Add exponential backoff to reading config file

src/Config/FileSystemRuntimeConfigLoader.cs

@@ -7,6 +7,7 @@
 using System.Reflection;
 using System.Text.Json;
 using Azure.DataApiBuilder.Config.ObjectModel;
+using Azure.DataApiBuilder.Config.Utilities;
 using Azure.DataApiBuilder.Service.Exceptions;
 using Microsoft.Extensions.Logging;
 
@@ -199,8 +200,31 @@ public bool TryLoadConfig(
             // and ensures the file handle is released immediately after reading.
             // Previous usage of File.Open may cause file locking issues when
             // actively using hot-reload and modifying the config file in a text editor.
-            string json = _fileSystem.File.ReadAllText(path);
-            if (TryParseConfig(json, out RuntimeConfig, connectionString: _connectionString, replaceEnvVar: replaceEnvVar))
+            // Includes an exponential back-off retry mechanism to accommodate
+            // circumstances where the file may be in use by another process.
+            int runCount = 1;
+            string json = string.Empty;
+            while (runCount <= FileUtilities.RunLimit)
+            {
+                try
+                {
+                    json = _fileSystem.File.ReadAllText(path);
+                    break;
+                }
+                catch (IOException ex)
+                {
+                    Console.WriteLine($"IO Exception, retrying due to {ex.Message}");
+                    if (runCount == FileUtilities.RunLimit)
+                    {
+                        throw;
+                    }
+
+                    Thread.Sleep(TimeSpan.FromSeconds(Math.Pow(FileUtilities.ExponentialRetryBase, runCount)));
+                    runCount++;
+                }
+            }
+
+            if (!string.IsNullOrEmpty(json) && TryParseConfig(json, out RuntimeConfig, connectionString: _connectionString, replaceEnvVar: replaceEnvVar))
             {
                 if (TrySetupConfigFileWatcher())
                 {

src/Config/Utilities/FileUtilities.cs

@@ -11,14 +11,14 @@ internal class FileUtilities
     /// <summary>
     /// Limit the number of retries when failures occur.
     /// </summary>
-    private static readonly int _runLimit = 3;
+    public static readonly int RunLimit = 3;
 
     /// <summary>
     /// Base of the exponential retry back-off.
     /// -> base ^ runCount
     /// e.g. 2^1 , 2^2, 2^3, etc.
     /// </summary>
-    private static readonly int _exponentialRetryBase = 2;
+    public static readonly int ExponentialRetryBase = 2;
 
     /// <summary>
     /// Computes the SHA256 hash for the input data (file contents) at the given path.
@@ -41,8 +41,8 @@ public static byte[] ComputeHash(IFileSystem fileSystem, string filePath)
         // Exponential back-off retry mechanism.
         int runCount = 1;
 
-        // Maximum 2^_runLimit seconds of wait time due to retries.
-        while (runCount <= _runLimit)
+        // Maximum 2^RunLimit seconds of wait time due to retries.
+        while (runCount <= RunLimit)
         {
             try
             {
@@ -68,12 +68,12 @@ public static byte[] ComputeHash(IFileSystem fileSystem, string filePath)
             catch (IOException ex)
             {
                 Console.WriteLine($"IO Exception, retrying due to {ex.Message}");
-                if (runCount == 3)
+                if (runCount == RunLimit)
                 {
                     throw;
                 }
 
-                Thread.Sleep(TimeSpan.FromSeconds(Math.Pow(_exponentialRetryBase, runCount)));
+                Thread.Sleep(TimeSpan.FromSeconds(Math.Pow(ExponentialRetryBase, runCount)));
                 runCount++;
             }
         }