Update S3-Conditional-Operations.md

S3-Conditional-Operations.md

@@ -14,12 +14,49 @@ ring changes.
 
 ## Supported Conditional Headers
 
-| Header | Applies To | Condition | Use Case |
-|--------|------------|-----------|----------|
-| **If-Match** | GET, PUT, COPY | ETag matches | Ensure object hasn't changed |
-| **If-None-Match** | GET, PUT, COPY | ETag doesn't match | Prevent overwrites, caching |
-| **If-Modified-Since** | GET, COPY | Modified after date | Conditional downloads |
-| **If-Unmodified-Since** | GET, PUT, COPY | Not modified after date | Safe modifications |
+These conditions act on the **target object** (the object being read or
+written). All four are supported on GET, HEAD, PUT, POST
+(`CompleteMultipartUpload`), and COPY. DELETE supports only `If-Match`.
+
+| Header | Type | Applies to | Special values |
+|--------|------|------------|----------------|
+| `If-Match` | ETag | GET, HEAD, PUT, POST, COPY, **DELETE** | `*` matches any existing object; comma-separated ETag list is honored |
+| `If-None-Match` | ETag | GET, HEAD, PUT, POST, COPY | `*` matches only when the object does **not** exist (compare-and-create); comma-separated ETag list is honored |
+| `If-Modified-Since` | RFC 1123 date | GET, HEAD, PUT, POST, COPY | — |
+| `If-Unmodified-Since` | RFC 1123 date | GET, HEAD, PUT, POST, COPY | — |
+
+### Copy-source conditionals
+
+`CopyObject` and `UploadPartCopy` accept a second set of headers that
+condition on the **source** object. They are independent of the four
+standard headers above (which condition on the destination), so a single
+copy request can carry conditions for both ends.
+
+| Header | Type |
+|--------|------|
+| `x-amz-copy-source-if-match` | ETag |
+| `x-amz-copy-source-if-none-match` | ETag |
+| `x-amz-copy-source-if-modified-since` | RFC 1123 date |
+| `x-amz-copy-source-if-unmodified-since` | RFC 1123 date |
+
+### Evaluation order
+
+Per [RFC 7232 §6](https://datatracker.ietf.org/doc/html/rfc7232#section-6)
+and AWS S3, the conditions are evaluated in this order; later conditions
+are skipped when an earlier one is present:
+
+1. `If-Match`
+2. `If-Unmodified-Since` — ignored if `If-Match` is present
+   ([RFC 7232 §3.4](https://datatracker.ietf.org/doc/html/rfc7232#section-3.4))
+3. `If-None-Match`
+4. `If-Modified-Since` — ignored if `If-None-Match` is present
+   ([RFC 7232 §3.3](https://datatracker.ietf.org/doc/html/rfc7232#section-3.3))
+
+### Not supported
+
+* `If-Range`. A `Range` request without `If-Range` works normally; there is
+  no header to fall back to a full-object response when the source has
+  changed.
 
 ## HTTP Examples
 
@@ -78,15 +115,35 @@ curl -X PUT \
   -H "x-amz-copy-source: /source-bucket/source-file.txt" \
   -H "x-amz-copy-source-if-match: \"source-etag\"" \
   "http://localhost:8333/dest-bucket/dest-file.txt"
+
+# Same copy, but also refuse to overwrite the destination
+curl -X PUT \
+  -H "x-amz-copy-source: /source-bucket/source-file.txt" \
+  -H "x-amz-copy-source-if-match: \"source-etag\"" \
+  -H "If-None-Match: *" \
+  "http://localhost:8333/dest-bucket/dest-file.txt"
 ```
 
+### Conditional DELETE
+
+```bash
+# Delete only if the object's ETag still matches what we read
+curl -X DELETE \
+  -H "If-Match: \"d41d8cd98f00b204e9800998ecf8427e\"" \
+  "http://localhost:8333/mybucket/myfile.txt"
+```
+
+DELETE supports `If-Match` only; the other three conditional headers are
+ignored on DELETE per the AWS S3 spec.
+
 ## HTTP Status Codes
 
-| Status Code | Condition | Meaning |
-|-------------|-----------|---------|
-| **200 OK** | Condition met | Operation succeeded |
-| **304 Not Modified** | If-None-Match matched | Object unchanged (GET only) |
-| **412 Precondition Failed** | Condition not met | Operation blocked by condition |
+| Status Code | When | Meaning |
+|-------------|------|---------|
+| **200 OK** | All conditions met | Operation succeeded normally |
+| **304 Not Modified** | GET/HEAD with `If-None-Match` match, or `If-Modified-Since` not exceeded | Object unchanged; response carries the ETag header but no body |
+| **400 Invalid Request** | `If-Modified-Since` or `If-Unmodified-Since` is not a valid RFC 1123 date | Header is malformed |
+| **412 Precondition Failed** | Any other failed condition (all writes; GET/HEAD with `If-Match` mismatch or `If-Unmodified-Since` exceeded) | Operation blocked by condition |
 
 ## Atomicity and concurrency