[Bug] - posix w/awscli version 2.23.0 and up - files written as null bytes

[lrm25]

Describe the bug
Also see aws/aws-cli#9214. An added checksum verification to awscli somehow results in files being written to the posix gateway as null bytes, the file size being accurate but the bytes themselves obviously not.

This issue can currently be bypassed with the env var AWS_REQUEST_CHECKSUM_CALCULATION=WHEN_REQUIRED (except with s3 cp, see below).

To Reproduce

1. Download and use a version of awscli greater than 2.23.0.
2. Start versitygw with a posix gateway, e.g.: ./versitygw --access=******** --secret=******** --region=us-east-1 --iam-dir=/Users/lukemccrone/devel/versitygw/iam --cert /Users/lukemccrone/devel/versitygw/cert.pem --key /Users/lukemccrone/devel/versitygw/versitygw.pem posix --versioning-dir /tmp/versioning /tmp/gw
3. Create a bucket, e.g.: aws --no-verify-ssl s3api create-bucket --bucket versity-gwtest-bucket-one-1
4. Create a file with non-null bytes, such as a text file, and copy it, e.g.: aws --no-verify-ssl s3api put-object --body {file location} --bucket versity-gwtest-bucket-one-1 --key bucket_file.
5. Check the file data in /tmp/gw/<bucket name>.

Expected behavior
The file data matches what you added in step 4, rather than just containing null bytes.

Server Version
Version : v1.0.9
Build : 5f95d42
BuildTime: 2025-01-16_05:26:13PM
Darwin Lukes-MacBook-Pro.local 23.4.0 Darwin Kernel Version 23.4.0: Fri Mar 15 00:12:25 PDT 2024; root:xnu-10063.101.17~1/RELEASE_ARM64_T6030 arm64

[lrm25]

aws/aws-cli#9214 (comment) - as with here, though, s3 cp is not working, even with the env var set.