Support \if ... \elif ... \else ... \endif in psql scripting.

This patch adds nestable conditional blocks to psql.  The control
structure feature per se is complete, but the boolean expressions
understood by \if and \elif are pretty primitive; basically, after
variable substitution and backtick expansion, the result has to be
"true" or "false" or one of the other standard spellings of a boolean
value.  But that's enough for many purposes, since you can always
do the heavy lifting on the server side; and we can extend it later.

Along the way, pay down some of the technical debt that had built up
around psql/command.c:
* Refactor exec_command() into a function per command, instead of
being a 1500-line monstrosity.  This makes the file noticeably longer
because of repetitive function header/trailer overhead, but it seems
much more readable.
* Teach psql_get_variable() and psqlscanslash.l to suppress variable
substitution and backtick expansion on the basis of the conditional
stack state, thereby allowing removal of the OT_NO_EVAL kluge.
* Fix the no-doubt-once-expedient hack of sometimes silently substituting
mainloop.c's previous_buf for query_buf when calling HandleSlashCmds.
(It's a bit remarkable that commands like \r worked at all with that.)
Recall of a previous query is now done explicitly in the slash commands
where that should happen.

Corey Huinker, reviewed by Fabien Coelho, further hacking by me

Discussion: https://postgr.es/m/CADkLM=c94OSRTnat=LX0ivNq4pxDNeoomFfYvBKM5N_xfmLtAA@mail.gmail.com

1 files changed, 89 insertions, 30 deletions

diff --git a/src/bin/psql/mainloop.c b/src/bin/psql/mainloop.c
index 6e358e2e1b8..2bc2f43b4e7 100644
--- a/src/bin/psql/mainloop.c
+++ b/src/bin/psql/mainloop.c
@@ -35,6 +35,7 @@ int
 MainLoop(FILE *source)
 {
 	PsqlScanState scan_state;	/* lexer working state */
+	ConditionalStack cond_stack;	/* \if status stack */
 	volatile PQExpBuffer query_buf;		/* buffer for query being accumulated */
 	volatile PQExpBuffer previous_buf;	/* if there isn't anything in the new
 										 * buffer yet, use this one for \e,
@@ -50,16 +51,15 @@ MainLoop(FILE *source)
 	volatile promptStatus_t prompt_status = PROMPT_READY;
 	volatile int count_eof = 0;
 	volatile bool die_on_error = false;
-
-	/* Save the prior command source */
 	FILE	   *prev_cmd_source;
 	bool		prev_cmd_interactive;
 	uint64		prev_lineno;
 
-	/* Save old settings */
+	/* Save the prior command source */
 	prev_cmd_source = pset.cur_cmd_source;
 	prev_cmd_interactive = pset.cur_cmd_interactive;
 	prev_lineno = pset.lineno;
+	/* pset.stmt_lineno does not need to be saved and restored */
 
 	/* Establish new source */
 	pset.cur_cmd_source = source;
@@ -69,6 +69,8 @@ MainLoop(FILE *source)
 
 	/* Create working state */
 	scan_state = psql_scan_create(&psqlscan_callbacks);
+	cond_stack = conditional_stack_create();
+	psql_scan_set_passthrough(scan_state, (void *) cond_stack);
 
 	query_buf = createPQExpBuffer();
 	previous_buf = createPQExpBuffer();
@@ -122,7 +124,19 @@ MainLoop(FILE *source)
 			cancel_pressed = false;
 
 			if (pset.cur_cmd_interactive)
+			{
 				putc('\n', stdout);
+
+				/*
+				 * if interactive user is in an \if block, then Ctrl-C will
+				 * exit from the innermost \if.
+				 */
+				if (!conditional_stack_empty(cond_stack))
+				{
+					psql_error("\\if: escaped\n");
+					conditional_stack_pop(cond_stack);
+				}
+			}
 			else
 			{
 				successResult = EXIT_USER;
@@ -140,7 +154,8 @@ MainLoop(FILE *source)
 			/* May need to reset prompt, eg after \r command */
 			if (query_buf->len == 0)
 				prompt_status = PROMPT_READY;
-			line = gets_interactive(get_prompt(prompt_status), query_buf);
+			line = gets_interactive(get_prompt(prompt_status, cond_stack),
+									query_buf);
 		}
 		else
 		{
@@ -286,8 +301,10 @@ MainLoop(FILE *source)
 				(scan_result == PSCAN_EOL && pset.singleline))
 			{
 				/*
-				 * Save query in history.  We use history_buf to accumulate
-				 * multi-line queries into a single history entry.
+				 * Save line in history.  We use history_buf to accumulate
+				 * multi-line queries into a single history entry.  Note that
+				 * history accumulation works on input lines, so it doesn't
+				 * matter whether the query will be ignored due to \if.
 				 */
 				if (pset.cur_cmd_interactive && !line_saved_in_history)
 				{
@@ -296,22 +313,36 @@ MainLoop(FILE *source)
 					line_saved_in_history = true;
 				}
 
-				/* execute query */
-				success = SendQuery(query_buf->data);
-				slashCmdStatus = success ? PSQL_CMD_SEND : PSQL_CMD_ERROR;
-				pset.stmt_lineno = 1;
-
-				/* transfer query to previous_buf by pointer-swapping */
+				/* execute query unless we're in an inactive \if branch */
+				if (conditional_active(cond_stack))
 				{
-					PQExpBuffer swap_buf = previous_buf;
+					success = SendQuery(query_buf->data);
+					slashCmdStatus = success ? PSQL_CMD_SEND : PSQL_CMD_ERROR;
+					pset.stmt_lineno = 1;
 
-					previous_buf = query_buf;
-					query_buf = swap_buf;
-				}
-				resetPQExpBuffer(query_buf);
+					/* transfer query to previous_buf by pointer-swapping */
+					{
+						PQExpBuffer swap_buf = previous_buf;
 
-				added_nl_pos = -1;
-				/* we need not do psql_scan_reset() here */
+						previous_buf = query_buf;
+						query_buf = swap_buf;
+					}
+					resetPQExpBuffer(query_buf);
+
+					added_nl_pos = -1;
+					/* we need not do psql_scan_reset() here */
+				}
+				else
+				{
+					/* if interactive, warn about non-executed query */
+					if (pset.cur_cmd_interactive)
+						psql_error("query ignored; use \\endif or Ctrl-C to exit current \\if block\n");
+					/* fake an OK result for purposes of loop checks */
+					success = true;
+					slashCmdStatus = PSQL_CMD_SEND;
+					pset.stmt_lineno = 1;
+					/* note that query_buf doesn't change state */
+				}
 			}
 			else if (scan_result == PSCAN_BACKSLASH)
 			{
@@ -343,21 +374,24 @@ MainLoop(FILE *source)
 
 				/* execute backslash command */
 				slashCmdStatus = HandleSlashCmds(scan_state,
-												 query_buf->len > 0 ?
-												 query_buf : previous_buf);
+												 cond_stack,
+												 query_buf,
+												 previous_buf);
 
 				success = slashCmdStatus != PSQL_CMD_ERROR;
-				pset.stmt_lineno = 1;
 
-				if ((slashCmdStatus == PSQL_CMD_SEND || slashCmdStatus == PSQL_CMD_NEWEDIT) &&
-					query_buf->len == 0)
-				{
-					/* copy previous buffer to current for handling */
-					appendPQExpBufferStr(query_buf, previous_buf->data);
-				}
+				/*
+				 * Resetting stmt_lineno after a backslash command isn't
+				 * always appropriate, but it's what we've done historically
+				 * and there have been few complaints.
+				 */
+				pset.stmt_lineno = 1;
 
 				if (slashCmdStatus == PSQL_CMD_SEND)
 				{
+					/* should not see this in inactive branch */
+					Assert(conditional_active(cond_stack));
+
 					success = SendQuery(query_buf->data);
 
 					/* transfer query to previous_buf by pointer-swapping */
@@ -374,6 +408,8 @@ MainLoop(FILE *source)
 				}
 				else if (slashCmdStatus == PSQL_CMD_NEWEDIT)
 				{
+					/* should not see this in inactive branch */
+					Assert(conditional_active(cond_stack));
 					/* rescan query_buf as new input */
 					psql_scan_finish(scan_state);
 					free(line);
@@ -429,8 +465,17 @@ MainLoop(FILE *source)
 		if (pset.cur_cmd_interactive)
 			pg_send_history(history_buf);
 
-		/* execute query */
-		success = SendQuery(query_buf->data);
+		/* execute query unless we're in an inactive \if branch */
+		if (conditional_active(cond_stack))
+		{
+			success = SendQuery(query_buf->data);
+		}
+		else
+		{
+			if (pset.cur_cmd_interactive)
+				psql_error("query ignored; use \\endif or Ctrl-C to exit current \\if block\n");
+			success = true;
+		}
 
 		if (!success && die_on_error)
 			successResult = EXIT_USER;
@@ -439,6 +484,19 @@ MainLoop(FILE *source)
 	}
 
 	/*
+	 * Check for unbalanced \if-\endifs unless user explicitly quit, or the
+	 * script is erroring out
+	 */
+	if (slashCmdStatus != PSQL_CMD_TERMINATE &&
+		successResult != EXIT_USER &&
+		!conditional_stack_empty(cond_stack))
+	{
+		psql_error("reached EOF without finding closing \\endif(s)\n");
+		if (die_on_error && !pset.cur_cmd_interactive)
+			successResult = EXIT_USER;
+	}
+
+	/*
 	 * Let's just make real sure the SIGINT handler won't try to use
 	 * sigint_interrupt_jmp after we exit this routine.  If there is an outer
 	 * MainLoop instance, it will reset sigint_interrupt_jmp to point to
@@ -452,6 +510,7 @@ MainLoop(FILE *source)
 	destroyPQExpBuffer(history_buf);
 
 	psql_scan_destroy(scan_state);
+	conditional_stack_destroy(cond_stack);
 
 	pset.cur_cmd_source = prev_cmd_source;
 	pset.cur_cmd_interactive = prev_cmd_interactive;