just serve: allow clients to access execution endpoint with a different address

To properly use `just serve`, both the client and the serve instance
must talk to the very same execution endpoint. Typically, both the
client and serve can reach out to the execution endpoint via the same
IP address. However, it might be possible that the client and a serve
instance know the same execution endpoint by means of differnet IP
addresses. For example, the client knows the execution endpoint
address through an _external_ IP address, while the serve instance,
deployed within the same network infrastructure, only knows the
_internal_ IP address.

This patch adds the subkey `"client address"` -- of the key
`"execution endpoint"` -- in the serve configuration file, to specify
the alternative pair `address:port` used by the client.

8 files changed, 61 insertions, 8 deletions

diff --git a/src/buildtool/common/cli.hpp b/src/buildtool/common/cli.hpp
index f486a5fd..ef6c043c 100644
--- a/src/buildtool/common/cli.hpp
+++ b/src/buildtool/common/cli.hpp
@@ -186,6 +186,8 @@ struct ServeArguments {
     std::optional<std::string> remote_serve_address;
     // repositories populated from just-serve config file
     std::vector<std::filesystem::path> repositories;
+    // remote execution endpoint as seen by the client
+    std::optional<std::string> client_remote_address;
 };
 
 struct GcArguments {
diff --git a/src/buildtool/main/main.cpp b/src/buildtool/main/main.cpp
index 72a5b060..5acba963 100644
--- a/src/buildtool/main/main.cpp
+++ b/src/buildtool/main/main.cpp
@@ -192,6 +192,7 @@ void SetupLogging(LogArguments const& clargs) {
     -> std::optional<RemoteServeConfig> {
     RemoteServeConfig::Builder builder;
     builder.SetRemoteAddress(srvargs.remote_serve_address)
+        .SetClientExecutionAddress(srvargs.client_remote_address)
         .SetKnownRepositories(srvargs.repositories)
         .SetJobs(cargs.jobs)
         .SetActionTimeout(bargs.timeout)
diff --git a/src/buildtool/main/serve.cpp b/src/buildtool/main/serve.cpp
index 531074c3..5d5f49fb 100644
--- a/src/buildtool/main/serve.cpp
+++ b/src/buildtool/main/serve.cpp
@@ -415,6 +415,22 @@ void ReadJustServeConfig(gsl::not_null<CommandLineArguments*> const& clargs) {
             }
             clargs->endpoint.remote_execution_address = address->String();
         }
+        // read the address used by the client
+        auto client_address =
+            exec_endpoint->Get("client address", Expression::none_t{});
+        if (client_address.IsNotNull()) {
+            if (not client_address->IsString()) {
+                Logger::Log(
+                    LogLevel::Error,
+                    "In serve service config file {}:\nValue for execution "
+                    "endpoint key \"client address\" has to be a string, but "
+                    "found {}",
+                    clargs->serve.config.string(),
+                    client_address->ToString());
+                std::exit(kExitFailure);
+            }
+            clargs->serve.client_remote_address = client_address->String();
+        }
         if (not ParseRetryCliOptions(serve_config, &clargs->retry)) {
             std::exit(kExitFailure);
         }
diff --git a/src/buildtool/serve_api/remote/config.hpp b/src/buildtool/serve_api/remote/config.hpp
index 582727e0..82119828 100644
--- a/src/buildtool/serve_api/remote/config.hpp
+++ b/src/buildtool/serve_api/remote/config.hpp
@@ -34,9 +34,12 @@
 struct RemoteServeConfig final {
     class Builder;
 
-    // Server address of remote execution.
+    // Server address of the serve endpoint.
     std::optional<ServerAddress> const remote_address;
 
+    // Execution endpoint used by the client.
+    std::optional<ServerAddress> const client_execution_address;
+
     // Known Git repositories to serve server.
     std::vector<std::filesystem::path> const known_repositories;
 
@@ -56,6 +59,13 @@ struct RemoteServeConfig final {
 class RemoteServeConfig::Builder final {
   public:
     // Set remote execution and cache address, unsets if parsing `address` fails
+    auto SetClientExecutionAddress(std::optional<std::string> value) noexcept
+        -> Builder& {
+        client_execution_address_ = std::move(value);
+        return *this;
+    }
+
+    // Set remote execution and cache address, unsets if parsing `address` fails
     auto SetRemoteAddress(std::optional<std::string> value) noexcept
         -> Builder& {
         remote_address_ = std::move(value);
@@ -110,7 +120,15 @@ class RemoteServeConfig::Builder final {
                                 *remote_address_)};
             }
         }
-
+        auto client_execution_address = default_config.client_execution_address;
+        if (client_execution_address_.has_value()) {
+            client_execution_address = ParseAddress(*client_execution_address_);
+            if (not client_execution_address) {
+                return unexpected{
+                    fmt::format("Setting client execution address '{}' failed.",
+                                *client_execution_address_)};
+            }
+        }
         auto known_repositories = default_config.known_repositories;
         if (known_repositories_.has_value()) {
             try {
@@ -154,6 +172,7 @@ class RemoteServeConfig::Builder final {
 
         return RemoteServeConfig{
             .remote_address = std::move(remote_address),
+            .client_execution_address = std::move(client_execution_address),
             .known_repositories = std::move(known_repositories),
             .jobs = jobs,
             .build_jobs = build_jobs,
@@ -162,9 +181,12 @@ class RemoteServeConfig::Builder final {
     }
 
   private:
-    // Server address of remote execution.
+    // Server address of the serve endpoint.
     std::optional<std::string> remote_address_;
 
+    // Execution endpoint used by the client.
+    std::optional<std::string> client_execution_address_;
+
     // Known Git repositories to serve server.
     std::optional<std::vector<std::filesystem::path>> known_repositories_;
 
diff --git a/src/buildtool/serve_api/serve_service/TARGETS b/src/buildtool/serve_api/serve_service/TARGETS
index 7b459f60..2a2928e5 100644
--- a/src/buildtool/serve_api/serve_service/TARGETS
+++ b/src/buildtool/serve_api/serve_service/TARGETS
@@ -170,6 +170,7 @@
     , ["@", "gsl", "", "gsl"]
     , ["src/buildtool/crypto", "hash_function"]
     , ["src/buildtool/execution_api/remote", "config"]
+    , ["src/buildtool/serve_api/remote", "config"]
     ]
   , "stage": ["src", "buildtool", "serve_api", "serve_service"]
   , "private-deps":
diff --git a/src/buildtool/serve_api/serve_service/configuration.cpp b/src/buildtool/serve_api/serve_service/configuration.cpp
index 2c1a14e4..503267f9 100644
--- a/src/buildtool/serve_api/serve_service/configuration.cpp
+++ b/src/buildtool/serve_api/serve_service/configuration.cpp
@@ -27,7 +27,13 @@ auto ConfigurationService::RemoteExecutionEndpoint(
     const ::justbuild::just_serve::RemoteExecutionEndpointRequest* /*request*/,
     ::justbuild::just_serve::RemoteExecutionEndpointResponse* response)
     -> ::grpc::Status {
-    auto address = remote_config_.remote_address;
+    std::optional<ServerAddress> address;
+    if (serve_config_.client_execution_address.has_value()) {
+        address = serve_config_.client_execution_address;
+    }
+    else {
+        address = remote_config_.remote_address;
+    }
     response->set_address(address ? address->ToJson().dump() : std::string());
     return ::grpc::Status::OK;
 }
diff --git a/src/buildtool/serve_api/serve_service/configuration.hpp b/src/buildtool/serve_api/serve_service/configuration.hpp
index 35431b7f..ce37c31d 100644
--- a/src/buildtool/serve_api/serve_service/configuration.hpp
+++ b/src/buildtool/serve_api/serve_service/configuration.hpp
@@ -22,6 +22,7 @@
 #include "justbuild/just_serve/just_serve.pb.h"
 #include "src/buildtool/crypto/hash_function.hpp"
 #include "src/buildtool/execution_api/remote/config.hpp"
+#include "src/buildtool/serve_api/remote/config.hpp"
 
 // This service can be used by the client to double-check the server
 // configuration.
@@ -30,9 +31,11 @@ class ConfigurationService final
   public:
     explicit ConfigurationService(
         HashFunction::Type hash_type,
-        gsl::not_null<RemoteExecutionConfig const*> const&
-            remote_config) noexcept
-        : hash_type_{hash_type}, remote_config_{*remote_config} {};
+        gsl::not_null<RemoteExecutionConfig const*> const& remote_config,
+        gsl::not_null<RemoteServeConfig const*> const& serve_config) noexcept
+        : hash_type_{hash_type},
+          remote_config_{*remote_config},
+          serve_config_{*serve_config} {}
 
     // Returns the address of the associated remote endpoint, if set,
     // or an empty string signaling that the serve endpoint acts also
@@ -58,6 +61,7 @@ class ConfigurationService final
   private:
     HashFunction::Type hash_type_;
     RemoteExecutionConfig const& remote_config_;
+    RemoteServeConfig const& serve_config_;
 };
 
 #endif  // INCLUDED_SRC_BUILD_SERVE_API_SERVE_SERVICE_CONFIGURATION_HPP
diff --git a/src/buildtool/serve_api/serve_service/serve_server_implementation.cpp b/src/buildtool/serve_api/serve_service/serve_server_implementation.cpp
index f6405651..c8426469 100644
--- a/src/buildtool/serve_api/serve_service/serve_server_implementation.cpp
+++ b/src/buildtool/serve_api/serve_service/serve_server_implementation.cpp
@@ -138,7 +138,8 @@ auto ServeServerImpl::Run(
                      &apis,
                      lock_,
                      serve ? &*serve : nullptr};
-    ConfigurationService cs{hash_type, remote_context->exec_config};
+    ConfigurationService cs{
+        hash_type, remote_context->exec_config, &serve_config};
 
     // For the SourceTreeService we need to always have access to a native
     // storage. In compatible mode, this requires creating a second local