feat(storage): add signed post policy v4

src/storage/src/error.rs

@@ -272,13 +272,13 @@ pub enum WriteError {
 
 type BoxError = Box<dyn std::error::Error + Send + Sync + 'static>;
 
-/// Signed URL creation errors.
+/// Signed URL and signed POST policy creation errors.
 #[derive(thiserror::Error, Debug)]
 #[error(transparent)]
 pub struct SigningError(SigningErrorKind);
 
 impl SigningError {
-    /// Returns true if the error was due to a problem signing the URL.
+    /// Returns true if the error was due to a problem signing the request.
     pub fn is_signing(&self) -> bool {
         matches!(self.0, SigningErrorKind::Signing(_))
     }
@@ -288,15 +288,15 @@ impl SigningError {
         matches!(self.0, SigningErrorKind::InvalidParameter(_, _))
     }
 
-    /// A problem to sign the URL.
+    /// A problem to sign the request.
     pub(crate) fn signing<T>(source: T) -> SigningError
     where
         T: Into<BoxError>,
     {
         SigningError(SigningErrorKind::Signing(source.into()))
     }
 
-    /// A problem to sign the URL due to invalid input.
+    /// A problem to sign the request due to invalid input.
     pub(crate) fn invalid_parameter<S: Into<String>, T>(field: S, source: T) -> SigningError
     where
         T: Into<BoxError>,
@@ -315,7 +315,7 @@ enum SigningErrorKind {
     #[error("signing failed: {0}")]
     Signing(#[source] BoxError),
 
-    /// An invalid input was provided to generate a signed URL.
+    /// An invalid input was provided to generate a signed URL or signed POST policy.
     #[error("invalid `{0}` parameter: {1}")]
     InvalidParameter(String, #[source] BoxError),
 }

src/storage/src/lib.rs

@@ -21,6 +21,7 @@
 //! * [Storage][client::Storage]
 //! * [StorageControl][client::StorageControl]
 //! * [SignedUrlBuilder][builder::storage::SignedUrlBuilder]
+//! * [PostPolicyV4Builder][builder::storage::PostPolicyV4Builder]
 //!
 //! **NOTE:** This crate used to contain a different implementation, with a
 //! different surface. [@yoshidan](https://github.com/yoshidan) generously
@@ -58,6 +59,7 @@ pub(crate) use google_cloud_gax::response::Response;
 
 pub mod backoff_policy;
 pub mod object_descriptor;
+pub mod post_policy_v4;
 pub mod read_object;
 pub mod read_resume_policy;
 pub mod retry_policy;
@@ -96,6 +98,7 @@ pub mod builder {
         //! Request builders for [Storage][crate::client::Storage].
         pub use crate::storage::client::ClientBuilder;
         pub use crate::storage::open_object::OpenObject;
+        pub use crate::storage::post_policy_v4::PostPolicyV4Builder;
         pub use crate::storage::read_object::ReadObject;
         pub use crate::storage::signed_url::SignedUrlBuilder;
         pub use crate::storage::write_object::WriteObject;

src/storage/src/post_policy_v4.rs

@@ -0,0 +1,19 @@
+// Copyright 2026 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//! Types related to signed V4 POST policies.
+//!
+//! See [PostPolicyV4Builder][crate::builder::storage::PostPolicyV4Builder].
+
+pub use crate::storage::post_policy_v4::{PolicyV4Fields, PostPolicyV4, PostPolicyV4Condition};

src/storage/src/storage.rs

@@ -18,6 +18,7 @@ pub(crate) mod client;
 pub(crate) mod common_options;
 pub(crate) mod open_object;
 pub(crate) mod perform_upload;
+pub(crate) mod post_policy_v4;
 pub(crate) mod read_object;
 pub mod request_options;
 pub(crate) mod signed_url;