Test the processing and upstream connection limits (closes #64)

Failing tests for two limits that do not exist yet. Config:
max_concurrent_processing and upstream_connections, their defaults, and
valid and invalid values from the file and the environment. Image
processor: never more images at once than its limit, waiting and then
failing with ErrTooManyImages when no slot frees, and freeing its slot on
every error. Fetcher: connections to all hosts counted together, apart
from the per-host limit, and freed on errors. Both image routes answer
503 when either wait gives up. TestEnvironmentSetsEveryKey sets the two
new variables, as it compares the whole config. The tests do not compile
until the limits exist.

Model: opus-5-5

internal/httpfetcher/max_connections_internal_test.go

@@ -0,0 +1,128 @@
+package httpfetcher
+
+import (
+	"errors"
+	"net"
+	"strconv"
+	"testing"
+	"time"
+)
+
+// imageURLOnPort is the fake upstream's image route on testPublicHost at
+// port. Each port is a different host to the per-host limit, while the test
+// dialer sends every port to the one test server.
+func imageURLOnPort(port int) string {
+	return "http://" + net.JoinHostPort(testPublicHost, strconv.Itoa(port)) +
+		"/image"
+}
+
+func TestDefaultConfigMaxConnections(t *testing.T) {
+	t.Parallel()
+
+	if got := DefaultConfig().MaxConnections; got != DefaultMaxConnections {
+		t.Errorf("MaxConnections = %d, want %d", got, DefaultMaxConnections)
+	}
+}
+
+// TestFetchLimitsConnectionsToAllHostsTogether checks that MaxConnections
+// counts the fetches to every host together, apart from the per-host
+// limit: with MaxConnections at 2 and two responses open from two hosts, a
+// fetch from a third host, which has nothing open, waits the whole wait
+// timeout and fails with ErrTooManyConnections. Closing one response lets
+// it through.
+func TestFetchLimitsConnectionsToAllHostsTogether(t *testing.T) {
+	t.Parallel()
+
+	srv := startUpstream(t)
+
+	cfg := DefaultConfig()
+	cfg.MaxConnections = 2
+
+	f, _ := newServerFetcher(t, srv, cfg)
+	f.connectionWaitTimeout = 100 * time.Millisecond
+
+	first, err := f.Fetch(testContext(t), imageURLOnPort(81))
+	if err != nil {
+		t.Fatalf("first Fetch() error = %v", err)
+	}
+
+	second, err := f.Fetch(testContext(t), imageURLOnPort(82))
+	if err != nil {
+		t.Fatalf("second Fetch() error = %v", err)
+	}
+
+	defer func() { _ = second.Content.Close() }()
+
+	start := time.Now()
+
+	_, err = f.Fetch(testContext(t), imageURLOnPort(83))
+	if !errors.Is(err, ErrTooManyConnections) {
+		t.Fatalf("third Fetch() error = %v, want ErrTooManyConnections", err)
+	}
+
+	if waited := time.Since(start); waited < f.connectionWaitTimeout {
+		t.Errorf("third Fetch() failed after %v, before waiting %v",
+			waited, f.connectionWaitTimeout)
+	}
+
+	if held := semLen(f, testPublicHost+":83"); held != 0 {
+		t.Errorf("the refused fetch kept its host's slot: %d held", held)
+	}
+
+	err = first.Content.Close()
+	if err != nil {
+		t.Fatalf("close first body: %v", err)
+	}
+
+	third, err := f.Fetch(testContext(t), imageURLOnPort(83))
+	if err != nil {
+		t.Fatalf("Fetch() after a response was closed: error = %v", err)
+	}
+
+	_ = third.Content.Close()
+}
+
+// TestFetchReleasesConnectionOnError checks that a fetch that fails after
+// taking its connection gives it back: with MaxConnections at 1, the slot
+// must be free after the failure and the next fetch must succeed.
+func TestFetchReleasesConnectionOnError(t *testing.T) {
+	t.Parallel()
+
+	cases := []struct {
+		name string
+		url  string
+		want error
+	}{
+		{"upstream answers 500", upstreamURL("/status/500"), ErrUpstreamError},
+		{"upstream sends HTML", upstreamURL("/html"), ErrInvalidContentType},
+		// 198.51.100.7 (TEST-NET-2) passes the SSRF checks, and the test
+		// dialer refuses every host but testPublicHost.
+		{"connecting fails", "http://198.51.100.7/image", errUnexpectedDial},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			srv := startUpstream(t)
+
+			cfg := DefaultConfig()
+			cfg.MaxConnections = 1
+
+			f, _ := newServerFetcher(t, srv, cfg)
+			f.connectionWaitTimeout = 100 * time.Millisecond
+
+			_, err := f.Fetch(testContext(t), tc.url)
+			if !errors.Is(err, tc.want) {
+				t.Fatalf("Fetch() error = %v, want %v", err, tc.want)
+			}
+
+			if held := len(f.allHostsSemaphore); held != 0 {
+				t.Fatalf("connection still held after the error: %d held", held)
+			}
+
+			res := fetchImage(t, f, "/image")
+			_ = res.Content.Close()
+		})
+	}
+}