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// 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:")
}
|
