ClangFormat: Add caching of current style

Previously, ClangFormat was getting the style for a file
every time styleForFile was called, which could cause
unnecessary delays and impact performance. This commit
adds caching of the current style with a timeout of 1s,
so that ClangFormat can quickly access the cached style
instead of recalculating it each time.

Change-Id: I33c114d51d6ce1acd0b6d9d2a28e2b6712e149dd
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/clangformat/clangformatbaseindenter.cpp

@@ -774,10 +774,28 @@ clang::format::FormatStyle overrideStyle(const Utils::FilePath &fileName)
     return currentSettingsStyle;
 }
 
-clang::format::FormatStyle ClangFormatBaseIndenter::styleForFile() const
+static std::chrono::milliseconds getCacheTimeout()
 {
-    if (getCurrentOverriddenSettings(m_fileName))
-        return overrideStyle(m_fileName);
+    using namespace std::chrono_literals;
+    bool ok = false;
+    const int envCacheTimeout = qEnvironmentVariableIntValue("CLANG_FORMAT_CACHE_TIMEOUT", &ok);
+    return ok ? std::chrono::milliseconds(envCacheTimeout) : 1s;
+}
+
+const clang::format::FormatStyle &ClangFormatBaseIndenter::styleForFile() const
+{
+    using namespace std::chrono_literals;
+    static const std::chrono::milliseconds cacheTimeout = getCacheTimeout();
+
+    QDateTime time = QDateTime::currentDateTime();
+    if (m_cachedStyle.expirationTime > time && !(m_cachedStyle.style == clang::format::getNoStyle()))
+        return m_cachedStyle.style;
+
+    if (getCurrentOverriddenSettings(m_fileName)) {
+        clang::format::FormatStyle style = overrideStyle(m_fileName);
+        m_cachedStyle.setCache(style, cacheTimeout);
+        return m_cachedStyle.style;
+    }
 
     llvm::Expected<clang::format::FormatStyle> styleFromProjectFolder
         = clang::format::getStyle("file",
@@ -788,14 +806,17 @@ clang::format::FormatStyle ClangFormatBaseIndenter::styleForFile() const
 
     if (styleFromProjectFolder && !(*styleFromProjectFolder == clang::format::getNoStyle())) {
         addQtcStatementMacros(*styleFromProjectFolder);
-        return *styleFromProjectFolder;
+        m_cachedStyle.setCache(*styleFromProjectFolder, cacheTimeout);
+        return m_cachedStyle.style;
     }
 
     handleAllErrors(styleFromProjectFolder.takeError(), [](const llvm::ErrorInfoBase &) {
         // do nothing
     });
 
-    return qtcStyle();
+
+    m_cachedStyle.setCache(qtcStyle(), 0ms);
+    return m_cachedStyle.style;
 }
 
 } // namespace ClangFormat

src/plugins/clangformat/clangformatbaseindenter.h

@@ -46,7 +46,7 @@ public:
 
     std::optional<int> margin() const override;
 
-    clang::format::FormatStyle styleForFile() const;
+    const clang::format::FormatStyle &styleForFile() const;
 
 protected:
     virtual bool formatCodeInsteadOfIndent() const { return false; }
@@ -71,6 +71,18 @@ private:
                                            ReplacementsToKeep replacementsToKeep,
                                            const QChar &typedChar = QChar::Null,
                                            bool secondTry = false) const;
+
+    struct CachedStyle {
+        clang::format::FormatStyle style = clang::format::getNoStyle();
+        QDateTime expirationTime;
+        void setCache(clang::format::FormatStyle newStyle, std::chrono::milliseconds timeout)
+        {
+            style = newStyle;
+            expirationTime = QDateTime::currentDateTime() + timeout;
+        }
+    };
+
+    mutable CachedStyle m_cachedStyle;
 };
 
 } // namespace ClangFormat

src/plugins/clangformat/clangformatutils.cpp

@@ -31,7 +31,7 @@ using namespace Utils;
 
 namespace ClangFormat {
 
-clang::format::FormatStyle qtcStyle()
+clang::format::FormatStyle calculateQtcStyle()
 {
     clang::format::FormatStyle style = getLLVMStyle();
     style.Language = FormatStyle::LK_Cpp;
@@ -179,6 +179,12 @@ clang::format::FormatStyle qtcStyle()
     return style;
 }
 
+clang::format::FormatStyle qtcStyle()
+{
+    static clang::format::FormatStyle style = calculateQtcStyle();
+    return style;
+}
+
 QString projectUniqueId(ProjectExplorer::Project *project)
 {
     if (!project)