1 files changed, 196 insertions, 0 deletions
diff --git a/core/http.go b/core/http.go
new file mode 100644
index 0000000..8629676
--- /dev/null
+++ b/core/http.go
@@ -0,0 +1,196 @@
+// HTTP client with exponential backoff retry.
+//
+// Handles transient network failures, timeouts, and rate limiting.
+//   - Backoff: 500ms → 1s → 2s → 4s max
+//   - Jitter prevents thundering herd
+//   - Respects 429 Retry-After header
+package core
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"math/rand"
+	"net"
+	"net/http"
+	"os"
+	"strconv"
+	"strings"
+	"time"
+)
+
+
+// ============================================================================
+// ╻ ╻╺┳╸╺┳╸┏━┓   ┏━┓┏━╸╺┳╸┏━┓╻ ╻
+// ┣━┫ ┃  ┃ ┣━┛   ┣┳┛┣╸  ┃ ┣┳┛┗┳┛
+// ╹ ╹ ╹  ╹ ╹     ╹┗╸┗━╸ ╹ ╹┗╸ ╹
+// ============================================================================
+
+
+const PoliteUserAgent = "scholscan/1.0 (https://github.com/mrichman/scholscan; mailto:matt@mrichman.net)"
+
+var DefaultHTTPClient = &http.Client{
+	Timeout: 30 * time.Second,
+}
+
+var (
+	retryMaxAttempts    = 4
+	retryInitialBackoff = 500 * time.Millisecond
+	retryMaxBackoff     = 5 * time.Second
+)
+
+// Makes HTTP request with exponential backoff retry
+func DoRequestWithRetry(
+	ctx context.Context,
+	client *http.Client,
+	req *http.Request,
+) (*http.Response, error) {
+	if client == nil {
+		client = DefaultHTTPClient
+	}
+	var lastErr error
+	backoff := retryInitialBackoff
+
+	for attempt := 1; attempt <= retryMaxAttempts; attempt++ {
+		// Make the request cancellable
+		reqWithCtx := req.WithContext(ctx)
+		resp, err := client.Do(reqWithCtx)
+		if err == nil {
+			if isRetriableStatus(resp.StatusCode) {
+				retryAfter := parseRetryAfter(resp.Header.Get("Retry-After"))
+				_ = resp.Body.Close()
+				sleep := backoff
+				if retryAfter > sleep {
+					sleep = retryAfter
+				}
+
+				// Add jitter to avoid thundering herd.
+				jitter := time.Duration(rand.Intn(int(backoff / 2)))
+				sleep += jitter
+
+				// Make sleep cancellable
+				timer := time.NewTimer(sleep)
+				select {
+				case <-ctx.Done():
+					timer.Stop()
+					return nil, ctx.Err()
+				case <-timer.C:
+				}
+
+				backoff = minDuration(backoff*2, retryMaxBackoff)
+				continue
+			}
+			return resp, nil
+		}
+		// Check for context cancellation
+		if ctx.Err() != nil {
+			return nil, ctx.Err()
+		}
+		// Network error: retry on timeouts, context deadline, transient net errors, and HTTP/2 stream errors
+		if os.IsTimeout(err) || errors.Is(err, context.DeadlineExceeded) || isTransientNetError(err) || isHTTP2StreamErr(err) {
+			lastErr = err
+
+			// Add jitter to avoid thundering herd.
+			jitter := time.Duration(rand.Intn(int(backoff / 2)))
+			sleep := backoff + jitter
+
+			// Make sleep cancellable
+			timer := time.NewTimer(sleep)
+			select {
+			case <-ctx.Done():
+				timer.Stop()
+				return nil, ctx.Err()
+			case <-timer.C:
+			}
+
+			backoff = minDuration(backoff*2, retryMaxBackoff)
+			continue
+		}
+		// Non-retriable error
+		return nil, err
+	}
+	if lastErr == nil {
+		lastErr = fmt.Errorf("request retries exhausted")
+	}
+	return nil, lastErr
+}
+
+
+// ============================================================================
+// ╻ ╻┏━╸╻  ┏━┓┏━╸┏━┓┏━┓
+// ┣━┫┣╸ ┃  ┣━┛┣╸ ┣┳┛┗━┓
+// ╹ ╹┗━╸┗━╸╹  ┗━╸╹┗╸┗━┛
+// ============================================================================
+
+
+func isRetriableStatus(code int) bool {
+	if code == http.StatusTooManyRequests {
+		return true
+	}
+	return code >= 500 && code != http.StatusNotImplemented
+}
+
+func parseRetryAfter(v string) time.Duration {
+	if v == "" {
+		return 0
+	}
+	if secs, err := strconv.Atoi(strings.TrimSpace(v)); err == nil && secs > 0 {
+		return time.Duration(secs) * time.Second
+	}
+	if t, err := http.ParseTime(v); err == nil {
+		if d := time.Until(t); d > 0 {
+			return d
+		}
+	}
+	return 0
+}
+
+func minDuration(a, b time.Duration) time.Duration {
+	if a < b {
+		return a
+	}
+	return b
+}
+
+// isTransientNetError returns true for network errors which are commonly transient,
+// such as timeouts and common connection reset/closed cases.
+func isTransientNetError(err error) bool {
+	if err == nil {
+		return false
+	}
+	var ne net.Error
+	if errors.As(err, &ne) {
+		if ne.Timeout() {
+			return true
+		}
+	}
+	msg := strings.ToLower(err.Error())
+	switch {
+	case strings.Contains(msg, "use of closed network connection"):
+		return true
+	case strings.Contains(msg, "connection reset by peer"):
+		return true
+	case strings.Contains(msg, "connection aborted"):
+		return true
+	case strings.Contains(msg, "broken pipe"):
+		return true
+	case strings.Contains(msg, "eof"):
+		// Treat unexpected EOFs as transient when occurring at transport level.
+		return true
+	default:
+		return false
+	}
+}
+
+// isHTTP2StreamErr detects HTTP/2 stream-level errors which are often transient.
+func isHTTP2StreamErr(err error) bool {
+	if err == nil {
+		return false
+	}
+	msg := strings.ToLower(err.Error())
+	return strings.Contains(msg, "stream error") ||
+		strings.Contains(msg, "internal_error") ||
+		strings.Contains(msg, "rst_stream") ||
+		strings.Contains(msg, "goaway") ||
+		strings.Contains(msg, "http2:")
+}