cmd/hcldec: opt-in JSON-formatted diagnostics

By default we generate human-readable diagnostics on the assumption that
the caller is a simple program that is capturing stdin via a pipe and
letting stderr go to the terminal.

More sophisticated callers may wish to analyze the diagnostics themselves
and perhaps present them in a different way, such as via a GUI.

cmd/hcldec/diags_json.go

@@ -0,0 +1,101 @@
+package main
+
+import (
+	"encoding/json"
+	"io"
+
+	"github.com/hashicorp/hcl2/hcl"
+)
+
+type jsonDiagWriter struct {
+	w     io.Writer
+	diags hcl.Diagnostics
+}
+
+var _ hcl.DiagnosticWriter = &jsonDiagWriter{}
+
+func (wr *jsonDiagWriter) WriteDiagnostic(diag *hcl.Diagnostic) error {
+	wr.diags = append(wr.diags, diag)
+	return nil
+}
+
+func (wr *jsonDiagWriter) WriteDiagnostics(diags hcl.Diagnostics) error {
+	wr.diags = append(wr.diags, diags...)
+	return nil
+}
+
+func (wr *jsonDiagWriter) Flush() error {
+	if len(wr.diags) == 0 {
+		return nil
+	}
+
+	type PosJSON struct {
+		Line   int `json:"line"`
+		Column int `json:"column"`
+		Byte   int `json:"byte"`
+	}
+	type RangeJSON struct {
+		Filename string  `json:"filename"`
+		Start    PosJSON `json:"start"`
+		End      PosJSON `json:"end"`
+	}
+	type DiagnosticJSON struct {
+		Severity string     `json:"severity"`
+		Summary  string     `json:"summary"`
+		Detail   string     `json:"detail,omitempty"`
+		Subject  *RangeJSON `json:"subject,omitempty"`
+	}
+	type DiagnosticsJSON struct {
+		Diagnostics []DiagnosticJSON `json:"diagnostics"`
+	}
+
+	diagsJSON := make([]DiagnosticJSON, 0, len(wr.diags))
+	for _, diag := range wr.diags {
+		var diagJSON DiagnosticJSON
+
+		switch diag.Severity {
+		case hcl.DiagError:
+			diagJSON.Severity = "error"
+		case hcl.DiagWarning:
+			diagJSON.Severity = "warning"
+		default:
+			diagJSON.Severity = "(unknown)" // should never happen
+		}
+
+		diagJSON.Summary = diag.Summary
+		diagJSON.Detail = diag.Detail
+		if diag.Subject != nil {
+			diagJSON.Subject = &RangeJSON{}
+			sJSON := diagJSON.Subject
+			rng := diag.Subject
+			sJSON.Filename = rng.Filename
+			sJSON.Start.Line = rng.Start.Line
+			sJSON.Start.Column = rng.Start.Column
+			sJSON.Start.Byte = rng.Start.Byte
+			sJSON.End.Line = rng.End.Line
+			sJSON.End.Column = rng.End.Column
+			sJSON.End.Byte = rng.End.Byte
+		}
+
+		diagsJSON = append(diagsJSON, diagJSON)
+	}
+
+	src, err := json.MarshalIndent(DiagnosticsJSON{diagsJSON}, "", "  ")
+	if err != nil {
+		return err
+	}
+	_, err = wr.w.Write(src)
+	wr.w.Write([]byte{'\n'})
+	return err
+}
+
+type flusher interface {
+	Flush() error
+}
+
+func flush(maybeFlusher interface{}) error {
+	if f, ok := maybeFlusher.(flusher); ok {
+		return f.Flush()
+	}
+	return nil
+}

cmd/hcldec/main.go

@@ -26,6 +26,7 @@ var vars = &varSpecs{}
 var (
 	specFile    = flag.StringP("spec", "s", "", "path to spec file (required)")
 	outputFile  = flag.StringP("out", "o", "", "write to the given file, instead of stdout")
+	diagsFormat = flag.StringP("diags", "", "", "format any returned diagnostics in the given format; currently only \"json\" is accepted")
 	showVarRefs = flag.BoolP("var-refs", "", false, "rather than decoding input, produce a JSON description of the variables referenced by it")
 	showVersion = flag.BoolP("version", "v", false, "show the version number and immediately exit")
 )
@@ -35,13 +36,6 @@ var diagWr hcl.DiagnosticWriter // initialized in init
 
 func init() {
 	flag.VarP(vars, "vars", "V", "provide variables to the given configuration file(s)")
-
-	color := terminal.IsTerminal(int(os.Stderr.Fd()))
-	w, _, err := terminal.GetSize(int(os.Stdout.Fd()))
-	if err != nil {
-		w = 80
-	}
-	diagWr = hcl.NewDiagnosticTextWriter(os.Stderr, parser.Files(), uint(w), color)
 }
 
 func main() {
@@ -55,6 +49,21 @@ func main() {
 
 	args := flag.Args()
 
+	switch *diagsFormat {
+	case "":
+		color := terminal.IsTerminal(int(os.Stderr.Fd()))
+		w, _, err := terminal.GetSize(int(os.Stdout.Fd()))
+		if err != nil {
+			w = 80
+		}
+		diagWr = hcl.NewDiagnosticTextWriter(os.Stderr, parser.Files(), uint(w), color)
+	case "json":
+		diagWr = &jsonDiagWriter{w: os.Stderr}
+	default:
+		fmt.Fprintf(os.Stderr, "Invalid diagnostics format %q: only \"json\" is supported.\n", *diagsFormat)
+		os.Exit(2)
+	}
+
 	err := realmain(args)
 
 	if err != nil {
@@ -75,6 +84,7 @@ func realmain(args []string) error {
 	diags = append(diags, specDiags...)
 	if specDiags.HasErrors() {
 		diagWr.WriteDiagnostics(diags)
+		flush(diagWr)
 		os.Exit(2)
 	}
 
@@ -146,6 +156,7 @@ func realmain(args []string) error {
 
 	if diags.HasErrors() {
 		diagWr.WriteDiagnostics(diags)
+		flush(diagWr)
 		os.Exit(2)
 	}
 
@@ -170,6 +181,7 @@ func realmain(args []string) error {
 
 	if diags.HasErrors() {
 		diagWr.WriteDiagnostics(diags)
+		flush(diagWr)
 		os.Exit(2)
 	}