Fix: Handle failed gzip reader creation gracefully

[AngelMariages]

I've been seeing that, when a gziped response was returned by some backend where the client didn't actually Accept that encoding, the service raised a panic and it crashed.

The example that I was able to reproduce is by making the backend return a gzip response but making the request only accept identity requests.

The panic produced is because the gzip.NewReader is unable to be created, so it returns a nil pointer; this nil pointer is then what causes the crash when trying to call the defered reader.Close function.

Stacktrace:

panic: runtime error: invalid memory address or nil pointer dereference
   panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x68 pc=0x148e3ce]
goroutine 28 [running]:
compress/gzip.(*Reader).Close(0xc000542e00?)
   /usr/local/go/src/compress/gzip/gunzip.go:290 +0xe
panic({0x325b2e0?, 0x54c2bb0?})
   /usr/local/go/src/runtime/panic.go:770 +0x132
compress/gzip.(*Reader).Read(0x11c85d6?, {0xc000992000?, 0xc0008a1b98?, 0x11b9fc5?})
   /usr/local/go/src/compress/gzip/gunzip.go:247 +0x22
encoding/json.(*Decoder).refill(0xc0005ec000)
   /usr/local/go/src/encoding/json/stream.go:165 +0x188
encoding/json.(*Decoder).readValue(0xc0005ec000)
   /usr/local/go/src/encoding/json/stream.go:140 +0x85
encoding/json.(*Decoder).Decode(0xc0005ec000, {0x30b7a60, 0xc000012060})
   /usr/local/go/src/encoding/json/stream.go:63 +0x75
github.com/luraproject/lura/v2/encoding.JSONCollectionDecoder({0x3f56fc0, 0x0}, 0xc00020a038)
   /go/pkg/mod/github.com/luraproject/lura/v2@v2.9.0/encoding/encoding.go:58 +0x8a
github.com/luraproject/lura/v2/proxy.NewHTTPProxyWithHTTPExecutor.DefaultHTTPResponseParserFactory.func2({0x3f7a8a8?, 0xc0000660f0?}, 0xc000896000)
   /go/pkg/mod/github.com/luraproject/lura/v2@v2.9.0/proxy/http_response.go:47 +0x179
github.com/luraproject/lura/v2/proxy.NewHTTPProxyWithHTTPExecutor.NewHTTPProxyDetailed.func3({0x3f7a8a8, 0xc0000660f0}, 0xc000201360)
   /go/pkg/mod/github.com/luraproject/lura/v2@v2.9.0/proxy/http.go:97 +0x3a2
github.com/luraproject/lura/v2/proxy.NewLoadBalancedMiddlewareWithSubscriberAndLogger.newLoadBalancedMiddleware.func1.1({0x3f7a8a8, 0xc0000660f0}, 0xc000201360)
   /go/pkg/mod/github.com/luraproject/lura/v2@v2.9.0/proxy/balancing.go:110 +0x1d9
github.com/luraproject/lura/v2/proxy.defaultFactory.newStack.NewRequestBuilderMiddlewareWithLogger.newRequestBuilderMiddleware.func5.1({0x3f7a8a8, 0xc0000660f0}, 0xc000201360)
   /go/pkg/mod/github.com/luraproject/lura/v2@v2.9.0/proxy/http.go:120 +0xb3
github.com/luraproject/lura/v2/proxy.requestPart({0x3f7a918, 0xc000933030}, 0xc0007550b0, 0xc000201360, 0xc0001455c0, 0xc000145620)
   /go/pkg/mod/github.com/luraproject/lura/v2@v2.9.0/proxy/merging.go:347 +0x7b
created by github.com/luraproject/lura/v2/proxy.NewMergeDataMiddleware.func1.parallelMerge.3 in goroutine 26
   /go/pkg/mod/github.com/luraproject/lura/v2@v2.9.0/proxy/merging.go:165 +0xca

Config for KrakenD to reproduce it

{
	"$schema": "https://www.krakend.io/schema/v2.9/krakend.json",
	"version": 3,
	"name": "KrakenD Test",
	"port": 8080,
	"endpoints": [
		{
			"endpoint": "/test_endpoint",
			"method": "GET",
			"input_query_strings": ["featureFlags"],
			"output_encoding": "json-collection",
			"input_headers": ["*"],
			"backend": [
				{
					"url_pattern": "/test_response",
					"host": ["http://host.docker.internal:8082"],
					"encoding": "json",
					"is_collection": true,
					"extra_config": {
						"modifier/martian": {
							"fifo.Group": {
								"scope": ["request"],
								"modifiers": [
									{
										"header.Modifier": {
											"scope": ["request"],
											"name": "Accept-Encoding",
											"value": "identity;q=0"
										}
									}
								]
							}
						}
					}
				},
				{
					"url_pattern": "/test_response_2",
					"host": ["http://host.docker.internal:8082"]
				}
			]
		}
	],
	"extra_config": {
		"router": {
			"hide_version_header": true,
			"auto_options": true,
			"disable_access_log": false
		},
		"telemetry/logging": {
			"level": "DEBUG",
			"stdout": true
		}
	}
}

Minimal server:

import gzip
from io import BytesIO
from http.server import BaseHTTPRequestHandler, HTTPServer
import random

class MalformedGzipHandler(BaseHTTPRequestHandler):
    def do_GET(self):
        if self.path == '/test_response':
            self.send_response(200)
            self.send_header('Content-Encoding', 'gzip')
            self.send_header('Content-Type', 'text/plain')
            self.end_headers()

            self.wfile.write(b"This is not gzip data")
        elif self.path == '/test_response_2':
            self.send_response(200)
            self.send_header('Content-Type', 'text/plain')
            self.end_headers()

            self.wfile.write(b"This is valid data")
        else:
            self.send_response(404)
            self.end_headers()

def run(server_class=HTTPServer, handler_class=MalformedGzipHandler, port=8082):
    server_address = ('', port)
    httpd = server_class(server_address, handler_class)
    print(f"Starting server on port {port}...")
    httpd.serve_forever()

if __name__ == '__main__':
    run()

[AngelMariages]

@alombarte @kpacha whenever you can, please review this PR, as I can't add anyone as reviewer.

[alombarte]

Thanks for your contribution @ AngelMariages, we will have a look when we can. Thanks!