fix: handle potential DB conflict due to concurrent upload requests in postFile (#19005)

This issue manifests when users have multiple templates which rely on
the same files, for example see:
https://github.com/coder/coder/issues/17442

---------

Signed-off-by: Callum Styan <callumstyan@gmail.com>

coderd/files.go

@@ -118,11 +118,23 @@ func (api *API) postFile(rw http.ResponseWriter, r *http.Request) {
 		Data:      data,
 	})
 	if err != nil {
-		httpapi.Write(ctx, rw, http.StatusInternalServerError, codersdk.Response{
-			Message: "Internal error saving file.",
-			Detail:  err.Error(),
-		})
-		return
+		if database.IsUniqueViolation(err, database.UniqueFilesHashCreatedByKey) {
+			// The file was uploaded by some concurrent process since the last time we checked for it, fetch it again.
+			file, err = api.Database.GetFileByHashAndCreator(ctx, database.GetFileByHashAndCreatorParams{
+				Hash:      hash,
+				CreatedBy: apiKey.UserID,
+			})
+			api.Logger.Info(ctx, "postFile handler hit UniqueViolation trying to upload file after already checking for the file existence", slog.F("hash", hash), slog.F("created_by_id", apiKey.UserID))
+		}
+		// At this point the first error was either not the UniqueViolation OR there's still an error even after we
+		// attempt to fetch the file again, so we should return here.
+		if err != nil {
+			httpapi.Write(ctx, rw, http.StatusInternalServerError, codersdk.Response{
+				Message: "Internal error saving file.",
+				Detail:  err.Error(),
+			})
+			return
+		}
 	}
 
 	httpapi.Write(ctx, rw, http.StatusCreated, codersdk.UploadResponse{

coderd/files_test.go

@@ -5,6 +5,7 @@ import (
 	"bytes"
 	"context"
 	"net/http"
+	"sync"
 	"testing"
 
 	"github.com/google/uuid"
@@ -69,6 +70,30 @@ func TestPostFiles(t *testing.T) {
 		_, err = client.Upload(ctx, codersdk.ContentTypeTar, bytes.NewReader(data))
 		require.NoError(t, err)
 	})
+	t.Run("InsertConcurrent", func(t *testing.T) {
+		t.Parallel()
+		client := coderdtest.New(t, nil)
+		_ = coderdtest.CreateFirstUser(t, client)
+
+		ctx, cancel := context.WithTimeout(context.Background(), testutil.WaitLong)
+		defer cancel()
+
+		var wg sync.WaitGroup
+		var end sync.WaitGroup
+		wg.Add(1)
+		end.Add(3)
+		for range 3 {
+			go func() {
+				wg.Wait()
+				data := make([]byte, 1024)
+				_, err := client.Upload(ctx, codersdk.ContentTypeTar, bytes.NewReader(data))
+				end.Done()
+				require.NoError(t, err)
+			}()
+		}
+		wg.Done()
+		end.Wait()
+	})
 }
 
 func TestDownload(t *testing.T) {