fix(retry): retry transient edge answers centrally, fix stale task test (#57)

- Route HTTP helpers (getJSON/formRequest/deleteReq/postJSON/putJSON/
  multipart upload), Query and AuthenticateContext/ensureToken through the
  deterministic 429/502/503/504 retry (retryRaw), so the integration suite
  no longer fails on the shared openresty rate limit under parallel runs.
- Fix stale cmd/office/fetch integration test: loader.TaskFields was
  replaced by loader.DetailForm (broke go vet -tags=integration).
- Add unit tests for Transient/DoRetry.

retry_test.go

@@ -0,0 +1,88 @@
+package onlyoffice
+
+import (
+	"context"
+	"errors"
+	"testing"
+	"time"
+)
+
+func TestTransient(t *testing.T) {
+	cases := []struct {
+		err  error
+		want bool
+	}{
+		{nil, false},
+		{errors.New("boom"), false},
+		{errors.New("GET /api/2.0/files/1.json: 429 Too Many Requests"), true},
+		{errors.New("auth: 502 bad gateway"), true},
+		{errors.New("POST /x: 503 service unavailable"), true},
+		{errors.New("PUT /x: 504 gateway timeout"), true},
+		{errors.New("GET /x: 404 not found"), false},
+		{errors.New("GET /x: 4290 not a code"), false},
+	}
+	for _, c := range cases {
+		if got := Transient(c.err); got != c.want {
+			t.Errorf("Transient(%v) = %v, want %v", c.err, got, c.want)
+		}
+	}
+}
+
+func TestDoRetryRetriesTransientThenSucceeds(t *testing.T) {
+	p := RetryPolicy{Attempts: 5, Base: time.Millisecond, Max: 5 * time.Millisecond}
+	attempts := 0
+	err := DoRetry(context.Background(), p, func() error {
+		attempts++
+		if attempts < 3 {
+			return errors.New("GET /x: 429 Too Many Requests")
+		}
+		return nil
+	})
+	if err != nil {
+		t.Fatalf("DoRetry: %v", err)
+	}
+	if attempts != 3 {
+		t.Fatalf("attempts = %d, want 3", attempts)
+	}
+}
+
+func TestDoRetryStopsOnPermanentError(t *testing.T) {
+	p := RetryPolicy{Attempts: 5, Base: time.Millisecond, Max: time.Millisecond}
+	attempts := 0
+	err := DoRetry(context.Background(), p, func() error {
+		attempts++
+		return errors.New("GET /x: 404 not found")
+	})
+	if err == nil {
+		t.Fatal("want error")
+	}
+	if attempts != 1 {
+		t.Fatalf("attempts = %d, want 1", attempts)
+	}
+}
+
+func TestDoRetryExhaustsAttemptsOnPersistentTransient(t *testing.T) {
+	p := RetryPolicy{Attempts: 3, Base: time.Millisecond, Max: time.Millisecond}
+	attempts := 0
+	err := DoRetry(context.Background(), p, func() error {
+		attempts++
+		return errors.New("GET /x: 429 Too Many Requests")
+	})
+	if !Transient(err) {
+		t.Fatalf("want transient error, got %v", err)
+	}
+	if attempts != 3 {
+		t.Fatalf("attempts = %d, want 3", attempts)
+	}
+}
+
+func TestDoRetryHonoursContextCancellation(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+	err := DoRetry(ctx, RetryPolicy{Attempts: 3, Base: time.Millisecond, Max: time.Millisecond}, func() error {
+		return errors.New("GET /x: 429 Too Many Requests")
+	})
+	if !errors.Is(err, context.Canceled) {
+		t.Fatalf("want context.Canceled, got %v", err)
+	}
+}