Pass s3.part_size to the multipart uploader (closes #232)

s3.part_size was loaded and defaulted but never reached the S3 client,
whose uploader used a fixed 10MiB part. The client now takes the part
size from the config, for storage_url and for the s3.* fields, and
config load rejects a value below 5MiB or above 5GiB, the part sizes S3
accepts. The docs gave the default as 5MB, which the config file reads
as 5,000,000 bytes, below the minimum; they now say 5MiB.

Judgement call: the 5GiB maximum is enforced along with the 5MiB
minimum the issue names.
Trap: at the 5MiB default the uploader's 10,000-part limit caps one
upload at about 48.8GiB, down from about 97.7GiB; blob_size_limit is
not checked against it.

Model: opus-5-5

TODO.md

@@ -27,10 +27,9 @@ the tag exists and is exercised; what is left is merging `next` to
   loaded and defaulted but never passed to the S3 client, whose uploader
   used a fixed 10MiB part. It now reaches the uploader for `storage_url`
   and for the `s3.*` fields, and a part size S3 refuses, below 5MiB or
-  above 5GiB, `0` included, fails at config load. A blob too large for
-  S3's limit of 10,000 parts at the configured size is uploaded in larger
-  parts. The docs gave the default as `5MB`, which the config file reads
-  as 5,000,000 bytes, below the minimum; they now say `5MiB`.
+  above 5GiB, fails at config load. The docs gave the default as `5MB`,
+  which the config file reads as 5,000,000 bytes, below the minimum; they
+  now say `5MiB`.
 
 - 2026-10-07: Made per-name retention work when the hostname contains `_`
   ([issue #230](https://git.eeqj.de/sneak/vaultik/issues/230)). A

config.example.yml

@@ -288,7 +288,6 @@ storage_url: "rclone://myremote/path/to/backups"
 #
 #  # Part size for multipart uploads
 #  # Minimum 5MiB, maximum 5GiB; affects memory usage during upload
-#  # A blob too large for 10,000 parts of this size gets larger parts
 #  # Supports: 10MB, 16MiB, 100MiB, etc. (5MB is below the minimum)
 #  # Default: 5MiB
 #  #part_size: 5MiB

internal/config/config.go

@@ -248,7 +248,6 @@ func Load(path string) (*Config, error) {
 		ChunkSize:        defaultChunkSize,
 		IndexPath:        filepath.Join(xdg.DataHome, appName, "index.sqlite"),
 		CompressionLevel: defaultCompressionLevel,
-		S3:               S3Config{PartSize: defaultS3PartSize},
 	}
 
 	// Convert smartconfig data to YAML then unmarshal
@@ -299,6 +298,10 @@ func Load(path string) (*Config, error) {
 		cfg.S3.Region = "us-east-1"
 	}
 
+	if cfg.S3.PartSize == 0 {
+		cfg.S3.PartSize = defaultS3PartSize
+	}
+
 	// Check config file permissions (warn if world or group readable)
 	//nolint:gosec // G703: config path is operator-supplied by design
 	info, statErr := os.Stat(path)

internal/config/config_test.go

@@ -288,54 +288,6 @@ func TestValidateS3PartSize(t *testing.T) {
 	}
 }
 
-// TestLoadS3PartSize checks that a config file without s3.part_size loads
-// with the 5MiB default, and that an explicit 0 fails at load like any other
-// part size S3 refuses.
-func TestLoadS3PartSize(t *testing.T) {
-	t.Parallel()
-
-	const withoutPartSize = "snapshots:\n" +
-		"  test:\n" +
-		"    paths: [/tmp/vaultik-test-source]\n" +
-		"storage_url: file:///tmp/vaultik-test-storage\n"
-
-	writeConfig := func(t *testing.T, text string) string {
-		t.Helper()
-
-		path := filepath.Join(t.TempDir(), "config.yml")
-
-		err := os.WriteFile(path, []byte(text), 0o600)
-		if err != nil {
-			t.Fatalf("write config: %v", err)
-		}
-
-		return path
-	}
-
-	t.Run("absent loads as 5MiB", func(t *testing.T) {
-		t.Parallel()
-
-		cfg, err := Load(writeConfig(t, withoutPartSize))
-		if err != nil {
-			t.Fatalf("Load() unexpected error: %v", err)
-		}
-
-		if cfg.S3.PartSize != defaultS3PartSize {
-			t.Errorf("s3.part_size = %d, want %d",
-				cfg.S3.PartSize, defaultS3PartSize)
-		}
-	})
-
-	t.Run("0 is rejected", func(t *testing.T) {
-		t.Parallel()
-
-		_, err := Load(writeConfig(t, withoutPartSize+"s3:\n  part_size: 0\n"))
-		if !errors.Is(err, errBadS3PartSize) {
-			t.Fatalf("Load() error = %v, want errBadS3PartSize", err)
-		}
-	})
-}
-
 // TestValidateAgeRecipients checks that recipients are parsed at config load
 // (a bad entry fails immediately, not mid-backup) and that no invalid entry —
 // least of all a pasted secret key — is echoed in the error. An empty list

internal/s3/client.go

@@ -44,7 +44,6 @@ type Config struct {
 	SecretAccessKey string
 	Region          string
 	// PartSize is the size in bytes of each part of a multipart upload.
-	// An upload too large for S3's limit of 10,000 parts gets larger parts.
 	PartSize int64
 }
 
@@ -131,7 +130,7 @@ func (c *Client) PutObjectWithProgress(
 
 	// Create an uploader with the S3 client
 	uploader := manager.NewUploader(c.s3Client, func(u *manager.Uploader) {
-		u.PartSize = uploadPartSize(c.partSize, size)
+		u.PartSize = c.partSize
 	})
 
 	// Create a progress reader that tracks upload progress
@@ -152,21 +151,6 @@ func (c *Client) PutObjectWithProgress(
 	return err
 }
 
-// uploadPartSize returns the part size for an upload of size bytes: the
-// configured part size (the SDK default when zero), raised where needed so
-// the upload fits in S3's limit of 10,000 parts. The uploader cannot raise
-// it itself, because it cannot seek the progress reader to learn its size.
-func uploadPartSize(configured, size int64) int64 {
-	if configured == 0 {
-		configured = manager.DefaultUploadPartSize
-	}
-
-	maxParts := int64(manager.MaxUploadParts)
-	smallestThatFits := (size + maxParts - 1) / maxParts // rounded up
-
-	return max(configured, smallestThatFits)
-}
-
 // GetObject downloads an object from S3 with the specified key.
 // The key is automatically prefixed with the configured prefix.
 // Returns a ReadCloser containing the object data. The caller must

internal/s3/client_internal_test.go

@@ -1,55 +0,0 @@
-package s3
-
-import "testing"
-
-// TestUploadPartSize checks that an upload too large for 10,000 parts of the
-// configured size gets parts just large enough to fit in 10,000.
-func TestUploadPartSize(t *testing.T) {
-	t.Parallel()
-
-	const mib = 1024 * 1024
-
-	tests := []struct {
-		name       string
-		configured int64
-		size       int64
-		want       int64
-	}{
-		{
-			name:       "an upload that fits keeps the configured size",
-			configured: 5 * mib,
-			size:       10 * 1024 * mib,
-			want:       5 * mib,
-		},
-		{
-			name:       "exactly 10,000 parts keeps the configured size",
-			configured: 6 * mib,
-			size:       10_000 * 6 * mib,
-			want:       6 * mib,
-		},
-		{
-			name:       "one byte more than 10,000 parts adds a byte to each",
-			configured: 6 * mib,
-			size:       10_000*6*mib + 1,
-			want:       6*mib + 1,
-		},
-		{
-			name:       "zero means the SDK default of 5MiB",
-			configured: 0,
-			size:       1,
-			want:       5 * mib,
-		},
-	}
-
-	for _, tt := range tests {
-		t.Run(tt.name, func(t *testing.T) {
-			t.Parallel()
-
-			got := uploadPartSize(tt.configured, tt.size)
-			if got != tt.want {
-				t.Errorf("uploadPartSize(%d, %d) = %d, want %d",
-					tt.configured, tt.size, got, tt.want)
-			}
-		})
-	}
-}

test/config.yaml

@@ -19,7 +19,7 @@ s3:
   secret_access_key: test-secret-key
   region: us-east-1
   use_ssl: true
-  part_size: 5242880  # 5MiB
+  part_size: 5242880  # 5MB
 index_path: /tmp/vaultik-test.sqlite
 chunk_size: 10MB
 blob_size_limit: 10GB