cf: check downloads repo before attempting download of mod in modpack

src/main/java/me/itzg/helpers/curseforge/CurseForgeApiClient.java

@@ -218,18 +218,19 @@ public Mono<CurseForgeFile> getModFileInfo(
                 }
                 return e;
             })
-            .map(GetModFileResponse::getData);
+            .map(GetModFileResponse::getData)
+            .checkpoint();
     }
 
     public Mono<Path> download(CurseForgeFile cfFile, Path outputFile, FileDownloadStatusHandler handler) {
-        return preparedFetch.fetch(
-                cfFile.getDownloadUrl() != null ?
-                    normalizeDownloadUrl(cfFile.getDownloadUrl())
-                    : downloadFallbackUriBuilder.resolve(
-                        "/v1/mods/{modId}/files/{fileId}/download",
-                        cfFile.getModId(), cfFile.getId()
-                    )
-            )
+        final URI uriToDownload = cfFile.getDownloadUrl() != null ?
+            normalizeDownloadUrl(cfFile.getDownloadUrl())
+            : downloadFallbackUriBuilder.resolve(
+                "/v1/mods/{modId}/files/{fileId}/download",
+                cfFile.getModId(), cfFile.getId()
+            );
+        log.trace("Downloading cfFile={} from normalizedUrl={}", cfFile, uriToDownload);
+        return preparedFetch.fetch(uriToDownload)
             .toFile(outputFile)
             .skipExisting(true)
             .handleStatus(handler)

src/main/java/me/itzg/helpers/curseforge/CurseForgeInstaller.java

@@ -56,6 +56,7 @@
 import me.itzg.helpers.json.ObjectMappers;
 import org.apache.commons.compress.archivers.zip.ZipArchiveEntry;
 import org.apache.commons.compress.archivers.zip.ZipFile;
+import org.jetbrains.annotations.Nullable;
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
 import reactor.core.scheduler.Schedulers;
@@ -435,17 +436,19 @@ private Path locateWorldZipInRepo(String fileName) {
         return locateFileIn(fileName, downloadsRepo, downloadsRepo.resolve(REPO_SUBDIR_WORLDS));
     }
 
-    private static Path locateFileIn(String fileName, Path... dirs) {
+    private static Path locateFileIn(String fileName, @Nullable Path... dirs) {
         for (Path dir : dirs) {
-            final Path resolved = dir.resolve(fileName);
-            if (Files.exists(resolved)) {
-                return resolved;
-            }
+            if (dir != null) {
+                final Path resolved = dir.resolve(fileName);
+                if (Files.exists(resolved)) {
+                    return resolved;
+                }
 
-            // When downloading, the browser may replace spaces with +'s
-            final Path altResolved = dir.resolve(fileName.replace(' ', '+'));
-            if (Files.exists(altResolved)) {
-                return altResolved;
+                // When downloading, the browser may replace spaces with +'s
+                final Path altResolved = dir.resolve(fileName.replace(' ', '+'));
+                if (Files.exists(altResolved)) {
+                    return altResolved;
+                }
             }
         }
         return null;
@@ -581,6 +584,7 @@ private ModPackResults processModpack(InstallContext context,
                     excludeIncludeIds.getForceIncludeIds(),
                     context.categoryInfo
                 )
+                    .checkpoint()
             )
             .collectList()
             .block();
@@ -741,6 +745,7 @@ else if (category.getSlug().equals("worlds")) {
                         final Mono<ResolveResult> resolvedFileMono =
                             Mono.defer(() ->
                                     downloadOrResolveFile(context, modInfo, isWorld, outputDir, cfFile)
+                                        .checkpoint()
                                 )
                                 // retry the deferred part above if one of the expected failure cases
                                 .retryWhen(
@@ -795,7 +800,10 @@ private Mono<ResolveResult> downloadOrResolveFile(InstallContext context, CurseF
 
         // Will try to locate an existing file by alternate names that browser might create,
         // but only for non-world files of the modpack
-        final Path locatedFile = !isWorld ? locateFileIn(cfFile.getFileName(), outputDir) : null;
+        final Path locatedFile = !isWorld ? locateFileIn(cfFile.getFileName(),
+            outputDir,
+            downloadsRepo
+        ) : null;
 
         if (locatedFile != null) {
             log.info("Mod file {} already exists", locatedFile);

src/main/java/me/itzg/helpers/curseforge/FileHashVerifier.java

@@ -5,6 +5,7 @@
 import java.util.EnumMap;
 import java.util.List;
 import java.util.Map;
+import java.util.stream.Collectors;
 import lombok.extern.slf4j.Slf4j;
 import me.itzg.helpers.curseforge.model.FileHash;
 import me.itzg.helpers.curseforge.model.HashAlgo;
@@ -24,6 +25,10 @@ public class FileHashVerifier {
     }
 
     public static Mono<Path> verify(Path file, List<FileHash> hashes) {
+        if (hashes.isEmpty()) {
+            return Mono.just(file);
+        }
+
         for (final FileHash hash : hashes) {
             final ChecksumAlgo checksumAlgo = algos.get(hash.getAlgo());
             if (checksumAlgo != null) {
@@ -42,6 +47,10 @@ public static Mono<Path> verify(Path file, List<FileHash> hashes) {
             }
         }
 
-        return Mono.error(new IllegalArgumentException("Unable to find compatible checksum algorithm"));
+        return Mono.error(new IllegalArgumentException("Unable to find compatible checksum algorithm from " +
+            hashes.stream()
+                .map(FileHash::getAlgo)
+                .collect(Collectors.toList())
+            ));
     }
 }