[SPARK-17658][SPARKR] read.df/write.df API taking path optionally in SparkR

## What changes were proposed in this pull request?

`write.df`/`read.df` API require path which is not actually always necessary in Spark. Currently, it only affects the datasources implementing `CreatableRelationProvider`. Currently, Spark currently does not have internal data sources implementing this but it'd affect other external datasources.

In addition we'd be able to use this way in Spark's JDBC datasource after https://github.com/apache/spark/pull/12601 is merged.

**Before**

 - `read.df`

  ```r
> read.df(source = "json")
Error in dispatchFunc("read.df(path = NULL, source = NULL, schema = NULL, ...)",  :
  argument "x" is missing with no default
```

  ```r
> read.df(path = c(1, 2))
Error in dispatchFunc("read.df(path = NULL, source = NULL, schema = NULL, ...)",  :
  argument "x" is missing with no default
```

  ```r
> read.df(c(1, 2))
Error in invokeJava(isStatic = TRUE, className, methodName, ...) :
  java.lang.ClassCastException: java.lang.Double cannot be cast to java.lang.String
	at org.apache.spark.sql.execution.datasources.DataSource.hasMetadata(DataSource.scala:300)
	at
...
In if (is.na(object)) { :
...
```

 - `write.df`

  ```r
> write.df(df, source = "json")
Error in (function (classes, fdef, mtable)  :
  unable to find an inherited method for function ‘write.df’ for signature ‘"function", "missing"’
```

  ```r
> write.df(df, source = c(1, 2))
Error in (function (classes, fdef, mtable)  :
  unable to find an inherited method for function ‘write.df’ for signature ‘"SparkDataFrame", "missing"’
```

  ```r
> write.df(df, mode = TRUE)
Error in (function (classes, fdef, mtable)  :
  unable to find an inherited method for function ‘write.df’ for signature ‘"SparkDataFrame", "missing"’
```

**After**

- `read.df`

  ```r
> read.df(source = "json")
Error in loadDF : analysis error - Unable to infer schema for JSON at . It must be specified manually;
```

  ```r
> read.df(path = c(1, 2))
Error in f(x, ...) : path should be charactor, null or omitted.
```

  ```r
> read.df(c(1, 2))
Error in f(x, ...) : path should be charactor, null or omitted.
```

- `write.df`

  ```r
> write.df(df, source = "json")
Error in save : illegal argument - 'path' is not specified
```

  ```r
> write.df(df, source = c(1, 2))
Error in .local(df, path, ...) :
  source should be charactor, null or omitted. It is 'parquet' by default.
```

  ```r
> write.df(df, mode = TRUE)
Error in .local(df, path, ...) :
  mode should be charactor or omitted. It is 'error' by default.
```

## How was this patch tested?

Unit tests in `test_sparkSQL.R`

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #15231 from HyukjinKwon/write-default-r.

1 files changed, 35 insertions, 0 deletions

diff --git a/R/pkg/inst/tests/testthat/test_sparkSQL.R b/R/pkg/inst/tests/testthat/test_sparkSQL.R
index 9d874a0988..f5ab601f27 100644
--- a/R/pkg/inst/tests/testthat/test_sparkSQL.R
+++ b/R/pkg/inst/tests/testthat/test_sparkSQL.R
@@ -2544,6 +2544,41 @@ test_that("Spark version from SparkSession", {
   expect_equal(ver, version)
 })
 
+test_that("Call DataFrameWriter.save() API in Java without path and check argument types", {
+  df <- read.df(jsonPath, "json")
+  # This tests if the exception is thrown from JVM not from SparkR side.
+  # It makes sure that we can omit path argument in write.df API and then it calls
+  # DataFrameWriter.save() without path.
+  expect_error(write.df(df, source = "csv"),
+               "Error in save : illegal argument - 'path' is not specified")
+
+  # Arguments checking in R side.
+  expect_error(write.df(df, "data.tmp", source = c(1, 2)),
+               paste("source should be character, NULL or omitted. It is the datasource specified",
+                     "in 'spark.sql.sources.default' configuration by default."))
+  expect_error(write.df(df, path = c(3)),
+               "path should be charactor, NULL or omitted.")
+  expect_error(write.df(df, mode = TRUE),
+               "mode should be charactor or omitted. It is 'error' by default.")
+})
+
+test_that("Call DataFrameWriter.load() API in Java without path and check argument types", {
+  # This tests if the exception is thrown from JVM not from SparkR side.
+  # It makes sure that we can omit path argument in read.df API and then it calls
+  # DataFrameWriter.load() without path.
+  expect_error(read.df(source = "json"),
+               paste("Error in loadDF : analysis error - Unable to infer schema for JSON at .",
+                     "It must be specified manually"))
+  expect_error(read.df("arbitrary_path"), "Error in loadDF : analysis error - Path does not exist")
+
+  # Arguments checking in R side.
+  expect_error(read.df(path = c(3)),
+               "path should be charactor, NULL or omitted.")
+  expect_error(read.df(jsonPath, source = c(1, 2)),
+               paste("source should be character, NULL or omitted. It is the datasource specified",
+                     "in 'spark.sql.sources.default' configuration by default."))
+})
+
 unlink(parquetPath)
 unlink(orcPath)
 unlink(jsonPath)