feat: compute fallback Go versions

internal/server/backendinfo/fallback.go

@@ -0,0 +1,32 @@
+package backendinfo
+
+import (
+	"strconv"
+	"strings"
+)
+
+func prefillFallbacks(info *BackendVersions) {
+	if info.PreviousStable == "" {
+		info.PreviousStable = guessPreviousVersion(info.CurrentStable)
+	}
+
+	if info.Nightly == "" {
+		info.Nightly = "devel"
+	}
+}
+
+func guessPreviousVersion(baseVer string) string {
+	chunks := strings.Split(baseVer, ".")
+	if len(chunks) < 2 {
+		return baseVer
+	}
+
+	minorVer, err := strconv.Atoi(chunks[1])
+	if err != nil {
+		return baseVer
+	}
+
+	minorVer = max(0, minorVer-1)
+	return chunks[0] + "." + strconv.Itoa(minorVer) + ".0"
+}
+

internal/server/backendinfo/service.go

@@ -77,6 +77,10 @@ func (svc *BackendVersionService) visitCache() (*cacheEntry, error) {
 		return svc.memCache, nil
 	}
 
+	if svc.cfg.CacheFile == "" {
+		return nil, fs.ErrNotExist
+	}
+
 	f, err := os.Open(svc.cfg.CacheFile)
 	if err != nil {
 		return nil, err
@@ -132,6 +136,10 @@ func (svc *BackendVersionService) cacheVersions(versions *BackendVersions) error
 		Data:      *versions,
 	}
 
+	if svc.cfg.CacheFile == "" {
+		return nil
+	}
+
 	err := os.MkdirAll(filepath.Dir(svc.cfg.CacheFile), 0755)
 	if err != nil {
 		return fmt.Errorf("MkdirAll failed: %w", err)
@@ -174,8 +182,20 @@ func (svc *BackendVersionService) pullBackendVersions(ctx context.Context) (*Bac
 			svc.logger.Debug("Fetching go version for backend", zap.String("backend", e.backend))
 			result, err := svc.fetchGoBackendVersionWithRetry(gCtx, e.backend)
 			if err != nil {
-				return fmt.Errorf("failed to get Go version from Go playground server for backend %q: %w",
-					b.backend, err)
+				// Playground "gotip" and "goprev" backends are often broken
+				// and I'm getting tired of seeing 5xx responses if just one of them is dead.
+				//
+				// Throw only if stable version is down. For others - try to figure out fallback values.
+				if e.backend == goplay.BackendGoCurrent {
+					return fmt.Errorf("failed to get Go version from Go playground server for backend %q: %w",
+						b.backend, err)
+				}
+
+				svc.logger.Warn(
+					"can't fetch Go version for backend, will use fallback",
+					zap.String("backend", e.backend), zap.Error(err),
+				)
+				return nil
 			}
 
 			// We don't afraid race condition because each backend is written to a separate address
@@ -188,6 +208,7 @@ func (svc *BackendVersionService) pullBackendVersions(ctx context.Context) (*Bac
 		return nil, err
 	}
 
+	prefillFallbacks(versionInfo)
 	return versionInfo, nil
 }
 

internal/server/backendinfo/service_test.go

@@ -130,6 +130,27 @@ func TestBackendVersionService_GetRemoteVersions(t *testing.T) {
 				require.Equal(t, expect, dst.Data)
 			},
 		},
+		"should prefill fallbacks if one on backends is down": {
+			expect: BackendVersions{
+				CurrentStable:  "1.23.1",
+				PreviousStable: "1.22.0",
+				Nightly:        "devel",
+			},
+			constructor: func(t *testing.T, expect BackendVersions) *BackendVersionService {
+				srv := setupTestServer(BackendVersions{
+					CurrentStable: expect.CurrentStable,
+				})
+
+				logger := testutil.GetLogger(t).Desugar()
+				c := goplay.NewClient(srv.URL, "", 5 * time.Second)
+
+				svc := NewBackendVersionService(logger, c, ServiceConfig{
+					TTL: time.Hour,
+				})
+
+				return svc
+			},
+		},
 	}
 
 	for n, c := range cases {
@@ -177,6 +198,12 @@ func setupTestServer(expects BackendVersions) *httptest.Server {
 			msg = expects.CurrentStable
 		}
 
+		// Simulate prod down if empty
+		if msg == "" {
+			w.WriteHeader(http.StatusBadGateway)
+			return
+		}
+
 		rsp := goplay.CompileResponse{
 			Events: []*goplay.CompileEvent{
 				{