Aws::S3::Errors::IncompleteBody when passing IO at non-zero position to put_object since 1.210.0 #3346

Describe the bug

Since aws-sdk-s3 1.210.0, passing a File/IO object to put_object that isn't at position 0 results in Aws::S3::Errors::IncompleteBody. The SDK uploads 0 bytes while Content-Length reflects the actual file size.

This is a regression - prior versions automatically rewound the IO before upload.

Regression Issue

• Select this option if this issue appears to be a regression.

Expected Behavior

file = Tempfile.new
file.puts("some data")
# file position is now at EOF
client.put_object(bucket: 'my-bucket', key: 'test.txt', body: file)
# Should upload "some data\n"

Current Behavior

S3 returns IncompleteBody: You did not provide the number of bytes specified by the Content-Length HTTP header

Root Cause

PR #3323 changed PutObject to use trailer checksums (unsignedPayload: true). The AwsChunkedTrailerDigestIO wrapper doesn't rewind the underlying IO on initialization, so if the IO is at EOF, it immediately emits a zero-length chunk + trailer.

Notably, header checksums DO rewind (see calculate_checksum lines 359-363), making this inconsistent.

Reproduction Steps

require 'aws-sdk-s3'
require 'tempfile'

client = Aws::S3::Client.new(region: 'us-east-1')

file = Tempfile.new(['test', '.txt'])
file.puts("hello world")
# Don't rewind - file position at EOF

client.put_object(
  bucket: 'your-bucket',
  key: 'test.txt', 
  body: file
)
# => Aws::S3::Errors::IncompleteBody

Possible Solution

Add @io.rewind in AwsChunkedTrailerDigestIO#initialize, consistent with the pattern in calculate_checksum.

Gem version

aws-sdk-s3 1.210.0+, aws-sdk-core 3.241.0+

Environment

Ruby 3.4, macOS/Linux