Do not treat \. as an EOF marker in CSV mode for COPY IN.

Since backslash is (typically) not special in CSV data, we should
not be treating \. as special either.  The server historically did
this to keep CSV and TEXT modes more alike and to support V2 protocol;
but V2 protocol is long dead, and the inconsistency with CSV standards
is annoying.  Remove that behavior in CopyReadLineText, and make some
minor consequent code simplifications.

On the client side, we need to fix psql so that it does not check
for \. except when reading data from STDIN (that is, the script
source).  We must do that regardless of TEXT/CSV mode or there is
no way to end the COPY short of script EOF.  Also, be careful
not to send the \. to the server in that case.

This is a small compatibility break in that other applications
beside psql may need similar adjustment.  Also, using an older
version of psql with a v18 server may result in misbehavior
during CSV-mode COPY IN.

Daniel Vérité, reviewed by vignesh C, Robert Haas, and myself

Discussion: https://postgr.es/m/ed659f37-a9dd-42a7-82b9-0da562cc4006@manitou-mail.org

doc/src/sgml/libpq.sgml

@@ -7381,8 +7381,9 @@ int PQputline(PGconn *conn,
         <literal>\.</literal> as a final line to indicate to the server that it had
         finished sending <command>COPY</command> data.  While this still works, it is deprecated and the
         special meaning of <literal>\.</literal> can be expected to be removed in a
-        future release.  It is sufficient to call <xref linkend="libpq-PQendcopy"/> after
-        having sent the actual data.
+        future release.  (It already will misbehave in <literal>CSV</literal>
+        mode.)  It is sufficient to call <xref linkend="libpq-PQendcopy"/>
+        after having sent the actual data.
        </para>
       </note>
      </listitem>

doc/src/sgml/protocol.sgml

@@ -7606,8 +7606,9 @@ psql "dbname=postgres replication=database" -c "IDENTIFY_SYSTEM;"
    is a well-defined way to recover from errors during <command>COPY</command>.  The special
    <quote><literal>\.</literal></quote> last line is not needed anymore, and is not sent
    during <command>COPY OUT</command>.
-   (It is still recognized as a terminator during <command>COPY IN</command>, but its use is
-   deprecated and will eventually be removed.)  Binary <command>COPY</command> is supported.
+   (It is still recognized as a terminator during text-mode <command>COPY
+   IN</command>, but not in CSV mode.  The text-mode behavior is
+   deprecated and may eventually be removed.)  Binary <command>COPY</command> is supported.
    The CopyInResponse and CopyOutResponse messages include fields indicating
    the number of columns and the format of each column.
   </para>

doc/src/sgml/ref/copy.sgml

@@ -646,11 +646,16 @@ COPY <replaceable class="parameter">count</replaceable>
    </para>
 
    <para>
-    End of data can be represented by a single line containing just
+    End of data can be represented by a line containing just
     backslash-period (<literal>\.</literal>).  An end-of-data marker is
     not necessary when reading from a file, since the end of file
-    serves perfectly well; it is needed only when copying data to or from
-    client applications using pre-3.0 client protocol.
+    serves perfectly well; in that context this provision exists only for
+    backward compatibility.  However, <application>psql</application>
+    uses <literal>\.</literal> to terminate a <literal>COPY FROM
+    STDIN</literal> operation (that is, reading
+    in-line <command>COPY</command> data in a SQL script).  In that
+    context the rule is needed to be able to end the operation before the
+    end of the script.
    </para>
 
    <para>
@@ -811,16 +816,25 @@ COPY <replaceable class="parameter">count</replaceable>
 
    <para>
     Because backslash is not a special character in the <literal>CSV</literal>
-    format, <literal>\.</literal>, the end-of-data marker, could also appear
-    as a data value.  To avoid any misinterpretation, a <literal>\.</literal>
-    data value appearing as a lone entry on a line is automatically
-    quoted on output, and on input, if quoted, is not interpreted as the
-    end-of-data marker.  If you are loading a file created by another
-    application that has a single unquoted column and might have a
-    value of <literal>\.</literal>, you might need to quote that value in the
-    input file.
+    format, the end-of-data marker used in text mode (<literal>\.</literal>)
+    is not normally treated as special when reading <literal>CSV</literal>
+    data.  An exception is that <application>psql</application> will terminate
+    a <literal>COPY FROM STDIN</literal> operation (that is, reading
+    in-line <command>COPY</command> data in a SQL script) at a line containing
+    only <literal>\.</literal>, whether it is text or <literal>CSV</literal>
+    mode.
    </para>
 
+   <note>
+    <para>
+     <productname>PostgreSQL</productname> versions before v18 always
+     recognized unquoted <literal>\.</literal> as an end-of-data marker,
+     even when reading from a separate file.  For compatibility with older
+     versions, <command>COPY TO</command> will quote <literal>\.</literal>
+     when it's alone on a line, even though this is no longer necessary.
+    </para>
+   </note>
+
    <note>
     <para>
      In <literal>CSV</literal> format, all characters are significant. A quoted value

doc/src/sgml/ref/psql-ref.sgml

@@ -1135,7 +1135,8 @@ SELECT $1 \parse stmt1
 
         <para>
         For <literal>\copy ... from stdin</literal>, data rows are read from the same
-        source that issued the command, continuing until <literal>\.</literal>
+        source that issued the command, continuing until a line containing
+        only <literal>\.</literal>
         is read or the stream reaches <acronym>EOF</acronym>. This option is useful
         for populating tables in-line within an SQL script file.
         For <literal>\copy ... to stdout</literal>, output is sent to the same place
@@ -1179,10 +1180,6 @@ SELECT $1 \parse stmt1
         destination, because all data must pass through the client/server
         connection.  For large amounts of data the <acronym>SQL</acronym>
         command might be preferable.
-        Also, because of this pass-through method, <literal>\copy
-        ... from</literal> in <acronym>CSV</acronym> mode will erroneously
-        treat a <literal>\.</literal> data value alone on a line as an
-        end-of-input marker.
         </para>
         </tip>
 

src/backend/commands/copyfromparse.c

@@ -136,14 +136,6 @@ if (1) \
 	} \
 } else ((void) 0)
 
-/* Undo any read-ahead and jump out of the block. */
-#define NO_END_OF_COPY_GOTO \
-if (1) \
-{ \
-	input_buf_ptr = prev_raw_ptr + 1; \
-	goto not_end_of_copy; \
-} else ((void) 0)
-
 /* NOTE: there's a copy of this in copyto.c */
 static const char BinarySignature[11] = "PGCOPY\n\377\r\n\0";
 
@@ -1182,7 +1174,6 @@ CopyReadLineText(CopyFromState cstate)
 	bool		result = false;
 
 	/* CSV variables */
-	bool		first_char_in_line = true;
 	bool		in_quote = false,
 				last_was_esc = false;
 	char		quotec = '\0';
@@ -1268,12 +1259,12 @@ CopyReadLineText(CopyFromState cstate)
 		if (cstate->opts.csv_mode)
 		{
 			/*
-			 * If character is '\\' or '\r', we may need to look ahead below.
-			 * Force fetch of the next character if we don't already have it.
-			 * We need to do this before changing CSV state, in case one of
-			 * these characters is also the quote or escape character.
+			 * If character is '\r', we may need to look ahead below.  Force
+			 * fetch of the next character if we don't already have it.  We
+			 * need to do this before changing CSV state, in case '\r' is also
+			 * the quote or escape character.
 			 */
-			if (c == '\\' || c == '\r')
+			if (c == '\r')
 			{
 				IF_NEED_REFILL_AND_NOT_EOF_CONTINUE(0);
 			}
@@ -1377,10 +1368,10 @@ CopyReadLineText(CopyFromState cstate)
 		}
 
 		/*
-		 * In CSV mode, we only recognize \. alone on a line.  This is because
-		 * \. is a valid CSV data value.
+		 * Process backslash, except in CSV mode where backslash is a normal
+		 * character.
 		 */
-		if (c == '\\' && (!cstate->opts.csv_mode || first_char_in_line))
+		if (c == '\\' && !cstate->opts.csv_mode)
 		{
 			char		c2;
 
@@ -1398,12 +1389,6 @@ CopyReadLineText(CopyFromState cstate)
 			if (c2 == '.')
 			{
 				input_buf_ptr++;	/* consume the '.' */
-
-				/*
-				 * Note: if we loop back for more data here, it does not
-				 * matter that the CSV state change checks are re-executed; we
-				 * will come back here with no important state changed.
-				 */
 				if (cstate->eol_type == EOL_CRNL)
 				{
 					/* Get the next character */
@@ -1412,23 +1397,13 @@ CopyReadLineText(CopyFromState cstate)
 					c2 = copy_input_buf[input_buf_ptr++];
 
 					if (c2 == '\n')
-					{
-						if (!cstate->opts.csv_mode)
-							ereport(ERROR,
-									(errcode(ERRCODE_BAD_COPY_FILE_FORMAT),
-									 errmsg("end-of-copy marker does not match previous newline style")));
-						else
-							NO_END_OF_COPY_GOTO;
-					}
+						ereport(ERROR,
+								(errcode(ERRCODE_BAD_COPY_FILE_FORMAT),
+								 errmsg("end-of-copy marker does not match previous newline style")));
 					else if (c2 != '\r')
-					{
-						if (!cstate->opts.csv_mode)
-							ereport(ERROR,
-									(errcode(ERRCODE_BAD_COPY_FILE_FORMAT),
-									 errmsg("end-of-copy marker corrupt")));
-						else
-							NO_END_OF_COPY_GOTO;
-					}
+						ereport(ERROR,
+								(errcode(ERRCODE_BAD_COPY_FILE_FORMAT),
+								 errmsg("end-of-copy marker corrupt")));
 				}
 
 				/* Get the next character */
@@ -1437,14 +1412,9 @@ CopyReadLineText(CopyFromState cstate)
 				c2 = copy_input_buf[input_buf_ptr++];
 
 				if (c2 != '\r' && c2 != '\n')
-				{
-					if (!cstate->opts.csv_mode)
-						ereport(ERROR,
-								(errcode(ERRCODE_BAD_COPY_FILE_FORMAT),
-								 errmsg("end-of-copy marker corrupt")));
-					else
-						NO_END_OF_COPY_GOTO;
-				}
+					ereport(ERROR,
+							(errcode(ERRCODE_BAD_COPY_FILE_FORMAT),
+							 errmsg("end-of-copy marker corrupt")));
 
 				if ((cstate->eol_type == EOL_NL && c2 != '\n') ||
 					(cstate->eol_type == EOL_CRNL && c2 != '\n') ||
@@ -1467,7 +1437,7 @@ CopyReadLineText(CopyFromState cstate)
 				result = true;	/* report EOF */
 				break;
 			}
-			else if (!cstate->opts.csv_mode)
+			else
 			{
 				/*
 				 * If we are here, it means we found a backslash followed by
@@ -1475,23 +1445,11 @@ CopyReadLineText(CopyFromState cstate)
 				 * after a backslash is special, so we skip over that second
 				 * character too.  If we didn't do that \\. would be
 				 * considered an eof-of copy, while in non-CSV mode it is a
-				 * literal backslash followed by a period.  In CSV mode,
-				 * backslashes are not special, so we want to process the
-				 * character after the backslash just like a normal character,
-				 * so we don't increment in those cases.
+				 * literal backslash followed by a period.
 				 */
 				input_buf_ptr++;
 			}
 		}
-
-		/*
-		 * This label is for CSV cases where \. appears at the start of a
-		 * line, but there is more text after it, meaning it was a data value.
-		 * We are more strict for \. in CSV mode because \. could be a data
-		 * value, while in non-CSV mode, \. cannot be a data value.
-		 */
-not_end_of_copy:
-		first_char_in_line = false;
 	}							/* end of outer loop */
 
 	/*

src/backend/commands/copyto.c

@@ -1160,8 +1160,11 @@ CopyAttributeOutCSV(CopyToState cstate, const char *string,
 	if (!use_quote)
 	{
 		/*
-		 * Because '\.' can be a data value, quote it if it appears alone on a
-		 * line so it is not interpreted as the end-of-data marker.
+		 * Quote '\.' if it appears alone on a line, so that it will not be
+		 * interpreted as an end-of-data marker.  (PG 18 and up will not
+		 * interpret '\.' in CSV that way, except in embedded-in-SQL data; but
+		 * we want the data to be loadable by older versions too.  Also, this
+		 * avoids breaking clients that are still using PQgetline().)
 		 */
 		if (single_attr && strcmp(ptr, "\\.") == 0)
 			use_quote = true;

src/bin/psql/copy.c

@@ -620,20 +620,29 @@ handleCopyIn(PGconn *conn, FILE *copystream, bool isbinary, PGresult **res)
 				/* current line is done? */
 				if (buf[buflen - 1] == '\n')
 				{
-					/* check for EOF marker, but not on a partial line */
-					if (at_line_begin)
+					/*
+					 * When at the beginning of the line and the data is
+					 * inlined, check for EOF marker.  If the marker is found,
+					 * we must stop at this point.  If not, the \. line can be
+					 * sent to the server, and we let it decide whether it's
+					 * an EOF or not depending on the format: in TEXT mode, \.
+					 * will be interpreted as an EOF, in CSV, it will not.
+					 */
+					if (at_line_begin && copystream == pset.cur_cmd_source)
 					{
-						/*
-						 * This code erroneously assumes '\.' on a line alone
-						 * inside a quoted CSV string terminates the \copy.
-						 * https://www.postgresql.org/message-id/E1TdNVQ-0001ju-GO@wrigleys.postgresql.org
-						 *
-						 * https://www.postgresql.org/message-id/bfcd57e4-8f23-4c3e-a5db-2571d09208e2@beta.fastmail.com
-						 */
 						if ((linelen == 3 && memcmp(fgresult, "\\.\n", 3) == 0) ||
 							(linelen == 4 && memcmp(fgresult, "\\.\r\n", 4) == 0))
 						{
 							copydone = true;
+
+							/*
+							 * Remove the EOF marker from the data sent.  In
+							 * CSV mode, the EOF marker must be removed,
+							 * otherwise it would be interpreted by the server
+							 * as valid data.
+							 */
+							*fgresult = '\0';
+							buflen -= linelen;
 						}
 					}
 

src/test/regress/expected/copy.out

@@ -32,6 +32,24 @@ select * from copytest except select * from copytest2;
 -------+------+--------
 (0 rows)
 
+--- test unquoted \. as data inside CSV
+-- do not use copy out to export the data, as it would quote \.
+\o :filename
+\qecho line1
+\qecho '\\.'
+\qecho line2
+\o
+-- get the data back in with copy
+truncate copytest2;
+copy copytest2(test) from :'filename' csv;
+select test from copytest2 order by test collate "C";
+ test  
+-------
+ \.
+ line1
+ line2
+(3 rows)
+
 -- test header line feature
 create temp table copytest3 (
 	c1 int,

src/test/regress/sql/copy.sql

@@ -38,6 +38,18 @@ copy copytest2 from :'filename' csv quote '''' escape E'\\';
 
 select * from copytest except select * from copytest2;
 
+--- test unquoted \. as data inside CSV
+-- do not use copy out to export the data, as it would quote \.
+\o :filename
+\qecho line1
+\qecho '\\.'
+\qecho line2
+\o
+-- get the data back in with copy
+truncate copytest2;
+copy copytest2(test) from :'filename' csv;
+select test from copytest2 order by test collate "C";
+
 
 -- test header line feature