From 809c53287a5ed558617678d04b6a66a5bcd1a9fd Mon Sep 17 00:00:00 2001
From: Richard Smith <richard@metafoo.co.uk>
Date: Mon, 27 Jan 2025 07:36:09 -0800
Subject: [PATCH] Fix crash when driver fuzzer finds language-server. (#4847)

The driver fuzzer, unlike the actual driver, provides a null
`input_stream`, which caused the `language-server` subcommand to crash.
---
 toolchain/driver/driver_test.cpp                | 5 +++++
 toolchain/driver/language_server_subcommand.cpp | 1 +
 2 files changed, 6 insertions(+)

diff --git a/toolchain/driver/driver_test.cpp b/toolchain/driver/driver_test.cpp
index 4fba9e5210ff..2fd62947ba36 100644
--- a/toolchain/driver/driver_test.cpp
+++ b/toolchain/driver/driver_test.cpp
@@ -228,5 +228,10 @@ TEST_F(DriverTest, FileOutput) {
   EXPECT_THAT(ReadFile("test.s"), ContainsRegex("Main:"));
 }
 
+TEST_F(DriverTest, LanguageServerNoStdin) {
+  EXPECT_FALSE(driver_.RunCommand({"language-server"}).success);
+  EXPECT_THAT(test_error_stream_.TakeStr(), HasSubstr("requires input_stream"));
+}
+
 }  // namespace
 }  // namespace Carbon
diff --git a/toolchain/driver/language_server_subcommand.cpp b/toolchain/driver/language_server_subcommand.cpp
index 816a66184da9..a93eefc5a88f 100644
--- a/toolchain/driver/language_server_subcommand.cpp
+++ b/toolchain/driver/language_server_subcommand.cpp
@@ -22,6 +22,7 @@ auto LanguageServerSubcommand::Run(DriverEnv& driver_env) -> DriverResult {
   if (!driver_env.input_stream) {
     *driver_env.error_stream
         << "error: language-server requires input_stream\n";
+    return {.success = false};
   }
 
   auto err =