Display a more helpful message to the user if an update cannot be installed because the application was run from a read-only file system.

On Mac, this can happen if the user runs the app directly from a disk image.  On all platforms,
this can happen if the app is run from any kind of read-only media or network share.

 * Intercept IO errors during installation and try to find a more friendly alternative
   to the OS error string.

 * Catch any IO exceptions throw whilst reverting a partial update and log details.

src/FileUtils.cpp

@@ -24,28 +24,57 @@
 #endif
 
 FileUtils::IOException::IOException(const std::string& error)
-: m_errno(0)
+{
+	init(errno,error);
+}
+
+FileUtils::IOException::IOException(int errorCode, const std::string& error)
+{
+	init(errorCode,error);
+}
+
+void FileUtils::IOException::init(int errorCode, const std::string& error)
 {
 	m_error = error;
 
 #ifdef PLATFORM_UNIX
-	m_errno = errno;
+	m_errorCode = errorCode;
 
-	if (m_errno > 0)
+	if (m_errorCode > 0)
 	{
-		m_error += " details: " + std::string(strerror(m_errno));
+		m_error += " details: " + std::string(strerror(m_errorCode));
 	}
 #endif
 
 #ifdef PLATFORM_WINDOWS
+	m_errorCode = 0;
 	m_error += " GetLastError returned: " + intToStr(GetLastError());
 #endif
+
+	LOG(Info,"created IOException with errno " + intToStr(m_errorCode) + " rofs " + intToStr(EROFS));
 }
 
 FileUtils::IOException::~IOException() throw ()
 {
 }
 
+FileUtils::IOException::Type FileUtils::IOException::type() const
+{
+#ifdef PLATFORM_UNIX
+	switch (m_errorCode)
+	{
+		case 0:
+			return NoError;
+		case EROFS:
+			return ReadOnlyFileSystem;
+		default:
+			return Unknown;
+	}
+#else
+	return Unknown;
+#endif
+}
+
 bool FileUtils::fileExists(const char* path) throw (IOException)
 {
 #ifdef PLATFORM_UNIX

src/FileUtils.h

@@ -21,17 +21,32 @@ class FileUtils
 		{
 			public:
 				IOException(const std::string& error);
+				IOException(int errno, const std::string& error);
 
 				virtual ~IOException() throw ();
 
+				enum Type
+				{
+					NoError,
+					/** Unknown error type.  Call what() to get the description
+					  * provided by the OS.
+					  */
+					Unknown,
+					ReadOnlyFileSystem
+				};
+
 				virtual const char* what() const throw ()
 				{
 					return m_error.c_str();
 				}
 
+				Type type() const;
+
 			private:
+				void init(int errorCode, const std::string& error);
+
 				std::string m_error;
-				int m_errno;
+				int m_errorCode;
 		};
 
 		/** Remove a file.  Throws an exception if the file

src/UpdateInstaller.cpp

@@ -71,6 +71,29 @@ void UpdateInstaller::reportError(const std::string& error)
 	}
 }
 
+std::string UpdateInstaller::friendlyErrorForError(const FileUtils::IOException& exception) const
+{
+	std::string friendlyError;
+
+	switch (exception.type())
+	{
+		case FileUtils::IOException::ReadOnlyFileSystem:
+#ifdef PLATFORM_MAC
+			friendlyError = AppInfo::appName() + " was started from a read-only location.  "
+			                "Copy it to the Applications folder on your Mac and run "
+			                "it from there.";
+#else
+			friendlyError = AppInfo::appName() + " was started from a read-only location.  "
+							"Re-install it to a location that can be updated and run it from there.";
+#endif
+			break;
+		default:
+			break;
+	}
+
+	return friendlyError;
+}
+
 void UpdateInstaller::run() throw ()
 {
 	if (!m_script || !m_script->isValid())
@@ -144,7 +167,13 @@ void UpdateInstaller::run() throw ()
 	{
 		LOG(Info,"Starting update installation");
 
+		// the detailed error string returned by the OS
 		std::string error;
+		// the message to present to the user.  This may be the same
+		// as 'error' or may be different if a more helpful suggestion
+		// can be made for a particular problem
+		std::string friendlyError;
+
 		try
 		{
 			LOG(Info,"Installing new and updated files");
@@ -161,6 +190,7 @@ void UpdateInstaller::run() throw ()
 		catch (const FileUtils::IOException& exception)
 		{
 			error = exception.what();
+			friendlyError = friendlyErrorForError(exception);
 		}
 		catch (const std::string& genericError)
 		{
@@ -170,10 +200,23 @@ void UpdateInstaller::run() throw ()
 		if (!error.empty())
 		{
 			LOG(Error,std::string("Error installing update ") + error);
-			revert();
+
+			try
+			{
+				revert();
+			}
+			catch (const FileUtils::IOException& exception)
+			{
+				LOG(Error,"Error reverting partial update " + std::string(exception.what()));
+			}
+
 			if (m_observer)
 			{
-				m_observer->updateError(error);
+				if (friendlyError.empty())
+				{
+					friendlyError = error;
+				}
+				m_observer->updateError(friendlyError);
 			}
 		}
 

src/UpdateInstaller.h

@@ -1,6 +1,7 @@
 #pragma once
 
 #include "Platform.h"
+#include "FileUtils.h"
 #include "UpdateScript.h"
 
 #include <list>
@@ -52,6 +53,7 @@ class UpdateInstaller
 		void postInstallUpdate();
 
 		std::list<std::string> updaterArgs() const;
+		std::string friendlyErrorForError(const FileUtils::IOException& ex) const;
 
 		Mode m_mode;
 		std::string m_installDir;