json: implement {tags} template Tags is pretty easy to implement. Let's start there. The output is slightly different from `hg tags -Tjson`. For reference, the CLI has the following output: [ { "node": "e2049974f9a23176c2addb61d8f5b86e0d620490", "rev": 29880, "tag": "tip", "type": "" }, ... ] Our output has the format: { "node": "0aeb19ea57a6d223bacddda3871cb78f24b06510", "tags": [ { "node": "e2049974f9a23176c2addb61d8f5b86e0d620490", "tag": "tag1", "date": [1427775457.0, 25200] }, ... ] } "rev" is omitted because it isn't a reliable identifier. We shouldn't be exposing them in web APIs and giving the impression it remotely resembles a stable identifier. Perhaps we could one day hide this behind a config option (it might be useful to expose when running servers locally). The "type" of the tag isn't defined because this information isn't yet exposed to the hgweb templater (it could be in a follow-up) and because it is questionable whether different types should be exposed at all. (Should the web interface really be exposing "local" tags?) We use an object for the outer type instead of Array for a few reasons. First, it is extensible. If we ever need to throw more global properties into the output, we can do that without breaking backwards compatibility (property additions should be backwards compatible). Second, uniformity in web APIs is nice. Having everything return objects seems much saner than a mix of array and object. Third, there are security issues with arrays in older browsers. The JSON web services world almost never uses arrays as the main type for this reason. Another possibly controversial part about this patch is how dates are defined. While JSON has a Date type, it is based on the JavaScript Date type, which is widely considered a pile of garbage. It is a non-starter for this reason. Many of Mercurial's built-in date filters drop seconds resolution. So that's a non-starter as well, since we want the API to be lossless where possible. rfc3339date, rfc822date, isodatesec, and date are all lossless. However, they each require the client to perform string parsing on top of JSON decoding. While date parsing libraries are pretty ubiquitous, some languages don't have them out of the box. However, pretty much every programming language can deal with UNIX timestamps (which are just integers or floats). So, we choose to use Mercurial's internal date representation, which in JSON is modeled as float seconds since UNIX epoch and an integer timezone offset from UTC (keep in mind JavaScript/JSON models all "Numbers" as double prevision floating point numbers, so there isn't a difference between ints and floats in JSON).