Populate commit→PR cache from pull_request events

Fixes cache miss race condition where check_suite/check_run events arrive before
the cache is populated, causing them to be sent with repo URL only instead of PR URL.

Now when a pull_request event arrives:
- Extract commit SHA and PR number from the event
- Populate commitPRCache[owner/repo:sha] = [pr_number]
- Handle multiple PRs for same commit (append to existing list)

This ensures check events milliseconds later find the PR in cache, eliminating
the need for clients to do their own GitHub API lookups.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: 2 people

pkg/client/client.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"log/slog"
 	"os"
+	"strconv"
 	"strings"
 	"sync"
 	"time"
@@ -93,10 +94,12 @@ type Client struct {
 	retries    int
 
 	// Cache for commit SHA to PR number lookups (for check event race condition)
-	commitPRCache   map[string][]int // key: "owner/repo:sha", value: PR numbers
-	commitCacheKeys []string         // track insertion order for LRU eviction
-	cacheMu         sync.RWMutex
-	maxCacheSize    int
+	commitPRCache       map[string][]int     // key: "owner/repo:sha", value: PR numbers
+	commitPRCacheExpiry map[string]time.Time // key: "owner/repo:sha", value: expiry time (only for empty results)
+	commitCacheKeys     []string             // track insertion order for LRU eviction
+	cacheMu             sync.RWMutex
+	maxCacheSize        int
+	emptyResultTTL      time.Duration // TTL for empty results (to handle GitHub indexing race)
 }
 
 // New creates a new robust WebSocket client.
@@ -127,13 +130,15 @@ func New(config Config) (*Client, error) {
 	}
 
 	return &Client{
-		config:          config,
-		stopCh:          make(chan struct{}),
-		stoppedCh:       make(chan struct{}),
-		logger:          logger,
-		commitPRCache:   make(map[string][]int),
-		commitCacheKeys: make([]string, 0, 512),
-		maxCacheSize:    512,
+		config:              config,
+		stopCh:              make(chan struct{}),
+		stoppedCh:           make(chan struct{}),
+		logger:              logger,
+		commitPRCache:       make(map[string][]int),
+		commitPRCacheExpiry: make(map[string]time.Time),
+		commitCacheKeys:     make([]string, 0, 512),
+		maxCacheSize:        512,
+		emptyResultTTL:      30 * time.Second, // Retry empty results after 30s
 	}, nil
 }
 
@@ -640,6 +645,64 @@ func (c *Client) readEvents(ctx context.Context, ws *websocket.Conn) error {
 		eventNum := c.eventCount
 		c.mu.Unlock()
 
+		// Populate cache from pull_request events to prevent cache misses
+		// This ensures check events arriving shortly after PR creation can find the PR
+		if event.Type == "pull_request" && event.CommitSHA != "" && strings.Contains(event.URL, "/pull/") {
+			// Extract owner/repo/pr_number from URL
+			parts := strings.Split(event.URL, "/")
+			if len(parts) >= 7 && parts[2] == "github.com" && parts[5] == "pull" {
+				owner := parts[3]
+				repo := parts[4]
+				prNum, err := strconv.Atoi(parts[6])
+				if err == nil && prNum > 0 {
+					key := owner + "/" + repo + ":" + event.CommitSHA
+
+					c.cacheMu.Lock()
+					// Check if cache entry exists
+					existing, exists := c.commitPRCache[key]
+					if !exists {
+						// New cache entry
+						c.commitCacheKeys = append(c.commitCacheKeys, key)
+						c.commitPRCache[key] = []int{prNum}
+						c.logger.Debug("Populated cache from pull_request event",
+							"commit_sha", event.CommitSHA,
+							"owner", owner,
+							"repo", repo,
+							"pr_number", prNum)
+
+						// Evict oldest 25% if cache is full
+						if len(c.commitCacheKeys) > c.maxCacheSize {
+							n := c.maxCacheSize / 4
+							for i := range n {
+								delete(c.commitPRCache, c.commitCacheKeys[i])
+							}
+							c.commitCacheKeys = c.commitCacheKeys[n:]
+						}
+					} else {
+						// Check if PR number already in list
+						found := false
+						for _, existingPR := range existing {
+							if existingPR == prNum {
+								found = true
+								break
+							}
+						}
+						if !found {
+							// Add PR to existing cache entry
+							c.commitPRCache[key] = append(existing, prNum)
+							c.logger.Debug("Added PR to existing cache entry",
+								"commit_sha", event.CommitSHA,
+								"owner", owner,
+								"repo", repo,
+								"pr_number", prNum,
+								"total_prs", len(c.commitPRCache[key]))
+						}
+					}
+					c.cacheMu.Unlock()
+				}
+			}
+		}
+
 		// Handle check events with repo-only URLs (GitHub race condition)
 		// Automatically expand into per-PR events using GitHub API with caching
 		if (event.Type == "check_run" || event.Type == "check_suite") && event.CommitSHA != "" && !strings.Contains(event.URL, "/pull/") {