differences in how their global mutable state behaves when compared to managed providers. Namely, unmanaged providers are likely to retain global state when managed providers would have reset it. Developers relying on global state should be aware of this. |
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|---|---|---|
| .. | ||
| internal/plugin | ||
| .gitignore | ||
| LICENSE | ||
| README.md | ||
| client.go | ||
| discover.go | ||
| error.go | ||
| go.mod | ||
| go.sum | ||
| grpc_broker.go | ||
| grpc_client.go | ||
| grpc_controller.go | ||
| grpc_server.go | ||
| grpc_stdio.go | ||
| log_entry.go | ||
| mtls.go | ||
| mux_broker.go | ||
| plugin.go | ||
| process.go | ||
| process_posix.go | ||
| process_windows.go | ||
| protocol.go | ||
| rpc_client.go | ||
| rpc_server.go | ||
| server.go | ||
| server_mux.go | ||
| stream.go | ||
| testing.go | ||
README.md
Go Plugin System over RPC
go-plugin is a Go (golang) plugin system over RPC. It is the plugin system
that has been in use by HashiCorp tooling for over 4 years. While initially
created for Packer, it is additionally in use by
Terraform, Nomad, and
Vault.
While the plugin system is over RPC, it is currently only designed to work over a local [reliable] network. Plugins over a real network are not supported and will lead to unexpected behavior.
This plugin system has been used on millions of machines across many different projects and has proven to be battle hardened and ready for production use.
Features
The HashiCorp plugin system supports a number of features:
Plugins are Go interface implementations. This makes writing and consuming plugins feel very natural. To a plugin author: you just implement an interface as if it were going to run in the same process. For a plugin user: you just use and call functions on an interface as if it were in the same process. This plugin system handles the communication in between.
Cross-language support. Plugins can be written (and consumed) by almost every major language. This library supports serving plugins via gRPC. gRPC-based plugins enable plugins to be written in any language.
Complex arguments and return values are supported. This library
provides APIs for handling complex arguments and return values such
as interfaces, io.Reader/Writer, etc. We do this by giving you a library
(MuxBroker) for creating new connections between the client/server to
serve additional interfaces or transfer raw data.
Bidirectional communication. Because the plugin system supports complex arguments, the host process can send it interface implementations and the plugin can call back into the host process.
Built-in Logging. Any plugins that use the log standard library
will have log data automatically sent to the host process. The host
process will mirror this output prefixed with the path to the plugin
binary. This makes debugging with plugins simple. If the host system
uses hclog then the log data
will be structured. If the plugin also uses hclog, logs from the plugin
will be sent to the host hclog and be structured.
Protocol Versioning. A very basic "protocol version" is supported that can be incremented to invalidate any previous plugins. This is useful when interface signatures are changing, protocol level changes are necessary, etc. When a protocol version is incompatible, a human friendly error message is shown to the end user.
Stdout/Stderr Syncing. While plugins are subprocesses, they can continue
to use stdout/stderr as usual and the output will get mirrored back to
the host process. The host process can control what io.Writer these
streams go to to prevent this from happening.
TTY Preservation. Plugin subprocesses are connected to the identical
stdin file descriptor as the host process, allowing software that requires
a TTY to work. For example, a plugin can execute ssh and even though there
are multiple subprocesses and RPC happening, it will look and act perfectly
to the end user.
Host upgrade while a plugin is running. Plugins can be "reattached"
so that the host process can be upgraded while the plugin is still running.
This requires the host/plugin to know this is possible and daemonize
properly. NewClient takes a ReattachConfig to determine if and how to
reattach.
Cryptographically Secure Plugins. Plugins can be verified with an expected checksum and RPC communications can be configured to use TLS. The host process must be properly secured to protect this configuration.
Architecture
The HashiCorp plugin system works by launching subprocesses and communicating
over RPC (using standard net/rpc or gRPC). A single
connection is made between any plugin and the host process. For net/rpc-based
plugins, we use a connection multiplexing
library to multiplex any other connections on top. For gRPC-based plugins,
the HTTP2 protocol handles multiplexing.
This architecture has a number of benefits:
-
Plugins can't crash your host process: A panic in a plugin doesn't panic the plugin user.
-
Plugins are very easy to write: just write a Go application and
go build. Or use any other language to write a gRPC server with a tiny amount of boilerplate to support go-plugin. -
Plugins are very easy to install: just put the binary in a location where the host will find it (depends on the host but this library also provides helpers), and the plugin host handles the rest.
-
Plugins can be relatively secure: The plugin only has access to the interfaces and args given to it, not to the entire memory space of the process. Additionally, go-plugin can communicate with the plugin over TLS.
Usage
To use the plugin system, you must take the following steps. These are
high-level steps that must be done. Examples are available in the
examples/ directory.
-
Choose the interface(s) you want to expose for plugins.
-
For each interface, implement an implementation of that interface that communicates over a
net/rpcconnection or over a gRPC connection or both. You'll have to implement both a client and server implementation. -
Create a
Pluginimplementation that knows how to create the RPC client/server for a given plugin type. -
Plugin authors call
plugin.Serveto serve a plugin from themainfunction. -
Plugin users use
plugin.Clientto launch a subprocess and request an interface implementation over RPC.
That's it! In practice, step 2 is the most tedious and time consuming step.
Even so, it isn't very difficult and you can see examples in the examples/
directory as well as throughout our various open source projects.
For complete API documentation, see GoDoc.
Roadmap
Our plugin system is constantly evolving. As we use the plugin system for new projects or for new features in existing projects, we constantly find improvements we can make.
At this point in time, the roadmap for the plugin system is:
Semantic Versioning. Plugins will be able to implement a semantic version. This plugin system will give host processes a system for constraining versions. This is in addition to the protocol versioning already present which is more for larger underlying changes.
What About Shared Libraries?
When we started using plugins (late 2012, early 2013), plugins over RPC were the only option since Go didn't support dynamic library loading. Today, Go supports the plugin standard library with a number of limitations. Since 2012, our plugin system has stabilized from tens of millions of users using it, and has many benefits we've come to value greatly.
For example, we use this plugin system in Vault where dynamic library loading is not acceptable for security reasons. That is an extreme example, but we believe our library system has more upsides than downsides over dynamic library loading and since we've had it built and tested for years, we'll continue to use it.
Shared libraries have one major advantage over our system which is much higher performance. In real world scenarios across our various tools, we've never required any more performance out of our plugin system and it has seen very high throughput, so this isn't a concern for us at the moment.