reverseproxy: Dynamic upstreams (with SRV and A/AAAA support) (#4470)

* reverseproxy: Begin refactor to enable dynamic upstreams

Streamed here: https://www.youtube.com/watch?v=hj7yzXb11jU

* Implement SRV and A/AAA upstream sources

Also get upstreams at every retry loop iteration instead of just once
before the loop. See #4442.

* Minor tweaks from review

* Limit size of upstreams caches

* Add doc notes deprecating LookupSRV

* Provision dynamic upstreams

Still WIP, preparing to preserve health checker functionality

* Rejigger health checks

Move active health check results into handler-specific Upstreams.

Improve documentation regarding health checks and upstreams.

* Deprecation notice

* Add Caddyfile support, use `caddy.Duration`

* Interface guards

* Implement custom resolvers, add resolvers to http transport Caddyfile

* SRV: fix Caddyfile `name` inline arg, remove proto condition

* Use pointer receiver

* Add debug logs

Co-authored-by: Francis Lavoie <lavofr@gmail.com>

2 files changed, 123 insertions, 0 deletions

diff --git a/caddytest/integration/caddyfile_adapt/reverse_proxy_dynamic_upstreams.txt b/caddytest/integration/caddyfile_adapt/reverse_proxy_dynamic_upstreams.txt
new file mode 100644
index 0000000..2f2cbcd
--- /dev/null
+++ b/caddytest/integration/caddyfile_adapt/reverse_proxy_dynamic_upstreams.txt
@@ -0,0 +1,116 @@
+:8884 {
+	reverse_proxy {
+		dynamic a foo 9000
+	}
+
+	reverse_proxy {
+		dynamic a {
+			name foo
+			port 9000
+			refresh 5m
+			resolvers 8.8.8.8 8.8.4.4
+			dial_timeout 2s
+			dial_fallback_delay 300ms
+		}
+	}
+}
+
+:8885 {
+	reverse_proxy {
+		dynamic srv _api._tcp.example.com
+	}
+
+	reverse_proxy {
+		dynamic srv {
+			service api
+			proto tcp
+			name example.com
+			refresh 5m
+			resolvers 8.8.8.8 8.8.4.4
+			dial_timeout 1s
+			dial_fallback_delay -1s
+		}
+	}
+}
+
+----------
+{
+	"apps": {
+		"http": {
+			"servers": {
+				"srv0": {
+					"listen": [
+						":8884"
+					],
+					"routes": [
+						{
+							"handle": [
+								{
+									"dynamic_upstreams": {
+										"name": "foo",
+										"port": "9000",
+										"source": "a"
+									},
+									"handler": "reverse_proxy"
+								},
+								{
+									"dynamic_upstreams": {
+										"dial_fallback_delay": 300000000,
+										"dial_timeout": 2000000000,
+										"name": "foo",
+										"port": "9000",
+										"refresh": 300000000000,
+										"resolver": {
+											"addresses": [
+												"8.8.8.8",
+												"8.8.4.4"
+											]
+										},
+										"source": "a"
+									},
+									"handler": "reverse_proxy"
+								}
+							]
+						}
+					]
+				},
+				"srv1": {
+					"listen": [
+						":8885"
+					],
+					"routes": [
+						{
+							"handle": [
+								{
+									"dynamic_upstreams": {
+										"name": "_api._tcp.example.com",
+										"source": "srv"
+									},
+									"handler": "reverse_proxy"
+								},
+								{
+									"dynamic_upstreams": {
+										"dial_fallback_delay": -1000000000,
+										"dial_timeout": 1000000000,
+										"name": "example.com",
+										"proto": "tcp",
+										"refresh": 300000000000,
+										"resolver": {
+											"addresses": [
+												"8.8.8.8",
+												"8.8.4.4"
+											]
+										},
+										"service": "api",
+										"source": "srv"
+									},
+									"handler": "reverse_proxy"
+								}
+							]
+						}
+					]
+				}
+			}
+		}
+	}
+}
\ No newline at end of file
diff --git a/caddytest/integration/caddyfile_adapt/reverse_proxy_options.txt b/caddytest/integration/caddyfile_adapt/reverse_proxy_options.txt
index 70e7af6..e41b900 100644
--- a/caddytest/integration/caddyfile_adapt/reverse_proxy_options.txt
+++ b/caddytest/integration/caddyfile_adapt/reverse_proxy_options.txt
@@ -17,6 +17,7 @@ https://example.com {
 			dial_fallback_delay 5s
 			response_header_timeout 8s
 			expect_continue_timeout 9s
+			resolvers 8.8.8.8 8.8.4.4
 
 			versions h2c 2
 			compression off
@@ -88,6 +89,12 @@ https://example.com {
 														"max_response_header_size": 30000000,
 														"protocol": "http",
 														"read_buffer_size": 10000000,
+														"resolver": {
+															"addresses": [
+																"8.8.8.8",
+																"8.8.4.4"
+															]
+														},
 														"response_header_timeout": 8000000000,
 														"versions": [
 															"h2c",