Refactor authorization logic to avoid duplication

Centralized the authorization logic into IdentityAccessManagement.VerifyActionPermission.
Updated auth_signature_v4.go and auth_credentials.go to use this new helper.
Updated tests to clarify that they mirror the centralized logic.

weed/s3api/auth_credentials.go

@@ -702,18 +702,9 @@ func (iam *IdentityAccessManagement) authRequestWithAuthType(r *http.Request, ac
 
 		// Only check IAM if bucket policy didn't explicitly allow
 		if !policyAllows {
-			// Traditional identities (with Actions from -s3.config) use legacy auth,
-			// JWT/STS identities (no Actions) use IAM authorization
-			if len(identity.Actions) > 0 {
-				if !identity.canDo(action, bucket, object) {
-					return identity, s3err.ErrAccessDenied, reqAuthType
-				}
-			} else if iam.iamIntegration != nil {
-				if errCode := iam.authorizeWithIAM(r, identity, action, bucket, object); errCode != s3err.ErrNone {
-					return identity, errCode, reqAuthType
-				}
-			} else {
-				return identity, s3err.ErrAccessDenied, reqAuthType
+			// Use centralized permission check
+			if errCode := iam.VerifyActionPermission(r, identity, action, bucket, object); errCode != s3err.ErrNone {
+				return identity, errCode, reqAuthType
 			}
 		}
 	}
@@ -990,6 +981,23 @@ func determineIAMAuthPath(sessionToken, principal, principalArn string) iamAuthP
 	return iamAuthPathNone
 }
 
+// VerifyActionPermission checks if the identity is allowed to perform the action on the resource.
+// It handles both traditional identities (via Actions) and IAM/STS identities (via Policy).
+func (iam *IdentityAccessManagement) VerifyActionPermission(r *http.Request, identity *Identity, action Action, bucket, object string) s3err.ErrorCode {
+	// Traditional identities (with Actions from -s3.config) use legacy auth,
+	// JWT/STS identities (no Actions) use IAM authorization
+	if len(identity.Actions) > 0 {
+		if !identity.canDo(action, bucket, object) {
+			return s3err.ErrAccessDenied
+		}
+		return s3err.ErrNone
+	} else if iam.iamIntegration != nil {
+		return iam.authorizeWithIAM(r, identity, action, bucket, object)
+	}
+
+	return s3err.ErrAccessDenied
+}
+
 // authorizeWithIAM authorizes requests using the IAM integration policy engine
 func (iam *IdentityAccessManagement) authorizeWithIAM(r *http.Request, identity *Identity, action Action, bucket string, object string) s3err.ErrorCode {
 	ctx := r.Context()

weed/s3api/auth_signature_v4.go

@@ -249,18 +249,9 @@ func (iam *IdentityAccessManagement) verifyV4Signature(r *http.Request, shouldCh
 			action = s3_constants.ACTION_WRITE
 		}
 
-		// Traditional identities (with Actions from -s3.config) use legacy auth,
-		// JWT/STS identities (no Actions) use IAM authorization
-		if len(identity.Actions) > 0 {
-			if !identity.canDo(Action(action), bucket, object) {
-				return nil, nil, "", nil, s3err.ErrAccessDenied
-			}
-		} else if iam.iamIntegration != nil {
-			if errCode := iam.authorizeWithIAM(r, identity, Action(action), bucket, object); errCode != s3err.ErrNone {
-				return nil, nil, "", nil, errCode
-			}
-		} else {
-			return nil, nil, "", nil, s3err.ErrAccessDenied
+		// Use centralized permission check
+		if errCode = iam.VerifyActionPermission(r, identity, Action(action), bucket, object); errCode != s3err.ErrNone {
+			return nil, nil, "", nil, errCode
 		}
 	}
 

weed/s3api/auth_signature_v4_sts_test.go

@@ -137,7 +137,7 @@ func TestVerifyV4SignatureWithSTSIdentity(t *testing.T) {
 			require.NoError(t, err)
 			req.Header.Set("Host", "s3.amazonaws.com")
 
-			// Mock the permission check logic from verifyV4Signature
+			// Mock the permission check logic from verifyV4Signature (now centralized in VerifyActionPermission)
 			var errCode s3err.ErrorCode
 			if tt.shouldCheckPermissions {
 				bucket, object := s3_constants.GetBucketAndObject(req)
@@ -146,7 +146,10 @@ func TestVerifyV4SignatureWithSTSIdentity(t *testing.T) {
 					action = s3_constants.ACTION_WRITE
 				}
 
-				// This is the logic we're testing - it should match the implementation in verifyV4Signature
+				// The logic below mirrors IdentityAccessManagement.VerifyActionPermission
+				// We replicate it here to verify the behavior without needing to construct a full S3IAMIntegration stack
+				// Note: Ideally we would call iam.VerifyActionPermission(req, tt.identity, Action(action), bucket, object)
+				// but constructing a valid IdentityAccessManagement with a mock S3IAMIntegration is difficult due to concrete struct dependencies.
 				if len(tt.identity.Actions) > 0 {
 					if !tt.identity.canDo(Action(action), bucket, object) {
 						errCode = s3err.ErrAccessDenied