COPY JSON: use trailing commas in FORCE_ARRAY output

Change COPY TO ... FORMAT JSON, FORCE_ARRAY output to place commas at
the end of each array element line, instead of at the beginning of the
next line.

Previously, output looked like this:
```
[
 {"id":1}
,{"id":2}
]
```

This is valid JSON, but it is an unusual formatting style and can be
surprising to readers. Make it emit the more conventional form instead:
```
[
 {"id":1},
 {"id":2}
]
```

Implement this without buffering the whole result by adjusting how JSON
rows are terminated and how the separator is emitted between rows.

Update the regression test output accordingly.

Author: Chao Li <lic@highgo.com>
Reviewed-by: Ayush Tiwari <ayushtiwari.slg01@gmail.com>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Alex Guo <guo.alex.hengchen@gmail.com>
Discussion: https://postgr.es/m/DFAC4097-2559-4DED-B7D5-EB53B02E9DA3@gmail.com

Author: Chao Li (Evan)

src/backend/commands/copyto.c

@@ -156,6 +156,7 @@ static void CopySendData(CopyToState cstate, const void *databuf, int datasize);
 static void CopySendString(CopyToState cstate, const char *str);
 static void CopySendChar(CopyToState cstate, char c);
 static void CopySendEndOfRow(CopyToState cstate);
+static void CopySendTextLikeLineTerminator(CopyToState cstate);
 static void CopySendTextLikeEndOfRow(CopyToState cstate);
 static void CopySendInt32(CopyToState cstate, int32 val);
 static void CopySendInt16(CopyToState cstate, int16 val);
@@ -349,6 +350,8 @@ CopyToJsonEnd(CopyToState cstate)
 {
 	if (cstate->opts.force_array)
 	{
+		if (cstate->json_row_delim_needed)
+			CopySendTextLikeLineTerminator(cstate);
 		CopySendChar(cstate, ']');
 		CopySendTextLikeEndOfRow(cstate);
 	}
@@ -418,7 +421,11 @@ CopyToJsonOneRow(CopyToState cstate, TupleTableSlot *slot)
 	if (cstate->opts.force_array)
 	{
 		if (cstate->json_row_delim_needed)
+		{
 			CopySendChar(cstate, ',');
+			CopySendTextLikeLineTerminator(cstate);
+			CopySendChar(cstate, ' ');
+		}
 		else
 		{
 			/* first row needs no delimiter */
@@ -447,7 +454,10 @@ CopyToJsonOneRow(CopyToState cstate, TupleTableSlot *slot)
 	else
 		CopySendData(cstate, cstate->json_buf->data, cstate->json_buf->len);
 
-	CopySendTextLikeEndOfRow(cstate);
+	if (cstate->opts.force_array)
+		CopySendEndOfRow(cstate);
+	else
+		CopySendTextLikeEndOfRow(cstate);
 }
 
 /*
@@ -659,11 +669,10 @@ CopySendEndOfRow(CopyToState cstate)
 }
 
 /*
- * Wrapper function of CopySendEndOfRow for text, CSV, and json formats. Sends the
- * line termination and do common appropriate things for the end of row.
+ * Append the platform-appropriate line termination for text-like output.
  */
-static inline void
-CopySendTextLikeEndOfRow(CopyToState cstate)
+static void
+CopySendTextLikeLineTerminator(CopyToState cstate)
 {
 	switch (cstate->copy_dest)
 	{
@@ -682,6 +691,16 @@ CopySendTextLikeEndOfRow(CopyToState cstate)
 		default:
 			break;
 	}
+}
+
+/*
+ * Wrapper function of CopySendEndOfRow for text, CSV, and json formats. Sends the
+ * line termination and do common appropriate things for the end of row.
+ */
+static inline void
+CopySendTextLikeEndOfRow(CopyToState cstate)
+{
+	CopySendTextLikeLineTerminator(cstate);
 
 	/* Now take the actions related to the end of a row */
 	CopySendEndOfRow(cstate);

src/test/regress/expected/copy.out

@@ -94,13 +94,13 @@ copy (values (1), (2)) TO stdout with (format json);
 {"column1":2}
 copy (select 1 union all select 2) to stdout with (format json, force_array true);
 [
- {"?column?":1}
-,{"?column?":2}
+ {"?column?":1},
+ {"?column?":2}
 ]
 copy (values (1), (2)) TO stdout with (format json, force_array true);
 [
- {"column1":1}
-,{"column1":2}
+ {"column1":1},
+ {"column1":2}
 ]
 copy copytest to stdout json;
 {"style":"DOS","test":"abc\r\ndef","filler":1}
@@ -159,17 +159,17 @@ ERROR:  COPY FORCE_ARRAY can only be used with JSON mode
 -- force_array variants
 copy copytest to stdout (format json, force_array);
 [
- {"style":"DOS","test":"abc\r\ndef","filler":1}
-,{"style":"Unix","test":"abc\ndef","filler":2}
-,{"style":"Mac","test":"abc\rdef","filler":3}
-,{"style":"esc\\ape","test":"a\\r\\\r\\\n\\nb","filler":4}
+ {"style":"DOS","test":"abc\r\ndef","filler":1},
+ {"style":"Unix","test":"abc\ndef","filler":2},
+ {"style":"Mac","test":"abc\rdef","filler":3},
+ {"style":"esc\\ape","test":"a\\r\\\r\\\n\\nb","filler":4}
 ]
 copy copytest(style, test) to stdout (format json, force_array true);
 [
- {"style":"DOS","test":"abc\r\ndef"}
-,{"style":"Unix","test":"abc\ndef"}
-,{"style":"Mac","test":"abc\rdef"}
-,{"style":"esc\\ape","test":"a\\r\\\r\\\n\\nb"}
+ {"style":"DOS","test":"abc\r\ndef"},
+ {"style":"Unix","test":"abc\ndef"},
+ {"style":"Mac","test":"abc\rdef"},
+ {"style":"esc\\ape","test":"a\\r\\\r\\\n\\nb"}
 ]
 copy copytest to stdout (format json, force_array false);
 {"style":"DOS","test":"abc\r\ndef","filler":1}