Adds X-Request-Id response header for Workhorse

This change configures workhorse to set the Correlation-ID value in
the response header. This is a common technique which allows errors to
be traced more easily.

This can be done through several methods: 1) users can review their
browser headers and report the value 2) nginx error pages could be
configured to report the correlation-id on the error page, allowing
users to easily report it.

Once the correlation-id has been reported, support engineers or
production staff can use this to further investigate issues using
logging infrastructure, sentry or distributed tracing.

main.go

@@ -152,7 +152,12 @@ func main() {
 		}
 	}
 
-	up := wrapRaven(correlation.InjectCorrelationID(upstream.NewUpstream(cfg)))
+	up := wrapRaven(
+		correlation.InjectCorrelationID(
+			upstream.NewUpstream(cfg),
+			correlation.WithSetResponseHeader(),
+		),
+	)
 
 	logger.Fatal(http.Serve(listener, up))
 }

vendor/gitlab.com/gitlab-org/labkit/correlation/inbound_http.go

@@ -11,7 +11,6 @@ import (
 // Whether the Correlation-ID is generated or propagated, once inside this handler the request context
 // will have a Correlation-ID associated with it.
 func InjectCorrelationID(h http.Handler, opts ...InboundHandlerOption) http.Handler {
-	// Currently we don't use any of the options available
 	config := applyInboundHandlerOptions(opts)
 
 	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
@@ -26,6 +25,10 @@ func InjectCorrelationID(h http.Handler, opts ...InboundHandlerOption) http.Hand
 			correlationID = generateRandomCorrelationIDWithFallback(r)
 		}
 
+		if config.sendResponseHeader {
+			setResponseHeader(w, correlationID)
+		}
+
 		ctx := ContextWithCorrelation(parent, correlationID)
 		h.ServeHTTP(w, r.WithContext(ctx))
 	})
@@ -34,3 +37,7 @@ func InjectCorrelationID(h http.Handler, opts ...InboundHandlerOption) http.Hand
 func extractFromRequest(r *http.Request) string {
 	return r.Header.Get(propagationHeader)
 }
+
+func setResponseHeader(w http.ResponseWriter, correlationID string) {
+	w.Header().Set(propagationHeader, correlationID)
+}

vendor/gitlab.com/gitlab-org/labkit/correlation/inbound_http_options.go

@@ -2,7 +2,8 @@ package correlation
 
 // The configuration for InjectCorrelationID
 type inboundHandlerConfig struct {
-	propagation bool
+	propagation        bool
+	sendResponseHeader bool
 }
 
 // InboundHandlerOption will configure a correlation handler
@@ -29,3 +30,11 @@ func WithPropagation() InboundHandlerOption {
 		config.propagation = true
 	}
 }
+
+// WithSetResponseHeader will configure the handler to set the correlation_id
+// in the http response headers
+func WithSetResponseHeader() InboundHandlerOption {
+	return func(config *inboundHandlerConfig) {
+		config.sendResponseHeader = true
+	}
+}

vendor/gitlab.com/gitlab-org/labkit/tracing/doc.go

+/*
+Package `tracing` is the primary entrypoint into LabKit's distributed tracing functionality.
+
+(This documentation assumes some minimal knowledge of Distributed Tracing, and uses
+tracing terminology without providing definitions. Please review
+https://opentracing.io/docs/overview/what-is-tracing/ for an broad overview of distributed
+tracing if you are not familiar with the technology)
+
+Internally the `tracing` package relies on Opentracing, but avoids leaking this abstraction.
+In theory, LabKit could replace Opentracing with another distributed tracing interface, such
+as Zipkin or OpenCensus, without needing to make changes to the application (other than vendoring
+in a new version of LabKit, of course).
+
+This design decision is deliberate: the package should not leak the underlying tracing implementation.
+
+The package provides three primary exports:
+
+* `tracing.Initialize()` for initializing the global tracer using the `GITLAB_TRACING` environment variable.
+* An HTTP Handler middleware, `tracing.Handler()`, for instrumenting incoming HTTP requests.
+* An HTTP RoundTripper, `tracing.NewRoundTripper()` for instrumenting outbound HTTP requests to other services.
+
+The provided example in `example_test.go` demonstrates usage of both the HTTP Middleware and the HTTP RoundTripper.
+
+*Initializing the global tracer*
+
+Opentracing makes use of a global tracer. Opentracing ships with a default NoOp tracer which does
+nothing at all. This is always configured, meaning that, without initialization, Opentracing does nothing and
+has a very low overhead.
+
+LabKit's tracing is configured through an environment variable, `GITLAB_TRACING`. This environment variable contains
+a "connection string"-like configuration, such as:
+
+* `opentracing://jaeger?udp_endpoint=localhost:6831`
+* `opentracing://datadog`
+* `opentracing://lightstep`
+
+The parameters for these connection-strings are implementation specific.
+
+This configuration is identical to the one used to configure GitLab's ruby tracing libraries in the `Gitlab::Tracing`
+package. Having a consistent configuration makes it easy to configure multiple processes at the same time. For example,
+in GitLab Development Kit, tracing can be configured with a single environment variable, `GITLAB_TRACING=... gdk run`,
+since `GITLAB_TRACING` will configure Workhorse (written in Go), Gitaly (written in Go) and GitLab's rails components,
+using the same configuration.
+
+*Compiling applications with Tracing support*
+
+Go's Opentracing interface does not allow tracing implementations to be loaded dynamically; implementations need to be
+compiled into the application. With LabKit, this is done conditionally, using build tags. Two build tags need to be
+specified:
+
+* `tracer_static` - this compiles in the static plugin registry support
+* `tracer_static_[DRIVER_NAME]` - this compile in support for the given driver.
+
+For example, to compile support for Jaeger, compile your Go app with `tracer_static,tracer_static_jaeger`
+
+Note that multiple (or all) drivers can be compiled in alongside one another: using the tags:
+`tracer_static,tracer_static_jaeger,tracer_static_lightstep,tracer_static_datadog`
+
+If the `GITLAB_TRACING` environment variable references an unknown or unregistered driver, it will log a message
+and continue without tracing. This is a deliberate decision: the risk of bringing down a cluster during a rollout
+with a misconfigured tracer configuration is greater than the risk of an operator loosing some time because
+their application was not compiled with the correct tracers.
+
+*Using the HTTP Handler middleware to instrument incoming HTTP requests*
+
+When an incoming HTTP request arrives on the server, it may already include Distributed Tracing headers,
+propagated from an upstream service.
+
+The tracing middleware will attempt to extract the tracing information from the headers (the exact headers used are
+tracing implementation specific), set up a span and pass the information through the request context.
+
+It is up to the Opentracing implementation to decide whether the span will be sent to the tracing infrastructure.
+This will be implementation-specific, but generally relies on server load, sampler configuration, whether an
+error occurred, whether certain spans took an anomalous amount of time, etc.
+
+*Using the  HTTP RoundTripper to instrument outgoing HTTP requests*
+
+The RoundTripper should be added to the HTTP client RoundTripper stack (see the example). When an outbound
+HTTP request is sent from the HTTP client, the RoundTripper will determine whether there is an active span
+and if so, will inject headers into the outgoing HTTP request identifying the span. The details of these
+headers is implementation specific.
+
+It is important to ensure that the context is passed into the outgoing request, using `req.WithContext(ctx)`
+so that the correct span information can be injected into the request headers.
+*/
+package tracing

vendor/vendor.json

@@ -603,58 +603,58 @@
 			"versionExact": "v0.71.0"
 		},
 		{
-			"checksumSHA1": "R6fNN36q3UydLODupK0vdx9h/CY=",
+			"checksumSHA1": "WMOuBgCyclqy+Mqunb0NbykaC4Y=",
 			"path": "gitlab.com/gitlab-org/labkit/correlation",
-			"revision": "2a3e1f5415a890402696dcbb2c31b5a79c17b579",
-			"revisionTime": "2019-01-08T10:46:58Z",
+			"revision": "4b0e4d5df664c0f0680f3479ce7cc3378f04d40e",
+			"revisionTime": "2019-02-11T18:00:21Z",
 			"version": "master",
 			"versionExact": "master"
 		},
 		{
 			"checksumSHA1": "UFBFulprWZHuL9GHhjCKoHXm+Ww=",
 			"path": "gitlab.com/gitlab-org/labkit/correlation/grpc",
-			"revision": "2a3e1f5415a890402696dcbb2c31b5a79c17b579",
-			"revisionTime": "2019-01-08T10:46:58Z",
+			"revision": "4b0e4d5df664c0f0680f3479ce7cc3378f04d40e",
+			"revisionTime": "2019-02-11T18:00:21Z",
 			"version": "master",
 			"versionExact": "master"
 		},
 		{
 			"checksumSHA1": "YyeJ2t/STDZj3X4xetPzTC0I9hU=",
 			"path": "gitlab.com/gitlab-org/labkit/correlation/raven",
-			"revision": "2a3e1f5415a890402696dcbb2c31b5a79c17b579",
-			"revisionTime": "2019-01-08T10:46:58Z",
+			"revision": "4b0e4d5df664c0f0680f3479ce7cc3378f04d40e",
+			"revisionTime": "2019-02-11T18:00:21Z",
 			"version": "master",
 			"versionExact": "master"
 		},
 		{
-			"checksumSHA1": "JAe9JIwWcJvTUxCbdOPAZxaXIz8=",
+			"checksumSHA1": "27mCUL3y0j+3NB/+YLWxQLTiSIU=",
 			"path": "gitlab.com/gitlab-org/labkit/tracing",
-			"revision": "2a3e1f5415a890402696dcbb2c31b5a79c17b579",
-			"revisionTime": "2019-01-08T10:46:58Z",
+			"revision": "4b0e4d5df664c0f0680f3479ce7cc3378f04d40e",
+			"revisionTime": "2019-02-11T18:00:21Z",
 			"version": "master",
 			"versionExact": "master"
 		},
 		{
 			"checksumSHA1": "uIvjqXAsMQK/Y5FgWRaGydYGbYs=",
 			"path": "gitlab.com/gitlab-org/labkit/tracing/connstr",
-			"revision": "2a3e1f5415a890402696dcbb2c31b5a79c17b579",
-			"revisionTime": "2019-01-08T10:46:58Z",
+			"revision": "4b0e4d5df664c0f0680f3479ce7cc3378f04d40e",
+			"revisionTime": "2019-02-11T18:00:21Z",
 			"version": "master",
 			"versionExact": "master"
 		},
 		{
 			"checksumSHA1": "IE38In/zPKpmKqvWAAThyaufQak=",
 			"path": "gitlab.com/gitlab-org/labkit/tracing/grpc",
-			"revision": "2a3e1f5415a890402696dcbb2c31b5a79c17b579",
-			"revisionTime": "2019-01-08T10:46:58Z",
+			"revision": "4b0e4d5df664c0f0680f3479ce7cc3378f04d40e",
+			"revisionTime": "2019-02-11T18:00:21Z",
 			"version": "master",
 			"versionExact": "master"
 		},
 		{
 			"checksumSHA1": "hB59Es/WTWfBPLSAheQaRyHGSXA=",
 			"path": "gitlab.com/gitlab-org/labkit/tracing/impl",
-			"revision": "2a3e1f5415a890402696dcbb2c31b5a79c17b579",
-			"revisionTime": "2019-01-08T10:46:58Z",
+			"revision": "4b0e4d5df664c0f0680f3479ce7cc3378f04d40e",
+			"revisionTime": "2019-02-11T18:00:21Z",
 			"version": "master",
 			"versionExact": "master"
 		},