feat(libstore): add S3 multipart upload configuration settings

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

@@ -61,6 +61,37 @@ 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.
+        )"};
+
+    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

@@ -1,6 +1,8 @@
 #include "nix/store/s3-binary-cache-store.hh"
 #include "nix/store/http-binary-cache-store.hh"
 #include "nix/store/store-registration.hh"
+#include "nix/util/error.hh"
+#include "nix/util/logging.hh"
 
 #include <cassert>
 #include <ranges>
@@ -27,6 +29,31 @@ S3BinaryCacheStoreConfig::S3BinaryCacheStoreConfig(
             cacheUri.query[key] = value;
         }
     }
+
+    const uint64_t AWS_MIN_PART_SIZE = 5 * 1024 * 1024;
+    const uint64_t AWS_MAX_PART_SIZE = 5ULL * 1024 * 1024 * 1024;
+
+    if (multipartChunkSize < AWS_MIN_PART_SIZE) {
+        throw UsageError(
+            "multipart-chunk-size must be at least 5 MiB (5242880 bytes), got %d bytes (%d MiB)",
+            multipartChunkSize.get(),
+            multipartChunkSize.get() / (1024 * 1024));
+    }
+
+    if (multipartChunkSize > AWS_MAX_PART_SIZE) {
+        throw UsageError(
+            "multipart-chunk-size must be at most 5 GiB (5368709120 bytes), got %d bytes (%d GiB)",
+            multipartChunkSize.get(),
+            multipartChunkSize.get() / (1024 * 1024 * 1024));
+    }
+
+    if (multipartUpload && multipartThreshold < multipartChunkSize) {
+        warn(
+            "multipart-threshold (%d MiB) is less than multipart-chunk-size (%d MiB), "
+            "which may result in single-part multipart uploads",
+            multipartThreshold.get() / (1024 * 1024),
+            multipartChunkSize.get() / (1024 * 1024));
+    }
 }
 
 std::string S3BinaryCacheStoreConfig::getHumanReadableURI() const