Add JSON mutations to raw HTTP

[gpahal]

This adds Json mutations to Dgraph's HTTP server.

Syntax:

{
    "set": [{ "name": "Alice" }],
    "delete": { "uid": "0x1", "name": null }
}

Possible limitations/problems:

• Parses the JSON twice: first for veryfing json and separating set and delete and second time to actually perform the mutation on api.Mutation
• Requires header indicating the type is JSON so that error handling is better and json mutations are not slowed down because of first checking for nquad mutations

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This change is 

[manishrjain]

Reviewed 3 of 3 files at r1.
Review status: all files reviewed at latest revision, all discussions resolved, some commit checks failed.

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dgraph/cmd/server/http.go, line 173 at r1 (raw file):

		// Parse JSON.
		ms := make(map[string]interface{})
		err := json.Unmarshal(m, &ms)

JSON unmarshal needs to be worked upon. So, let's consolidate that in one place. Can you use the same method that grpc uses to unmarshal JSON; so it's all in one place?

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Comments from Reviewable

[gpahal]

Review status: all files reviewed at latest revision, 1 unresolved discussion, some commit checks failed.

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dgraph/cmd/server/http.go, line 173 at r1 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

JSON unmarshal needs to be worked upon. So, let's consolidate that in one place. Can you use the same method that grpc uses to unmarshal JSON; so it's all in one place?

This JSON marshal only validates the JSON and checks its "set" and "delete" keys. It doesn't do anything with the values or numbers. And the rest of the function requires mu *api.Mutation which has SetJson and DeleteJson as unparsed JSON strings, so anyways we need the unparsed JSON. To avoid this, we would need to change the interface of edgraph.Mutate and nquadsToJson.

I didn't change much because I wanted to introduce this change without affecting any other code.

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Comments from Reviewable

[gpahal]

@manishrjain ^

[manishrjain]

Review status: all files reviewed at latest revision, all discussions resolved, some commit checks failed.

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dgraph/cmd/server/http.go, line 172 at r1 (raw file):

	if mType := r.Header.Get("X-Dgraph-MutationType"); mType == "json" {
		// Parse JSON.
		ms := make(map[string]interface{})

map[string][]byte? So, you don't need to marshal below.

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Comments from Reviewable

[gpahal]

Review status: 0 of 3 files reviewed at latest revision, 1 unresolved discussion.

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dgraph/cmd/server/http.go, line 172 at r1 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

map[string][]byte? So, you don't need to marshal below.

Done.

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Comments from Reviewable

[manishrjain]

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Review status: all files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable