Throw NotFoundException from GetPullRequestMergeBase

Previously null was being returned and information about what went wrong was lost.

Author: jcansdale

src/GitHub.App/Services/GitClient.cs

@@ -98,7 +98,7 @@ public Task Fetch(IRepository repository, string remoteName, params string[] ref
                     var remote = repository.Network.Remotes[remoteName];
                     repository.Network.Fetch(remote, refspecs, fetchOptions);
                 }
-                catch(Exception ex)
+                catch (Exception ex)
                 {
                     log.Error("Failed to fetch", ex);
 #if DEBUG
@@ -394,7 +394,7 @@ public async Task<string> GetPullRequestMergeBase(IRepository repo,
                 baseCommit = repo.Lookup<Commit>(baseSha);
                 if (baseCommit == null)
                 {
-                    return null;
+                    throw new NotFoundException($"Couldn't find {baseSha} after fetching from {baseCloneUrl}:{baseRef}.");
                 }
             }
 
@@ -405,14 +405,14 @@ public async Task<string> GetPullRequestMergeBase(IRepository repo,
                 headCommit = repo.Lookup<Commit>(headSha);
                 if (headCommit == null)
                 {
-                    return null;
+                    throw new NotFoundException($"Couldn't find {headSha} after fetching from {headCloneUrl}:{headRef}.");
                 }
             }
 
             var mergeBaseCommit = repo.ObjectDatabase.FindMergeBase(baseCommit, headCommit);
-            if(mergeBaseCommit == null)
+            if (mergeBaseCommit == null)
             {
-                return null;
+                throw new NotFoundException($"Couldn't find merge base between {baseCommit} and {headCommit}.");
             }
 
             return mergeBaseCommit.Sha;

src/GitHub.App/Services/PullRequestService.cs

@@ -316,11 +316,6 @@ public IObservable<string> ExtractFile(
                         pullRequest.Head.Sha,
                         pullRequest.Base.Ref,
                         pullRequest.Head.Ref);
-
-                    if (sha == null)
-                    {
-                        throw new NotFoundException($"Couldn't find merge base between {pullRequest.Base.Sha} and {pullRequest.Head.Sha}.");
-                    }
                 }
 
                 var file = await ExtractToTempFile(repo, pullRequest.Number, sha, fileName, encoding);
@@ -350,7 +345,7 @@ static bool HasPreamble(string file, Encoding encoding)
             {
                 foreach (var b in encoding.GetPreamble())
                 {
-                    if(b != stream.ReadByte())
+                    if (b != stream.ReadByte())
                     {
                         return false;
                     }
@@ -473,7 +468,7 @@ async Task<bool> IsBranchMarkedAsPullRequest(IRepository repo, string branchName
         {
             var prConfigKey = $"branch.{branchName}.{SettingGHfVSPullRequest}";
             var value = ParseGHfVSConfigKeyValue(await gitClient.GetConfig<string>(repo, prConfigKey));
-            return value != null && 
+            return value != null &&
                 value.Item1 == pullRequest.Base.RepositoryCloneUrl.Owner &&
                 value.Item2 == pullRequest.Number;
         }

src/GitHub.Exports.Reactive/Services/IGitClient.cs

@@ -197,6 +197,7 @@ public interface IGitClient
         /// <returns>
         /// The merge base SHA or null.
         /// </returns>
+        /// <exception cref="LibGit2Sharp.NotFoundException">Thrown when the merge base can't be found.</exception>
         Task<string> GetPullRequestMergeBase(IRepository repo, UriString baseCloneUrl, UriString headCloneUrl, string baseSha, string headSha, string baseRef, string headRef);
 
         /// Checks whether the current head is pushed to its remote tracking branch.