Add logic to pick alternative log path on error

This change causes the Logger class to pick an alternative path for the
log file if the desired location can't be used.  One case where this
occurs is when the host process is installed under the Program Files
folder.

Author: daviwil

src/PowerShellEditorServices/Utility/Logger.cs

@@ -126,15 +126,16 @@ public LogWriter(LogLevel minimumLogLevel, string logFilePath, bool deleteExisti
                         logFilePath);
             }
 
-            // Open the log file for writing with UTF8 encoding
-            this.textWriter =
-                new StreamWriter(
-                    new FileStream(
-                        logFilePath,
-                        deleteExisting ?
-                            FileMode.Create :
-                            FileMode.Append),
-                    Encoding.UTF8);
+            if (!this.TryOpenLogFile(logFilePath, deleteExisting))
+            {
+                // If the log file couldn't be opened at this location,
+                // try opening it in a more reliable path
+                this.TryOpenLogFile(
+                    Path.Combine(
+                        Environment.GetFolderPath(Environment.SpecialFolder.LocalApplicationData),
+                        Path.GetFileName(logFilePath)),
+                    deleteExisting);
+            }
         }
 
         public void Write(
@@ -144,7 +145,8 @@ public void Write(
             string callerSourceFile = null,
             int callerLineNumber = 0)
         {
-            if (logLevel >= this.minimumLogLevel)
+            if (this.textWriter != null &&
+                logLevel >= this.minimumLogLevel)
             {
                 // Print the timestamp and log level
                 this.textWriter.WriteLine(
@@ -176,5 +178,39 @@ public void Dispose()
                 this.textWriter = null;
             }
         }
+
+        private bool TryOpenLogFile(
+            string logFilePath, 
+            bool deleteExisting)
+        {
+            try
+            {
+                // Open the log file for writing with UTF8 encoding
+                this.textWriter =
+                    new StreamWriter(
+                        new FileStream(
+                            logFilePath,
+                            deleteExisting ?
+                                FileMode.Create :
+                                FileMode.Append),
+                        Encoding.UTF8);
+
+                return true;
+            }
+            catch (Exception e)
+            {
+                if (e is UnauthorizedAccessException ||
+                    e is IOException)
+                {
+                    // This exception is thrown when we can't open the file
+                    // at the path in logFilePath.  Return false to indicate
+                    // that the log file couldn't be created.
+                    return false;
+                }
+
+                // Unexpected exception, rethrow it
+                throw;
+            }
+        }
     }
 }