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.

2 files changed, 17 insertions, 0 deletions

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);
         }