Fix the endless loop in Join and Merge when both suffixes are the same

src/Microsoft.Data.Analysis/DataFrame.Join.cs

@@ -26,6 +26,15 @@ public partial class DataFrame
         private void SetSuffixForDuplicatedColumnNames(DataFrame dataFrame, DataFrameColumn column, string leftSuffix, string rightSuffix)
         {
             int index = dataFrame._columnCollection.IndexOf(column.Name);
+
+            // The loop below appends leftSuffix to the existing column and rightSuffix to the new one until the two
+            // names differ. If both suffixes are the same the names stay equal however many times they are appended,
+            // so there is no name that ends the loop.
+            if (index != -1 && leftSuffix == rightSuffix)
+            {
+                throw new ArgumentException(string.Format(Strings.SuffixesMustBeDifferent, nameof(leftSuffix), nameof(rightSuffix), column.Name), nameof(rightSuffix));
+            }
+
             while (index != -1)
             {
                 // Pre-existing column. Change name

src/Microsoft.Data.Analysis/Strings.resx

@@ -261,6 +261,9 @@
   <data name="StreamDoesntSupportReading" xml:space="preserve">
     <value>Stream doesn't support reading</value>
   </data>
+  <data name="SuffixesMustBeDifferent" xml:space="preserve">
+    <value>{0} and {1} must be different when both DataFrames contain a column called '{2}'</value>
+  </data>
   <data name="ValueNameAlreadyExists" xml:space="preserve">
     <value>Value name '{0}' matches an existing column name</value>
   </data>

test/Microsoft.Data.Analysis.Tests/DataFrameTests.Join.cs

@@ -82,6 +82,40 @@ public void TestJoin()
             VerifyJoin(join, left, right, JoinAlgorithm.Inner);
         }
 
+        [Theory]
+        [InlineData(JoinAlgorithm.Left)]
+        [InlineData(JoinAlgorithm.Right)]
+        [InlineData(JoinAlgorithm.FullOuter)]
+        [InlineData(JoinAlgorithm.Inner)]
+        public void TestJoin_ColumnNamesCollide_ThrowsOnSameSuffixOnBothSides(JoinAlgorithm joinAlgorithm)
+        {
+            DataFrame left = MakeDataFrameWithNumericColumns(3, false);
+            DataFrame right = MakeDataFrameWithNumericColumns(4, false);
+
+            // Both frames have a column called "Int", and one suffix cannot make the two names different,
+            // so the join used to keep renaming for ever instead of coming back
+            Assert.Throws<ArgumentException>(() => left.Join(right, "_same", "_same", joinAlgorithm));
+            Assert.Throws<ArgumentException>(() => left.Join(right, "", "", joinAlgorithm));
+
+            // different suffixes still work
+            DataFrame join = left.Join(right, "_left", "_right", joinAlgorithm);
+            Assert.Equal(left.Columns.Count + right.Columns.Count, join.Columns.Count);
+        }
+
+        [Fact]
+        public void TestJoin_SameSuffixWithNoSharedColumnNames()
+        {
+            DataFrame left = new DataFrame(new Int32DataFrameColumn("Left", new int?[] { 0, 1, 2 }));
+            DataFrame right = new DataFrame(new Int32DataFrameColumn("Right", new int?[] { 0, 1, 2 }));
+
+            // no name is shared, so nothing is renamed and the suffixes are never used
+            DataFrame join = left.Join(right, "_same", "_same");
+
+            Assert.Equal(2, join.Columns.Count);
+            Assert.Equal("Left", join.Columns[0].Name);
+            Assert.Equal("Right", join.Columns[1].Name);
+        }
+
         private void VerifyJoin(DataFrame join, DataFrame left, DataFrame right, JoinAlgorithm joinAlgorithm)
         {
             Int64DataFrameColumn mapIndices = new Int64DataFrameColumn("map", join.Rows.Count);

test/Microsoft.Data.Analysis.Tests/DataFrameTests.Merge.cs

@@ -1013,6 +1013,18 @@ public void TestMerge_CorrectColumnTypes()
             Assert.NotNull(merge.Columns.GetDateTimeColumn("DateTime_right"));
         }
 
+        [Fact]
+        public void TestMerge_ColumnNamesCollide_ThrowsOnSameSuffixOnBothSides()
+        {
+            DataFrame left = MakeDataFrameWithNumericColumns(3, false);
+            DataFrame right = MakeDataFrameWithNumericColumns(4, false);
+
+            Assert.Throws<ArgumentException>(() => left.Merge<int>(right, "Int", "Int", "_same", "_same"));
+
+            DataFrame merge = left.Merge<int>(right, "Int", "Int");
+            Assert.Equal(left.Columns.Count + right.Columns.Count, merge.Columns.Count);
+        }
+
         private void VerifyMerge(DataFrame merge, DataFrame left, DataFrame right, JoinAlgorithm joinAlgorithm)
         {
             if (joinAlgorithm == JoinAlgorithm.Left || joinAlgorithm == JoinAlgorithm.Inner)