Implement info.post() database method.

It adds an element to a JSON array corresponding to a
key in the info table. Errors out if the value is not
an array.

lib/www/server/lib/db/info/post.js

@@ -0,0 +1,41 @@
+const { setSurvey, transaction } = require('../connection');
+
+async function post (projectId, path, payload, opts = {}) {
+	const client = await setSurvey(projectId);
+	const [key, ...jsonpath] = (path||"").split("/").filter(i => i.length);
+
+	try {
+		const text = jsonpath.length
+			? `
+				INSERT INTO info (key, value)
+				VALUES ($2, jsonb_insert('${isNaN(Number(jsonpath[0])) ? "{}" : "[]"}'::jsonb, $3, $1))
+				ON CONFLICT (key) DO UPDATE
+					SET
+						key = $2,
+						value = jsonb_insert((SELECT value FROM info WHERE key = $2), $3, $1, true)
+				RETURNING *;
+			`
+			: `
+				INSERT INTO info (key, value)
+				VALUES ($2, jsonb_insert('[]'::jsonb, '{0}', $1))
+				ON CONFLICT (key) DO UPDATE
+					SET
+						key = $2,
+						value = jsonb_insert((SELECT value FROM info WHERE key = $2), '{-1}'::text[], $1, true)
+				RETURNING *;
+			`;
+		const values = jsonpath.length ? [JSON.stringify(payload), key, jsonpath] : [JSON.stringify(payload), key];
+		await client.query(text, values);
+	} catch (err) {
+		console.error("ERROR", err);
+		if (err.code == 22023) {
+			throw {status: 400, message: "Cannot post to non-array"};
+		} else {
+			throw err;
+		}
+	} finally {
+		client.release();
+	}
+}
+
+module.exports = post;