Go: cmd/go: get go list context function formatted as json

[jimmyfrasche]

The context template function available to go list has some useful data, even after ignoring the overlap with go env -json. There's no way to output the context as json, making it hard to use the results programatically.

cc: @bcmills @rsc @ianlancetaylor @alandonovan

[xuruiray]

go list should output a json array, but it output each element of array , i need to add , before every element

[jimmyfrasche]

@xuruiray that's unrelated to this issue. When go list -json returns a sequence of json documents. It's not returning an array in a single json document. This is extremely unlikely to change as much code relies on the current format.

[bcmills]

@xuruiray See https://github.com/golang/go/issues/27655#issuecomment-420993215.

[alandonovan]

We can't change the declaration of go/build.Context to have json: tags on its fields, as that is a backward incompatible change. I suppose it could have a MarshalJSON method.

As a workaround, can you extract the fields you need in a convenient format based on a command such as this one?

$ go list -f '{{printf "{GOROOT=%q, GOOS=%q, GOPATH=%q, CgoEnabled=%t, UseAllFiles=%t, Compiler=%q, BuildTags=%q, InstallSuffix=%q}" context.GOROOT context.GOOS context.GOPATH context.CgoEnabled context.UseAllFiles context.Compiler context.BuildTags context.InstallSuffix}}' errors

{GOROOT="/usr/local/google/home/adonovan/goroot", GOOS="linux", GOPATH="/usr/local/google/home/adonovan/go", CgoEnabled=true, UseAllFiles=false, Compiler="gc", BuildTags=[], InstallSuffix=""}

[bcmills]

We can't change the declaration of go/build.Context to have json: tags on its fields, as that is a backward incompatible change.

Could you give some more detail on that? We're allowed to add fields, so nobody should be reyling on convertibility anyway (especially given #16085), and we're allowed to add methods, so nobody should be relying on the fact that go/build.Context currently cannot be marshalled.

[alandonovan]

Oh, I forgot that we'd relax the convertibility rules. So then yes, we could add field tags to Context.

[jimmyfrasche]

The go list command actually copies the build.Context's contents into a locally defined struct for printing. It's the same as build.Context but without all the function pointers. It already has json tags, even.

[alandonovan]

So it does. One possible solution would be to add a "json" function to the set of functions available to the template, that would call json.Marshal on its operand. For example:

$ git diff
diff --git a/src/cmd/go/internal/list/list.go b/src/cmd/go/internal/list/list.go
index f3cb4e47ec..31512763df 100644
--- a/src/cmd/go/internal/list/list.go
+++ b/src/cmd/go/internal/list/list.go
@@ -344,7 +344,14 @@ func runList(cmd *base.Command, args []string) {
                fm := template.FuncMap{
                        "join":    strings.Join,
                        "context": context,
-                       "module":  modload.ModuleInfo,
+                       "json": func(x interface{}) (string, error) {
+                               y, err := json.MarshalIndent(x, "\t", "")
+                               if err != nil {
+                                       return "", err
+                               }
+                               return string(y), nil
+                       },
+                       "module": modload.ModuleInfo,
                }
                tmpl, err := template.New("main").Funcs(fm).Parse(*listFmt)
                if err != nil {
$ go list -f '{{context | json}}' errors
{
        "GOARCH": "amd64",
        "GOOS": "linux",
        "GOROOT": "/usr/local/google/home/adonovan/goroot",
        "GOPATH": "/usr/local/google/home/adonovan/go",
        "CgoEnabled": true,
        "Compiler": "gc",
        "ReleaseTags": [
        "go1.1",
        "go1.2",
        "go1.3",
        "go1.4",
        "go1.5",
        "go1.6",
        "go1.7",
        "go1.8",
        "go1.9",
        "go1.10",
        "go1.11"
        ]
        }

[jimmyfrasche]

Since this is the only thing you can't already get as json, maybe it should be a method on that local context type? Like `go list -f '{{context.Json}}'

[jimmyfrasche]

In the meantime, here's a workaround that formats BuildTags/ReleaseTags as json as well:

{{define "List"}}
    [
        {{range $i, $_ := .}}
            {{if $i}},{{end}}
            {{.|printf "%q"}}
        {{end}}
    ]
{{end}}
{{with context}}
    {
        "GOARCH":        {{.GOARCH|printf "%q"}},
        "GOOS":          {{.GOOS|printf "%q"}},
        "GOROOT":        {{.GOROOT|printf "%q"}},
        "GOPATH":        {{.GOPATH|printf "%q"}},
        "CgoEnabled":    {{.CgoEnabled}},
        "UseAllFiles":   {{.UseAllFiles}},
        "Compiler":      {{.Compiler|printf "%q"}},
        "BuildTags":     {{template "List" .BuildTags}},
        "ReleaseTags":   {{template "List" .ReleaseTags}},
        "InstallSuffix": {{.InstallSuffix|printf "%q"}}
    }
{{end}}