fix(gallery): add fallback URI resolution for backend installation (#8663)

* fix(gallery): add fallback URI resolution for backend installation

When a backend installation fails (e.g., due to missing 'latest-' tag),
try fallback URIs in order:
1. Replace 'latest-' with 'master-' in the URI
2. If that fails, append '-development' to the backend name

This fixes the issue where backend index entries don't match the
repository tags. For example, installing 'ace-step' tries to download
'latest-gpu-nvidia-cuda-13-ace-step' but only 'master-gpu-nvidia-cuda-13-ace-step'
exists in the quay.io registry.

Fixes: #8437
Signed-off-by: localai-bot <139863280+localai-bot@users.noreply.github.com>

* chore(gallery): make fallback URI patterns configurable via env vars

---------

Signed-off-by: localai-bot <139863280+localai-bot@users.noreply.github.com>

core/gallery/backends.go

@@ -25,6 +25,39 @@ const (
 	runFile      = "run.sh"
 )
 
+// Environment variables for configurable fallback URI patterns
+const (
+	// Default fallback tag values
+	defaultLatestTag = "latest"
+	defaultMasterTag = "master"
+	defaultDevSuffix = "development"
+
+	// Environment variable names
+	envLatestTag = "LOCALAI_BACKEND_IMAGES_RELEASE_TAG"
+	envMasterTag = "LOCALAI_BACKEND_IMAGES_BRANCH_TAG"
+	envDevSuffix = "LOCALAI_BACKEND_DEV_SUFFIX"
+)
+
+// getFallbackTagValues returns the configurable fallback tag values from environment variables
+func getFallbackTagValues() (latestTag, masterTag, devSuffix string) {
+	latestTag = os.Getenv(envLatestTag)
+	masterTag = os.Getenv(envMasterTag)
+	devSuffix = os.Getenv(envDevSuffix)
+
+	// Use defaults if environment variables are not set
+	if latestTag == "" {
+		latestTag = defaultLatestTag
+	}
+	if masterTag == "" {
+		masterTag = defaultMasterTag
+	}
+	if devSuffix == "" {
+		devSuffix = defaultDevSuffix
+	}
+
+	return latestTag, masterTag, devSuffix
+}
+
 // backendCandidate represents an installed concrete backend option for a given alias
 type backendCandidate struct {
 	name    string
@@ -139,6 +172,9 @@ func InstallBackendFromGallery(ctx context.Context, galleries []config.Gallery,
 }
 
 func InstallBackend(ctx context.Context, systemState *system.SystemState, modelLoader *model.ModelLoader, config *GalleryBackend, downloadStatus func(string, string, string, float64)) error {
+	// Get configurable fallback tag values from environment variables
+	latestTag, masterTag, devSuffix := getFallbackTagValues()
+
 	// Create base path if it doesn't exist
 	err := os.MkdirAll(systemState.Backend.BackendsPath, 0750)
 	if err != nil {
@@ -182,6 +218,36 @@ func InstallBackend(ctx context.Context, systemState *system.SystemState, modelL
 				}
 			}
 
+			// Try fallback: replace latestTag + "-" with masterTag + "-" in the URI
+			fallbackURI := strings.Replace(string(config.URI), latestTag + "-", masterTag + "-", 1)
+			if fallbackURI != string(config.URI) {
+				xlog.Debug("Trying fallback URI", "original", config.URI, "fallback", fallbackURI)
+				if err := downloader.URI(fallbackURI).DownloadFileWithContext(ctx, backendPath, "", 1, 1, downloadStatus); err == nil {
+					xlog.Debug("Downloaded backend using fallback URI", "uri", fallbackURI, "backendPath", backendPath)
+					success = true
+				} else {
+					// Try another fallback: add "-" + devSuffix suffix to the backend name
+					// For example: master-gpu-nvidia-cuda-13-ace-step -> master-gpu-nvidia-cuda-13-ace-step-development
+					if !strings.Contains(fallbackURI, "-" + devSuffix) {
+						// Extract backend name from URI and add -development
+						parts := strings.Split(fallbackURI, "-")
+						if len(parts) >= 2 {
+							// Find where the backend name ends (usually the last part before the tag)
+							// Pattern: quay.io/go-skynet/local-ai-backends:master-gpu-nvidia-cuda-13-ace-step
+							lastDash := strings.LastIndex(fallbackURI, "-")
+							if lastDash > 0 {
+								devFallbackURI := fallbackURI[:lastDash] + "-" + devSuffix
+								xlog.Debug("Trying development fallback URI", "fallback", devFallbackURI)
+								if err := downloader.URI(devFallbackURI).DownloadFileWithContext(ctx, backendPath, "", 1, 1, downloadStatus); err == nil {
+									xlog.Debug("Downloaded backend using development fallback URI", "uri", devFallbackURI, "backendPath", backendPath)
+									success = true
+								}
+							}
+						}
+					}
+				}
+			}
+
 			if !success {
 				xlog.Error("Failed to download backend", "uri", config.URI, "backendPath", backendPath, "error", err)
 				return fmt.Errorf("failed to download backend %q: %v", config.URI, err)