Clang: Disable internal threads in libclang

We already run all operations in threads to avoid blocking, there is no
need to start more threads within libclang. Before this change, a
reparse would trigger 3 threads to start:
  1. clangbackend: Utils::runAsync() for the job
  2. libclang-internal: thread for reparse
  3. libclang-internal: thread for annotating tokens (highlighting)

Ensure that we use the same stack size for our threads as libclang was
doing internally. C++ parsers usually have higher stack size
requirements.

Change-Id: I2f67602ddfbf77ea2c69144b56acf64ba08041f6
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>

src/libs/clangsupport/clangcodemodelconnectionclient.cpp

@@ -25,6 +25,7 @@
 
 #include "clangcodemodelconnectionclient.h"
 
+#include <utils/environment.h>
 #include <utils/temporarydirectory.h>
 
 #include <QCoreApplication>
@@ -50,6 +51,10 @@ ClangCodeModelConnectionClient::ClangCodeModelConnectionClient(
     m_processCreator.setTemporaryDirectoryPattern("clangbackend-XXXXXX");
     m_processCreator.setArguments({connectionName()});
 
+    Utils::Environment environment;
+    environment.set(QStringLiteral("LIBCLANG_NOTHREADS"), QString());
+    m_processCreator.setEnvironment(environment);
+
     stdErrPrefixer().setPrefix("clangbackend.stderr: ");
     stdOutPrefixer().setPrefix("clangbackend.stdout: ");
 }

src/libs/clangsupport/processcreator.cpp

@@ -56,6 +56,11 @@ void ProcessCreator::setArguments(const QStringList &arguments)
     m_arguments = arguments;
 }
 
+void ProcessCreator::setEnvironment(const Utils::Environment &environment)
+{
+    m_environment = environment;
+}
+
 std::future<QProcessUniquePointer> ProcessCreator::createProcess() const
 {
     return std::async(std::launch::async, [&] {
@@ -167,6 +172,10 @@ QProcessEnvironment ProcessCreator::processEnvironment() const
         processEnvironment.insert("TEMP", temporaryDirectoryPath);
     }
 
+    const Utils::Environment &env = m_environment;
+    for (auto it = env.constBegin(); it != env.constEnd(); ++it)
+        processEnvironment.insert(it.key(), it.value());
+
     return processEnvironment;
 }
 

src/libs/clangsupport/processcreator.h

@@ -29,6 +29,7 @@
 
 #include "processhandle.h"
 
+#include <utils/environment.h>
 #include <utils/temporarydirectory.h>
 
 #include <QStringList>
@@ -51,6 +52,7 @@ public:
     void setTemporaryDirectoryPattern(const QString &temporaryDirectoryPattern);
     void setProcessPath(const QString &m_processPath);
     void setArguments(const QStringList &m_arguments);
+    void setEnvironment(const Utils::Environment &environment);
     void setObserver(QObject *m_observer);
 
     std::future<QProcessUniquePointer> createProcess() const;
@@ -72,6 +74,7 @@ private:
     QString m_processPath;
     QString m_temporaryDirectoryPattern;
     QStringList m_arguments;
+    Utils::Environment m_environment;
     QObject *m_observer = nullptr;
 };
 

src/tools/clangbackend/source/clangasyncjob.h

@@ -58,7 +58,9 @@ public:
                          &QFutureWatcher<Result>::finished,
                          onFinished);
 
-        const QFuture<Result> future = Utils::runAsync(m_runner);
+        // Use 16MB stack size as clang_annotateTokens() would with an internal thread.
+        const Utils::StackSizeInBytes stackSize = 1024 * 1024 * 16;
+        const QFuture<Result> future = Utils::runAsync(stackSize, m_runner);
         m_futureWatcher.setFuture(future);
 
         return future;