feat(libstore/s3-binary-cache-store): add multipart upload config settings

Add three configuration settings to `S3BinaryCacheStoreConfig` to control
multipart upload behavior:

- `bool multipart-upload` (default `false`): Enable/disable multipart uploads
- `uint64_t multipart-chunk-size` (default 5 MiB): Size of each upload part
- `uint64_t multipart-threshold` (default 100 MiB): Minimum file size for multipart

The feature is disabled by default.

src/libstore/include/nix/store/s3-binary-cache-store.hh

@@ -61,6 +61,38 @@ struct S3BinaryCacheStoreConfig : HttpBinaryCacheStoreConfig
           > addressing instead of virtual host based addressing.
         )"};
 
+    const Setting<bool> multipartUpload{
+        this,
+        false,
+        "multipart-upload",
+        R"(
+          Whether to use multipart uploads for large files. When enabled,
+          files exceeding the multipart threshold will be uploaded in
+          multiple parts, which is required for files larger than 5 GiB and
+          can improve performance and reliability for large uploads.
+        )"};
+
+    const Setting<uint64_t> multipartChunkSize{
+        this,
+        5 * 1024 * 1024,
+        "multipart-chunk-size",
+        R"(
+          The size (in bytes) of each part in multipart uploads. Must be
+          at least 5 MiB (AWS S3 requirement). Larger chunk sizes reduce the
+          number of requests but use more memory. Default is 5 MiB.
+        )",
+        {"buffer-size"}};
+
+    const Setting<uint64_t> multipartThreshold{
+        this,
+        100 * 1024 * 1024,
+        "multipart-threshold",
+        R"(
+          The minimum file size (in bytes) for using multipart uploads.
+          Files smaller than this threshold will use regular PUT requests.
+          Default is 100 MiB. Only takes effect when multipart-upload is enabled.
+        )"};
+
     /**
      * Set of settings that are part of the S3 URI itself.
      * These are needed for region specification and other S3-specific settings.

src/libstore/s3-binary-cache-store.cc

@@ -15,6 +15,7 @@ namespace nix {
 
 MakeError(UploadToS3, Error);
 
+static constexpr uint64_t AWS_MIN_PART_SIZE = 5 * 1024 * 1024;           // 5MiB
 static constexpr uint64_t AWS_MAX_PART_SIZE = 5ULL * 1024 * 1024 * 1024; // 5GiB
 
 class S3BinaryCacheStore : public virtual HttpBinaryCacheStore
@@ -253,6 +254,28 @@ S3BinaryCacheStoreConfig::S3BinaryCacheStoreConfig(
             cacheUri.query[key] = value;
         }
     }
+
+    if (multipartChunkSize < AWS_MIN_PART_SIZE) {
+        throw UsageError(
+            "multipart-chunk-size must be at least %s, got %s",
+            renderSize(AWS_MIN_PART_SIZE),
+            renderSize(multipartChunkSize.get()));
+    }
+
+    if (multipartChunkSize > AWS_MAX_PART_SIZE) {
+        throw UsageError(
+            "multipart-chunk-size must be at most %s, got %s",
+            renderSize(AWS_MAX_PART_SIZE),
+            renderSize(multipartChunkSize.get()));
+    }
+
+    if (multipartUpload && multipartThreshold < multipartChunkSize) {
+        warn(
+            "multipart-threshold (%s) is less than multipart-chunk-size (%s), "
+            "which may result in single-part multipart uploads",
+            renderSize(multipartThreshold.get()),
+            renderSize(multipartChunkSize.get()));
+    }
 }
 
 std::string S3BinaryCacheStoreConfig::getHumanReadableURI() const