cmd/hcldec: Allow overriding the removal of nulls

We remove properties whose values are null by default in order to produce
output that is more convenient to consume in the common case.

However, sometimes those nulls are significant, so we'll allow the user
to opt in to retaining them, at the expense of producing a result that is
more noisy if the spec contains lots of optional attributes that are not
set.

cmd/hcldec/main.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"bytes"
 	"encoding/json"
 	"fmt"
 	"io/ioutil"
@@ -30,6 +31,7 @@ var (
 	showVarRefs = flag.BoolP("var-refs", "", false, "rather than decoding input, produce a JSON description of the variables referenced by it")
 	withType    = flag.BoolP("with-type", "", false, "include an additional object level at the top describing the HCL-oriented type of the result value")
 	showVersion = flag.BoolP("version", "v", false, "show the version number and immediately exit")
+	keepNulls   = flag.BoolP("keep-nulls", "", false, "retain object properties that have null as their value (they are removed by default)")
 )
 
 var parser = hclparse.NewParser()
@@ -207,7 +209,9 @@ func realmain(args []string) error {
 	// that refers to a missing item, but that'll probably be annoying for
 	// a consumer of our output to deal with so we'll just strip those
 	// out and reduce to only the non-null values.
-	out = stripJSONNullProperties(out)
+	if !*keepNulls {
+		out = stripJSONNullProperties(out)
+	}
 
 	target := os.Stdout
 	if *outputFile != "" {
@@ -331,8 +335,11 @@ func showVarRefsJSON(vars []hcl.Traversal, ctx *hcl.EvalContext) error {
 }
 
 func stripJSONNullProperties(src []byte) []byte {
+	dec := json.NewDecoder(bytes.NewReader(src))
+	dec.UseNumber()
+
 	var v interface{}
-	err := json.Unmarshal(src, &v)
+	err := dec.Decode(&v)
 	if err != nil {
 		// We expect valid JSON
 		panic(err)