Allow failing substituters

src/libstore/build/substitution-goal.cc

@@ -74,6 +74,9 @@ void PathSubstitutionGoal::tryNext()
     if (subs.size() == 0) {
         /* None left.  Terminate this goal and let someone else deal
            with it. */
+        if (subError.has_value()) {
+            throw std::move(*subError);
+        }
 
         /* Hack: don't indicate failure if there were no substituters.
            In that case the calling derivation should just do a
@@ -111,19 +114,12 @@ void PathSubstitutionGoal::tryNext()
     } catch (InvalidPath &) {
         tryNext();
         return;
-    } catch (SubstituterDisabled &) {
-        if (settings.tryFallback) {
-            tryNext();
-            return;
-        }
-        throw;
     } catch (Error & e) {
-        if (settings.tryFallback) {
-            logError(e.info());
-            tryNext();
-            return;
+        if (!subError.has_value()) {
+            subError = std::move(e);
         }
-        throw;
+        tryNext();
+        return;
     }
 
     if (info->path != storePath) {

src/libstore/build/substitution-goal.hh

@@ -22,6 +22,11 @@ struct PathSubstitutionGoal : public Goal
      */
     std::optional<StorePath> subPath;
 
+    /**
+     * The first error we came across while substituting.
+     */
+    std::optional<Error> subError;
+
     /**
      * The remaining substituters.
      */

src/libstore/http-binary-cache-store.cc

@@ -89,7 +89,7 @@ class HttpBinaryCacheStore : public virtual HttpBinaryCacheStoreConfig, public v
     void maybeDisable()
     {
         auto state(_state.lock());
-        if (state->enabled && settings.tryFallback) {
+        if (state->enabled) {
             int t = 60;
             printError("disabling binary cache '%s' for %s seconds", getUri(), t);
             state->enabled = false;

src/libstore/store-api.cc

@@ -592,10 +592,7 @@ void Store::querySubstitutablePathInfos(const StorePathCAMap & paths, Substituta
             } catch (InvalidPath &) {
             } catch (SubstituterDisabled &) {
             } catch (Error & e) {
-                if (settings.tryFallback)
-                    logError(e.info());
-                else
-                    throw;
+                logError(e.info());
             }
         }
     }