Preserve sub-second precision when serializing unixTimestamp parameters

[jonathan343]

Overview

Ports boto/botocore#3796 and fixes the CLI v2 equivalent of boto/botocore#3255.

CLI v2's vendored botocore serializes unixTimestamp request parameters as int(calendar.timegm(...)), dropping any sub-second precision the caller provided. This affects every json, rest-json, and smithy-rpc-v2-cbor body timestamp. iso8601 has always carried microseconds, and rfc822 (HTTP-date) only supports whole seconds, so only the epoch-seconds format had this issue.

This PR preserves the caller's sub-second precision for unixTimestamp. Whole-second timestamps serialize exactly as before.

The CBOR serializer now emits a tagged float64 for fractional timestamps and delegates integer encoding to the shared integer serializer, which also fixes an OverflowError for pre-1970 timestamps.

Testing

Reproduced the scenario from boto/botocore#3255 with kinesis-video-archived-media list-fragments, pointed at an unreachable endpoint so --debug logs the serialized body before the send fails:

aws kinesis-video-archived-media list-fragments \
  --stream-name some-stream \
  --fragment-selector '{
    "FragmentSelectorType": "PRODUCER_TIMESTAMP",
    "TimestampRange": {
      "StartTimestamp": "2024-09-12T10:49:36.500000Z",
      "EndTimestamp": "2024-09-12T10:49:38.833000Z"
    }
  }' \
  --debug 2>&1 | grep -o "'body': b'[^']*'"

Before (v2):

'body': b'{"StreamName": "some-stream", "FragmentSelector": {"FragmentSelectorType": "PRODUCER_TIMESTAMP", "TimestampRange": {"StartTimestamp": 1726138176, "EndTimestamp": 1726138178}}}'

After:

'body': b'{"StreamName": "some-stream", "FragmentSelector": {"FragmentSelectorType": "PRODUCER_TIMESTAMP", "TimestampRange": {"StartTimestamp": 1726138176.5, "EndTimestamp": 1726138178.833}}}'

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[Copilot]

🟢 Approval recommended

The focused implementation preserves compatibility for whole-second values and includes coverage for fractional and pre-epoch timestamps.

Pull request overview

Preserves fractional precision for Unix timestamp request parameters while retaining integer output for whole seconds.

Changes:

• Serializes fractional Unix timestamps without truncation.
• Supports fractional and negative timestamps in CBOR.
• Adds precision and encoding regression tests.

File summaries

File	Description
awscli/botocore/serialize.py	Updates Unix timestamp and CBOR serialization.
tests/unit/botocore/test_serialize.py	Adds timestamp precision and CBOR tests.
.changes/next-release/bugfix-Serialization-52170.json	Documents the serialization fix.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 0
• Review effort level: Balanced

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[GarrettBeatty]

code seems fine. just for my understanding why did we have all the _get_additional_info_and_num_bytes logic before and why can it be removed now?

[jonathan343]

The _serialize_type_integer we now use calls the same _get_additional_info_and_num_bytes helper under the hood. I'm honestly not sure why this wasn't called in the initial implementation.

But the current behavior is incorrect since we can't serialize negative timestamps. A timestamp value like datetime(1968, 12, 31, 23, 59, 59) currently results in a OverflowError: can't convert negative int to unsigned error which this new approach addresses.