s3: add s3:ExistingObjectTag condition support for bucket policies (#7677)

* s3: add s3:ExistingObjectTag condition support in policy engine

Add support for s3:ExistingObjectTag/<tag-key> condition keys in bucket
policies, allowing access control based on object tags.

Changes:
- Add ObjectEntry field to PolicyEvaluationArgs (entry.Extended metadata)
- Update EvaluateConditions to handle s3:ExistingObjectTag/<key> format
- Extract tag value from entry metadata using X-Amz-Tagging-<key> prefix

This enables policies like:
{
  "Condition": {
    "StringEquals": {
      "s3:ExistingObjectTag/status": ["public"]
    }
  }
}

Fixes: https://github.com/seaweedfs/seaweedfs/issues/7447

* s3: update EvaluatePolicy to accept object entry for tag conditions

Update BucketPolicyEngine.EvaluatePolicy to accept objectEntry parameter
(entry.Extended metadata) for evaluating tag-based policy conditions.

Changes:
- Add objectEntry parameter to EvaluatePolicy method
- Update callers in auth_credentials.go and s3api_bucket_handlers.go
- Pass nil for objectEntry in auth layer (entry fetched later in handlers)

For tag-based conditions to work, handlers should call EvaluatePolicy
with the object's entry.Extended after fetching the entry from filer.

* s3: add tests for s3:ExistingObjectTag policy conditions

Add comprehensive tests for object tag-based policy conditions:

- TestExistingObjectTagCondition: Basic tag matching scenarios
  - Matching/non-matching tag values
  - Missing tags, no tags, empty tags
  - Multiple tags with one matching

- TestExistingObjectTagConditionMultipleTags: Multiple tag conditions
  - Both tags match
  - Only one tag matches

- TestExistingObjectTagDenyPolicy: Deny policies with tag conditions
  - Default allow without tag
  - Deny when specific tag present

* s3: document s3:ExistingObjectTag support and feature status

Update policy engine documentation:

- Add s3:ExistingObjectTag/<tag-key> to supported condition keys
- Add 'Object Tag-Based Access Control' section with examples
- Add 'Feature Status' section with implemented and planned features

Planned features for future implementation:
- s3:RequestObjectTag/<key>
- s3:RequestObjectTagKeys
- s3:x-amz-server-side-encryption
- Cross-account access

* Implement tag-based policy re-check in handlers

- Add checkPolicyWithEntry helper to S3ApiServer for handlers to re-check
  policy after fetching object entry (for s3:ExistingObjectTag conditions)
- Add HasPolicyForBucket method to policy engine for efficient check
- Integrate policy re-check in GetObjectHandler after entry is fetched
- Integrate policy re-check in HeadObjectHandler after entry is fetched
- Update auth_credentials.go comments to explain two-phase evaluation
- Update documentation with supported operations for tag-based conditions

This implements 'Approach 1' where handlers re-check the policy with
the object entry after fetching it, allowing tag-based conditions to
be properly evaluated.

* Add integration tests for s3:ExistingObjectTag conditions

- Add TestCheckPolicyWithEntry: tests checkPolicyWithEntry helper with various
  tag scenarios (matching tags, non-matching tags, empty entry, nil entry)
- Add TestCheckPolicyWithEntryNoPolicyForBucket: tests early return when no policy
- Add TestCheckPolicyWithEntryNilPolicyEngine: tests nil engine handling
- Add TestCheckPolicyWithEntryDenyPolicy: tests deny policies with tag conditions
- Add TestHasPolicyForBucket: tests HasPolicyForBucket method

These tests cover the Phase 2 policy evaluation with object entry metadata,
ensuring tag-based conditions are properly evaluated.

* Address code review nitpicks

- Remove unused extractObjectTags placeholder function (engine.go)
- Add clarifying comment about s3:ExistingObjectTag/<key> evaluation
- Consolidate duplicate tag-based examples in README
- Factor out tagsToEntry helper to package level in tests

* Address code review feedback

- Fix unsafe type assertions in GetObjectHandler and HeadObjectHandler
  when getting identity from context (properly handle type assertion failure)
- Extract getConditionContextValue helper to eliminate duplicated logic
  between EvaluateConditions and EvaluateConditionsLegacy
- Ensure consistent handling of missing condition keys (always return
  empty slice)

* Fix GetObjectHandler to match HeadObjectHandler pattern

Add safety check for nil objectEntryForSSE before tag-based policy
evaluation, ensuring tag-based conditions are always evaluated rather
than silently skipped if entry is unexpectedly nil.

Addresses review comment from Copilot.

* Fix HeadObject action name in docs for consistency

Change 'HeadObject' to 's3:HeadObject' to match other action names.

* Extract recheckPolicyWithObjectEntry helper to reduce duplication

Move the repeated identity extraction and policy re-check logic from
GetObjectHandler and HeadObjectHandler into a shared helper method.

* Add validation for empty tag key in s3:ExistingObjectTag condition

Prevent potential issues with malformed policies containing
s3:ExistingObjectTag/ (empty tag key after slash).

weed/s3api/policy_engine/conditions.go

@@ -10,6 +10,7 @@ import (
 	"time"
 
 	"github.com/seaweedfs/seaweedfs/weed/glog"
+	"github.com/seaweedfs/seaweedfs/weed/s3api/s3_constants"
 )
 
 // LRUNode represents a node in the doubly-linked list for efficient LRU operations
@@ -705,8 +706,36 @@ func GetConditionEvaluator(operator string) (ConditionEvaluator, error) {
 	}
 }
 
+// ExistingObjectTagPrefix is the prefix for S3 policy condition keys
+const ExistingObjectTagPrefix = "s3:ExistingObjectTag/"
+
+// getConditionContextValue resolves the value(s) for a condition key.
+// For s3:ExistingObjectTag/<key> conditions, it looks up the tag in objectEntry.
+// For other condition keys, it looks up the value in contextValues.
+func getConditionContextValue(key string, contextValues map[string][]string, objectEntry map[string][]byte) []string {
+	if strings.HasPrefix(key, ExistingObjectTagPrefix) {
+		tagKey := key[len(ExistingObjectTagPrefix):]
+		if tagKey == "" {
+			return []string{} // Invalid: empty tag key
+		}
+		metadataKey := s3_constants.AmzObjectTaggingPrefix + tagKey
+		if objectEntry != nil {
+			if tagValue, exists := objectEntry[metadataKey]; exists {
+				return []string{string(tagValue)}
+			}
+		}
+		return []string{}
+	}
+
+	if vals, exists := contextValues[key]; exists {
+		return vals
+	}
+	return []string{}
+}
+
 // EvaluateConditions evaluates all conditions in a policy statement
-func EvaluateConditions(conditions PolicyConditions, contextValues map[string][]string) bool {
+// objectEntry is the object's metadata from entry.Extended (can be nil)
+func EvaluateConditions(conditions PolicyConditions, contextValues map[string][]string, objectEntry map[string][]byte) bool {
 	if len(conditions) == 0 {
 		return true // No conditions means always true
 	}
@@ -719,11 +748,7 @@ func EvaluateConditions(conditions PolicyConditions, contextValues map[string][]
 		}
 
 		for key, value := range conditionMap {
-			contextVals, exists := contextValues[key]
-			if !exists {
-				contextVals = []string{}
-			}
-
+			contextVals := getConditionContextValue(key, contextValues, objectEntry)
 			if !conditionEvaluator.Evaluate(value.Strings(), contextVals) {
 				return false // If any condition fails, the whole condition block fails
 			}
@@ -734,7 +759,8 @@ func EvaluateConditions(conditions PolicyConditions, contextValues map[string][]
 }
 
 // EvaluateConditionsLegacy evaluates conditions using the old interface{} format for backward compatibility
-func EvaluateConditionsLegacy(conditions map[string]interface{}, contextValues map[string][]string) bool {
+// objectEntry is the object's metadata from entry.Extended (can be nil)
+func EvaluateConditionsLegacy(conditions map[string]interface{}, contextValues map[string][]string, objectEntry map[string][]byte) bool {
 	if len(conditions) == 0 {
 		return true // No conditions means always true
 	}
@@ -753,11 +779,7 @@ func EvaluateConditionsLegacy(conditions map[string]interface{}, contextValues m
 		}
 
 		for key, value := range conditionMapTyped {
-			contextVals, exists := contextValues[key]
-			if !exists {
-				contextVals = []string{}
-			}
-
+			contextVals := getConditionContextValue(key, contextValues, objectEntry)
 			if !conditionEvaluator.Evaluate(value, contextVals) {
 				return false // If any condition fails, the whole condition block fails
 			}