XML-RPC Client for Go

This is an implementation of client-side part of XML-RPC protocol in Go.

Usage

Add dependency to your project:

go get -u alexejk.io/go-xmlrpc

Use it by creating an *xmlrpc.Client and firing RPC method calls with Call().

package main

import(
    "fmt"

    "alexejk.io/go-xmlrpc"
)

func main() {
    client, _ := xmlrpc.NewClient("https://bugzilla.mozilla.org/xmlrpc.cgi")
    defer client.Close()
	
    result := &struct {
        BugzillaVersion struct {
            Version string
        }
    }{}

    _ = client.Call("Bugzilla.version", nil, result)
    fmt.Printf("Version: %s\n", result.BugzillaVersion.Version)
}

Customization is supported by passing a list of Option to the NewClient function. For instance:

• To customize any aspect of http.Client used to perform requests, use HttpClient option, otherwise http.DefaultClient will be used
• To pass custom headers, make use of Headers option.
• To not fail parsing when unmapped fields exist in RPC responses, use SkipUnknownFields(true) option (default is false)

Argument encoding

Arguments to the remote RPC method are passed on as a *struct. This struct is encoded into XML-RPC types based on following rules:

• Order of fields in struct type matters - fields are taken in the order they are defined on the type.
• Numbers are to be specified as int (encoded as <int>) or float64 (encoded as <double>)
• Both pointer and value references are accepted (pointers are followed to actual values)
• map[string]any types are accepted and encoded into <struct>

Shortcut:
If a single <struct> argument is expected for the RPC method call, it is sometimes more convenient to pass a map[string]any as an argument without wrapping into struct{}. This map[string]any will be encoded into a single <struct> argument with <member> elements for each key-value pair. No other key types are supported and neither is it possible to apply this approach with multiple arguments (or other types).

Order preservation:
As per XML-RPC specification, the order of <member> elements in <struct> is not defined. When using maps, order of members in a struct is undeterministic, thus it is not guaranteed that the order of <member> elements will match the order of keys in the map (due to Go not preserving the order of keys). To preserve the order, use a struct type with fields defined in the desired order (order is inherited from the struct type itself, not the instance).

Response decoding

Response is decoded following similar rules to argument encoding.

• Order of fields is important.
• Outer struct should contain exported field for each response parameter (it is possible to ignore unknown structs with SkipUnknownFields option).
• Structs may contain pointers - they will be initialized if required.
• Structs may be parsed as map[string]any, in case struct member names are not known at compile time. Map keys are enforced to string type.

Handling of Empty Values

If XML-RPC response contains no value for well-known data-types, it will be decoded into the default "empty" values as per table below:

XML-RPC Value	Default Value
<string/>	""
<int/>, <i4/>	0
<boolean/>	false
<double/>	0.0
<dateTime.iso8601/>	time.Time{}
<base64/>	nil
<array><data/><array>	nil

As per XML-RPC specification, <struct> may not have an empty list of <member> elements, thus no default "empty" value is defined for it. Similarly, <array/> is considered invalid.

Field renaming

XML-RPC specification does not necessarily specify any rules for struct's member names. Some services allow struct member names to include characters not compatible with standard Go field naming. To support these use-cases, it is possible to remap the field by use of struct tag xmlrpc.

For example, if a response value is a struct that looks like this:

<struct>
    <member>
        <name>stringValue</name>
        <value><string>bar</string></value>
    </member>
    <member>
        <name>2_numeric.Value</name>
        <value><i4>2</i4></value>
    </member>
</struct>

it would be impossible to map the second value to a Go struct with a field 2_numeric.Value as it's not valid in Go. Instead, we can map it to any valid field as follows:

v := &struct {
    StringValue string
    SecondNumericValue string `xmlrpc:"2_numeric.Value"`
}{}

Similarly, request encoding honors xmlrpc tags.

Building

To build this project, simply run make all. If you prefer building in Docker instead - make build-in-docker is your friend.