internal/frontend: allow refetch of paths that previously failed

When a fetch request results in a 404, it's possible that the module
path will appear later in the index. Similarly, when a fetch request
results in a 480 or 500, it's possible that the error was transient or
has been fixed but not yet reprocessed.

In these cases, we should allow the user to refetch the path after the
taskIDChangeInterval has passed.

For golang/go#36811
For golang/go#37002

Change-Id: Ia4813140ca793b7b91d80bdc196f61af075766ca
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/244601
Reviewed-by: Jonathan Amsterdam <jba@google.com>

internal/frontend/fetch.go

@@ -187,7 +187,7 @@ func (s *Server) checkPossibleModulePaths(ctx context.Context, db *postgres.DB,
 			// Before enqueuing the module version to be fetched, check if we
 			// have already attempted to fetch it in the past. If so, just
 			// return the result from that fetch process.
-			fr := checkForPath(ctx, db, fullPath, modulePath, requestedVersion)
+			fr := checkForPath(ctx, db, fullPath, modulePath, requestedVersion, s.taskIDChangeInterval)
 			if !shouldQueue || fr.status != statusNotFoundInVersionMap {
 				results[i] = fr
 				return
@@ -200,7 +200,7 @@ func (s *Server) checkPossibleModulePaths(ctx context.Context, db *postgres.DB,
 			}
 			// After the fetch request is enqueued, poll the database until it has been
 			// inserted or the request times out.
-			fr = pollForPath(ctx, db, pollEvery, fullPath, modulePath, requestedVersion)
+			fr = pollForPath(ctx, db, pollEvery, fullPath, modulePath, requestedVersion, s.taskIDChangeInterval)
 			logf := log.Infof
 			if fr.status == http.StatusInternalServerError {
 				logf = log.Errorf
@@ -270,7 +270,7 @@ func displayPath(path, version string) string {
 
 // pollForPath polls the database until a row for fullPath is found.
 func pollForPath(ctx context.Context, db *postgres.DB, pollEvery time.Duration,
-	fullPath, modulePath, requestedVersion string) *fetchResult {
+	fullPath, modulePath, requestedVersion string, taskIDChangeInterval time.Duration) *fetchResult {
 	fr := &fetchResult{modulePath: modulePath}
 	defer derrors.Wrap(&fr.err, "pollForRedirectURL(%q, %q, %q)", modulePath, fullPath, requestedVersion)
 	ticker := time.NewTicker(pollEvery)
@@ -285,7 +285,7 @@ func pollForPath(ctx context.Context, db *postgres.DB, pollEvery time.Duration,
 		case <-ticker.C:
 			ctx2, cancel := context.WithTimeout(ctx, pollEvery)
 			defer cancel()
-			fr = checkForPath(ctx2, db, fullPath, modulePath, requestedVersion)
+			fr = checkForPath(ctx2, db, fullPath, modulePath, requestedVersion, taskIDChangeInterval)
 			if fr.status != statusNotFoundInVersionMap {
 				return fr
 			}
@@ -299,7 +299,8 @@ func pollForPath(ctx context.Context, db *postgres.DB, pollEvery time.Duration,
 // process that was initiated is not yet complete.  If the row exists version_map
 // but not paths, it means that a module was found at the requestedVersion, but
 // not the fullPath, so errPathDoesNotExistInModule is returned.
-func checkForPath(ctx context.Context, db *postgres.DB, fullPath, modulePath, requestedVersion string) (fr *fetchResult) {
+func checkForPath(ctx context.Context, db *postgres.DB,
+	fullPath, modulePath, requestedVersion string, taskIDChangeInterval time.Duration) (fr *fetchResult) {
 	defer func() {
 		// Based on
 		// https://github.com/lib/pq/issues/577#issuecomment-298341053, it seems
@@ -344,7 +345,26 @@ func checkForPath(ctx context.Context, db *postgres.DB, fullPath, modulePath, re
 		goModPath:  vm.GoModPath,
 	}
 	switch fr.status {
-	case http.StatusNotFound:
+	case http.StatusNotFound,
+		derrors.ToStatus(derrors.DBModuleInsertInvalid),
+		http.StatusInternalServerError:
+		if time.Since(vm.UpdatedAt) > taskIDChangeInterval {
+			// If the duration of taskIDChangeInterval has passed since
+			// a module_path was last inserted into version_map with a failed status,
+			// treat that data as expired.
+			//
+			// It is possible that the module has appeared in the Go Module
+			// Mirror during that time, the failure was transient, or the
+			// error has been fixed but the module version has not yet been
+			// reprocessed.
+			//
+			// Return statusNotFoundInVersionMap here, so that the fetch
+			// request will try to fetch this module version again.
+			// Since the taskIDChangeInterval has passed, it is now possible to
+			// enqueue that module version to the frontend task queue again.
+			fr.status = statusNotFoundInVersionMap
+			return fr
+		}
 		// The version_map indicates that the proxy returned a 404/410.
 		fr.err = errModuleDoesNotExist
 		return fr